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HL-LHC Analysis With ROOT
ROOT Project Input to the HL-LHC Computing Review Stage 2
The ROOT Team, September 2021
Bertrand Bellenot,aJakob Blomer,aPhilippe Canal,bOlivier Couet,aJavier
Lopez Gomez,aBernhard Manfred Gruber,aEnrico Guiraud,aJonas Hahnfeld,a
Sergey Linev,dLorenzo Moneta,aAxel Naumann,aVincenzo Eduardo
Padulano,aJonas Rembser,aAlja Mrak Tadel,cMatevz Tadel,cEnric Tejedor
and aVassil Vassileve
aCERN, Geneva, Switzerland
bFermi National Accelerator Laboratory, Batavia, USA
cUniversity of California San Diego, USA
dGSI, Darmstadt, Germany
ePrinceton University, USA
arXiv:2205.06121v1 [hep-ex] 12 May 2022
1 Introduction 1
2 Analysis Ecosystem 2
2.1 Machine Learning 2
2.2 Python 3
2.3 Data Format 5
3 Empowering Physicists 6
3.1 Analysis as Eﬀective Use of Luminosity, Eﬃciency of Physicists, Time
to Publication 6
3.2 Multi-Platform Support 7
3.3 Code Reuse and Preservation 8
3.4 Open Science 9
3.5 Training, Education, Support 10
4 Analysis Data 12
4.1 Data Models Optimized for Analysis 12
4.2 Machine Learning 12
4.3 Data Movement to GPU 13
4.4 Data Location, Transfer, Caching 14
5 Analysis Design 15
5.1 Declarative Analysis 16
5.2 Accelerators 18
6 Analysis Algorithms 18
6.1 Histogramming 18
6.2 Modeling and Fitting 19
6.3 Minimization 21
7 Visualization and Graphical Interfaces 21
7.1 Scientiﬁc Graphics 22
7.2 Graphical User Interfaces 23
7.3 Jupyter Notebooks 23
7.4 Event Display 23
– i –
8 Project Requirements 25
8.1 Organization 25
8.2 Development Team 26
8.3 Sustainability 27
8.4 Generational Handover 28
8.5 Cooperation & Innovation 29
8.6 Creating Opportunities for Externally Funded Projects 29
8.7 Summary of Risks, Functionality Gaps, Dependencies 30
9 Summary 41
10 Conclusion 42
The HL-LHC analysis is expected  to be similar to Run 3 analyses, with more
events (data and simulation), possibly higher number of samples (multiple back-
ground channels, side-band channels from data, comparing more collision event gen-
erators, higher number of data skims), higher complexity of ﬁts due to more precise
models with higher parameter counts, and more common usage of Machine Learning
to improve the physics performance of analysis algorithms. As a consequence of more
complex ﬁts and the usage of machine learning, ROOT expects a further increase in
the usage of GPUs and possibly other accelerators such as TPUs.
ROOT is preparing for HL-LHC analyses through a series of R&D tasks started
several years ago, to be able to address these challenges in the form of an integrated
solution. It will do this in time for HL-LHC, given the resources and ROOT’s expe-
rience with a sustainable feedback and development velocity, and the adoption rate
in the physics community. Several of these undertakings are massive (in ROOT’s
scale) with respect to the required eﬀort and expected impact.
Until now, ROOT has provided the core ingredients for virtually all LHC anal-
yses. Because of its role as ”integrated analysis foundation library”, much of ROOT
can be used by ROOT itself, increasing the impact of its own developments: all of
ROOT’s features are optimized with respect to I/O; many make use of ROOT’s visu-
alization abilities; fundamental features such as just-in-time compilation and network
calls are enabling fundamentally new features such as remote event displays.
Being a central element of the HEP software ecosystem, ROOT sees many con-
tributions, from volunteer, occasional improvements to institutionalized cooperation.
– 1 –
Whatever the contribution, it will become part of ROOT only if the contribution is
relevant to the community as a whole. Or, conversely, any successful contribution to
ROOT has a large impact on the community, multiplied through the tens of thousand
physicists and virtually all HEP experiments, world-wide.
Given the experience and expertise of the ROOT project, ROOT believes the
main HL-LHC analysis challenges to be:
•Data: high-throughput, local and remote data access to reduce the impact of
analyses’ traditional I/O bottleneck.
•Eﬃciency and simplicity: making it obvious for physicists how to write an
analysis with faster time-to-results.
•Robustness and correctness: providing results that can be trusted and pub-
•Interoperability: enabling physicists to use the language of choice, with the
packages of choice, without sacriﬁcing performance or simplicity.
•Sustainability and Innovation: investing in solutions that the community can
beneﬁt from for the next decades.
This part of the document introduces the main analysis challenges as perceived
by the ROOT project, alongside their potential remedies with associated risks.
2 Analysis Ecosystem
ROOT continues to be a catalyzer for an analysis ecosystem around it. At the same
time, Python is expected to become the dominating HL-LHC analysis language, and
ROOT is further integrating into a wider Python ecosystem. ROOT works towards
strengthening both aspects - by providing the integrated, fundamental ingredients
targeted at HEP that enable ecosystems to grow around it, and by providing smooth
interoperability to the Python analysis ecosystem in a reusable way for easy integra-
tion with emerging ecosystems such as Julia.
2.1 Machine Learning
Most primary machine learning (ML) interfaces are written in Python, encouraging
the transfer of large amounts of data for instance to GPUs which then internally
run non-Python code. ROOT works towards the integration with such frameworks,
speciﬁcally PyTorch and TensorFlow’s generator interface, to have simple and highly
eﬃcient user-facing interaction between ROOT’s analysis interface RDataFrame and
ML tools. This beneﬁts from ROOT’s ability to optimize the HEP I/O layer. Ex-
amples include ROOT’s ability to arrange for in-memory data layout and alignment
– 2 –
that corresponds to on-disk layouts, reducing or removing the need for CPU cycles
when transferring data into GPUs.
Investment in this area (1.5 FTE over 3 years) is needed for the analysis commu-
nity to beneﬁt from ROOT’s I/O layer, and to create a performance and usability
beneﬁt so signiﬁcant that analysts do not migrate to less performant options due
to a perceived increase of productivity. It is trivial for physicists to ﬁnd examples
for use of machine learning for instance from data in text format. Without addi-
tional beneﬁts and clear, communicated advantages, large parts of the community
will adopt less eﬃcient (in terms of CPU and storage-space requirements) approaches
simply because of their much higher online presence. This is an inherent risk of the
wider adoption of Python as the preferred analysis language, with a transforma-
tion away from the single, community-agreed, consistently designed, integrated and
performance-conscious analysis framework, to an ecosystem of literally thousands of
potential building blocks [https://pypi.org/ ]. Investment in this area (0.5FTE over
4 years) can help reduce the residual risk that comes with Python becoming the
primary analysis language, which is nowhere as visible as in the context of machine
learning, and which we expect to continue during HL-LHC.
ROOT’s role with ML must thus be to integrate external machine learning li-
braries with ROOT’s I/O and analysis facilities. This requires expertise in Python
and C++, ML, GPUs, as well as code and memory layout optimization, as an ongo-
ing eﬀort (0.5 FTE/y). ROOT’s R&D on RNTuple (1 FTE/y over 3 years), LLAMA
(0.5 FTE/y over 4 years), cling-CUDA integration (0.1FTE/y over 3 years) and au-
tomatic diﬀerentiation (0.5 FTE/y over 4 years) are signiﬁcant building blocks in
that respect. First results for RDataFrame-based training on GPUs are expected
in 2022; ML-optimized RNTuple-to-GPU transfer leveraging above mentioned R&D
will be implemented by 2024 (0.75 FTE over 2 years).
The optimization of machine-learning models is an interactive, user-facing activ-
ity. ROOT used to provide a GUI for the visual inspection of the training quality
(for instance versus epoch). This has many users, but needs to be improved and
ported to ROOT’s new web-based GUI, with all its advantages. Work for this is
underway (0.25FTE/y), and should complete by 2024.
Dissemination of optimal analysis approaches again underlines the necessity for
ROOT’s experts to be trusted and visible in the HEP ML and analysis community.
It requires the continuous investment of a signiﬁcant training eﬀort, by said ROOT
ML experts (0.25FTE/y).
Python is seeing a constantly increasing adoption rate as language of choice for
analyses. It is important to understand that this is the surface layer: just as bash
scripts steer programs for the computational work, Python code is ideally delegat-
ing resource-intense work to for instance C/C++ libraries. With PyROOT, ROOT
– 3 –
provides the ”glue” used by HEP to make HEP’s C++ libraries accessible transpar-
ently through Python. With signiﬁcantly more than ten years of experience, ROOT
has learned how to design C++ interfaces in a way that makes them good Python
interfaces, too. A good example is ROOT’s analysis interface RDataFrame, where
virtually identical user code is written, be it in C++ or Python . While much
of the (pseudo-) object oriented interfaces match nicely across languages, some id-
ioms are language-speciﬁc and need to be adapted. PyROOT enabled this through
a mechanism called ”pythonizations”: features added on top of the default bindings
to make ROOT easier to use from Python, or in a more intuitive way for Python
PyROOT relies on a project called cppyy , a tool used for instance in compu-
tational biology. Cppyy in turn relies on ROOT’s type description system and cling,
ROOT’s interpreter / JIT, to dynamically create the binding between Python and
C++. This binding layer needs to evolve with C++, supporting new language fea-
tures and data types. Signiﬁcant expertise is required for this, in the areas of Python,
and in cling and ROOT’s type description system, which provide PyROOT with an
answer to questions such as ”which methods does edm::Collection<edm::Jet>have”.
Due to a lack of past investment, this layer has accumulated signiﬁcant technical
debt. The current state is hindering gradual evolution with minimal investment, and
limits PyROOT’s performance and functionality. Addressing this would require an
investment of 1 FTE over 4 years.
The experiments’ frameworks are aggressively embracing new C++ standards,
cling and PyROOT are expected to support new features as quickly as possible. The
continuous investment of 0.5 FTE/y is necessary to prevent a reduction of function-
ality from Python, which in turn can mean usage migrating away from performant
ROOT’s interoperability with Python libraries such as NumPy is crucial for
high performance data processing. ROOT plans to invest in a C++ container that
allows data transfer directly into NumPy arrays. This work (0.25 FTE over 1 year)
is expected to conclude by 2023.
ROOT remains a C++ program and library; yet - seeing the widespread use
of Python in analyses - needs to ensure that ROOT can be used ”naturally” from
Python, in terms of interoperability (data formats such as NumPy), interface style,
Python syntax, object ownership. This requires investment in existing and future
C++ interfaces. It even stipulates the re-design of interfaces such as RooFit. With-
out such an investment, a fair part of the community is expected to migrate to
alternative solutions. The number of available alternatives - even today - will cause
a fragmentation of the community. This reduces the beneﬁts of central investment;
all known solutions have feature limitations, risking a reduced physics reach for
analyses; solutions will not be easily applicable to other problems due to the ex-
pected feature segregation. To counter these risks, ROOT has invested signiﬁcantly
– 4 –
in PyROOT and for instance RooFit, to improve ease of use, stability, performance,
support, documentation - in short: to reduce the physicists’ need to migrate away.
This eﬀort needs to be maintained (1 FTE/y), for ROOT to be able to continue to
compete, and to reduce the residual risk of community fragmentation. At the same
time, competition needs to be fostered, especially in the Python ecosystem, which
sees much innovation (especially in interfaces) and can be a fruitful, eﬀective ground
for software experiments. This competition is input to ROOT’s evolution, and allows
to benchmark ROOT against alternatives.
For several of years now, ROOT has been released also through Conda. This
gives many Python users trivial access to ROOT. The notion of ”ROOT is C++ and
thus awkward to use in Python” needs to be overcome, by continuing to invest in
simple distribution mechanisms and perfect embedding in Python ecosystems, mak-
ing ROOT accessible in Python ecosystems as easily as any other Python packaged
C++ library (continuous 0.1FTE/y).
What has been done for Python with PyROOT can be (and has been) done for
other languages, in a similar way. ROOT’s C++ interpreter / JIT compiler cling
allows ROOT to create such language bindings dynamically for languages that might
become relevant to HEP analyses in the future.
2.3 Data Format
Up until Run3, the LHC experiments used ROOT ﬁles and TTrees; several, mostly
small-scale, studies have questioned that and proposed alternatives. A more complete
and in-depth review showed the advantages of ROOT’s ﬁle format and TTree .
Nonetheless, these studies and for instance ROOT’s experience with supporting I/O
in multi-threaded environments have shown limitations to TTree. As ROOT’s I/O
subsystem guarantees backward compatibility (old ROOT ﬁles can be read with new
ROOT versions) as well as forward compatibility (new ROOT ﬁles can be read with
old ROOT versions), evolution of TTree is severely limited.
R&D on potential beneﬁts of a new data layout for HEP, called RNTuple ,
showed improvements to transfer rate, storage size eﬃciency (see Fig. 1), robustness,
and ﬂexibility that are suﬃciently signiﬁcant to warrant the introduction of a new,
evolved I/O subsystem for HL-LHC, see the Foundation part of the ROOT input.
ROOT caters both to frameworks and analysis physicists. It was thus paramount
to make RNTuple work exceptionally well also for analyses. RNTuple is expected
to have a signiﬁcant performance eﬀect on machine learning (more than a factor 10
read throughput in training compared to TTree) and RDataFrame, ROOT’s modern
analysis interface (factor 2 in read throughput). These two standard ingredients of
analyses are traditionally I/O limited and will beneﬁt directly from RNTuple.
– 5 –
Average event size [B]
Storage Efficiency CMS nanoAOD TTJet 13TeV June 2019
Storage Efficiency CMS nanoAOD TTJet 13TeV June 2019
uncompressed lz4 zstd zlib lzma
RNTuple / TTree
69% 73% 74% 76% 87%
Figure 1. Fraction of storage size RNTuple vs. TTree for identical data content (CMS
NanoAOD); lower is better.
ROOT expects to provide a tool converting TTrees to RNTuples in 2021 (0.2 FTE).
For several years now, ROOT has been advocating and training physicists in the use
of RDataFrame, also as an abstraction of the underlying data layout and to ease the
migration from TTree to RNTuple. RDataFrame can process ROOT ﬁles contain-
ing TTrees just as well as ROOT ﬁles containing RNTuples. ROOT will continue
to maintain the TTree interfaces. Nonetheless we expect to see a migration away
from the use of low level TTree interfaces to RDataFrame, enabling optimizations
and concurrency. After all, the data format will be deﬁned by the experiments; it is
ROOT’s task to ensure a smooth migration is possible for physicists.
Alternative data formats continue to be used in HEP, although it is unclear
whether their adoption is increasing. As analyses are probably the most agile part
of HEP’s software environment, they are expected to continue to try alternatives
(HDF5, Parquet, etc), also in function of the tools and libraries used by analyses
. ROOT’s goal is to preempt such re-formatting, which has consistently proven
as a bottleneck for analyses’ agility, preventing smooth integration of optimizations
in the ROOT-part of the analyses, and increasing the storage needs for analyses.
During the coming years, ROOT will continue to work on providing eﬃcient and
easy to use interfaces. Examples include interfaces to ML , for transparent use of
multi-core and GPUs [2,8], for distributed computing , to Python , and for
3 Empowering Physicists
ROOT’s role is to facilitate analyses. Helping physicists get analysis results quickly,
reliably, and with less resources, is what ROOT is striving to improve, constantly.
3.1 Analysis as Eﬀective Use of Luminosity, Eﬃciency of Physicists,
Time to Publication
There are two drawbacks in high turn-around times of analyses: on one hand, this
simply delays the results, pushing what could be published in 2025 to 2026. This
– 6 –
used to be perceived as a mere optimization issue. Being ﬁrst or a runner-up seems
to be of increasing signiﬁcance to funding agencies, the public, and experiments as
On the other hand, the analysis throughput of physicists is a limited resource.
Making physicists more productive simply means better use of luminosity. Or in
other terms, the analysis eﬃciency should be considered as an integral part of the
experiments’ overall eﬃciency: it should not stop at reconstruction.
Increasing the physics reach of a typical PhD analysis should thus be an impor-
tant goal for the community. This can be considered as part of the overall eﬃciency,
and included in optimizations and upgrades. As detailed below, ROOT has the
proven potential to measurably increase the eﬀectiveness of analyses by factors, at
a cost that is shared between experiments and that is far lower than investment in
3.2 Multi-Platform Support
Being mostly written in C++, ROOT’s code can make eﬃcient use of CPUs. This
brings a certain platform dependence: compiled C++ code will only work on a given
set of architectures; optimized code might restrict this even more. ROOT strives to
support as many platforms and operating systems as reasonably possible. It certainly
intends to support all architectures and operating systems that are in production use,
by experiments or a signiﬁcant number of physicists. It currently supports GCC and
clang compilers, up to the latest released versions; work is ongoing to also support
the Microsoft Visual Studio Compiler (0.2FTE/y). The supported operating systems
are Linux (Fedora, CentOS, Ubuntu, Debian) and macOS (up to the latest version,
including the ARM M1 version); work is ongoing to support Windows. Architectures
include Intel and AMD 64bit, Intel 32bit, ARM Aarch64; work is ongoing to ensure
continued support of Power8 and Power9. ROOT is released by Linux distributions
(or rather their package maintainers) on Fedora and CentOS EPEL, through Conda,
Homebrew, and Snap. This service has been provided by the community for many
years, and seems to demonstrate that ROOT is part of a solid, sustainable software
ecosystem. To facilitate all of this, ROOT maintains a Continuous Integration and
Testing / Continuous Release system (0.5 FTE/y).
In the past, being multi-platform caused ROOT to contain architecture-speciﬁc
code, such as X11 for Linux; Cocoa (including ObjectiveC code) for macOS, and
GDK for Windows. This is expertise not needed elsewhere in ROOT, and the evo-
lution of these platforms (such as macOS dropping X11 and GL support) forces
ROOT to invest in these areas of (for ROOT) niche expertise. To address this,
ROOT is developing HTML-based web-GUI and web-graphics systems, to succeed
and eventually replace the architecture-speciﬁc GUI and graphics backends.
This was again tested and validated by Apple’s recent move to its ARM-based
M1 architecture: within a few days, ROOT was passing most of its own, extensive
– 7 –
test suite (>600’000 lines of code) on M1. In that respect, ROOT’s Achilles’ heel
is the set of platforms supported by llvm and clang. It is reasonable to expect that
clang, one of the major C++ compilers and tooling ecosystems, will always support
HEP’s main architectures. So far this has always been the case; for instance, Power9
support and Apple ARM M1 support appeared in clang/llvm before they were needed
by ROOT. Nonetheless, ROOT’s extensive and special use requires expertise to adapt
to these new architectures and their support in clang/llvm.
Supported operating system Supported architectures
Fedora, CentOS, Ubuntu, Debian x86 64, i686, Aarch64, Power8, Power9
macOS x86 64, ARM M1
Windows 32bit, 64bit (work in
i686, x86 64
Table 1. Operating systems and their architectures as supported by ROOT.
3.3 Code Reuse and Preservation
Reuse of analysis code is common practice in HEP. Being a community-agreed stan-
dard tool with a strong focus on backward compatibility, ROOT helps with making
code reusable. Frameworks that attempt to formalize analyses, claiming to ease anal-
ysis reuse and preservation, often increase the eﬀective analysis complexity: they add
multiple ingredients to deﬁne an abstract analysis on top of a wealth of libraries and
software tools. ROOT prefers to reduce complexity, thereby guaranteeing evolution
and support of future architectures.
The platform independence and backwards compatibility of ROOT’s data format
helps analysis preservation: old ROOT ﬁles can be read without issues by the most
recent ROOT versions, even on the most recent chips such as Apple M1. By sup-
porting as many platforms as possible, ROOT aims to help with preservation: new
platforms will likely be similar to what ROOT already supports; migration should
thus be feasible.
Being able to run ROOT on a future platform is not suﬃcient for preservation
and longevity. The community must also retain suﬃcient technical expertise to
address future challenges, such as for reading ROOT ﬁles into a new memory layout.
The ROOT project’s core team of long term contributors is essential in this respect.
The move towards event data models (EDMs) that can be read without accom-
panying experiment libraries is helping with preservation: this data can be read with
ROOT alone, across decades. This approach has been beneﬁcial to HEP already
since PAW and ZEBRA - ﬁle formats that can be read and processed still today.
– 8 –
ROOT is engaging in discussions on a common likelihood interchange format for
HEP. On one hand, such a format is a useful and eﬀective documentation feature, for
instance for publications. On the other hand, it allows for optimized implementations
of likelihood functions outside of the highly general RooFit framework, which inspires
the continuous development and optimization of RooFit itself.
ROOT believes that analysis frameworks will continue to thrive: ROOT’s role is
to provide the building blocks that enable physics groups to tailor mini-frameworks to
their use. ROOT is continuing to ingest common functionality where it is of general
use to the community. This is especially called for where ROOT can implement
these tools in a better way, for instance regarding performance, maintainability, or
accessibility. Examples only from the RooFit context include the RooCrystalBall
and RooJohnson probability density functions; RooStats and HistFactory; ongoing
eﬀorts to integrate RooFit extensions developed within ATLAS; CMS’s and ATLAS’s
higher-level likelihood building tools; and RooUnfold.
3.4 Open Science
ROOT participates in CERN’s Open Science Policy Working Group. This is only
the most recent example of a long history of ROOT engaging with open science and
ROOT is engaged in the experiments’ open data eﬀorts, consulting on issues
with their data and analyses eﬃciencies (0.1 FTE/y). ROOT also beneﬁts itself
tremendously from said open data eﬀorts: these open data samples play a crucial
role for ROOT’s training sessions, example code, benchmarks and tests. One of the
limiting factors appears to be the available personpower from the experiments on the
open data side, and ROOT would be happy to intensify its involvement.
All of ROOT’s code is public. All changes to ROOT’s code are public; they can
be reviewed by anyone in the world. ROOT’s website is providing everything from
an introduction to ROOT up to the technical documentation of all of its interfaces.
ROOT sees contributions from high school students to university professors, with
changes in 2020 in ROOT itself that correspond to a diﬀ ﬁle of 31MB, with about
2800 lines changed each working day. ROOT follows the FAIR-Software approach,
making it as open as best practices recommend.
Multiple R&D projects are taking place in the context of ROOT, for instance
with the LLAMA / Alpaka group of the Helmholtz Center Dresden, Germany. While
CERN is providing the backbone of ROOT’s resources, crucial long-term contribu-
tions for speciﬁc areas come from Fermilab, GSI, Princeton, UCSD, and University
of Nebraska. Additional temporary commitments happen on a regular basis, in 2020
for instance by LAL, through Google Summer of Code as well as Season of Docs.
ROOT’s developments have an impact on other ﬁelds. A good example is
ROOT’s interpreter cling, which is famously serving as the engine behind Jupyter’s
C++ kernel and whose integration into llvm is ongoing; or ROOT’s work on an
– 9 –
improved, platform-independent RANLUX implementation  that we expect to
integrate into GCC’s standard library and the GNU Scientiﬁc Library. ROOT has
strong ties with the C++ committee; experience from HEP’s use of C++ frequently
leave traces in modiﬁcations of C++ std::simd , std::variant , C++ re-
ﬂection [14,15] and – due to the well-established connections – in the C++ im-
plementations such as std::any and std::unique ptr. ROOT’s I/O format for
the HL-LHC, RNTuple, will be accessible through a library that does not rely on
ROOT or its interpreter, with limited functionality. Nonetheless we expect that this
approach makes ROOT’s I/O layer signiﬁcantly more interesting for other sciences.
This means ROOT is itself an active element of open science. At the same
time, ROOT is used also outside HEP, for instance through BioDynamo  or in
quantitative ﬁnance research .
3.5 Training, Education, Support
ROOT’s advances will only have an impact on the community if the community is
aware of them. Dissemination is a core responsibility of ROOT; creating training
material and presentations for new features is time consuming. At the same time it
is extremely beneﬁcial also for the evolution of ROOT’s new interfaces, for instance
through feedback received during training sessions. In general there is a constant
tension between investing in ROOT’s evolution, or investing in talking about it; a
suitable balance has to be kept (1 FTE/y).
The ROOT team has started a ”Train the Trainer” series of events which will
be resumed after the COVID-related travel bans are reduced. Its purpose is to scale
training out: if the ROOT team cannot train the community the way it should, then
the ROOT team should train trainers who will then multiply the eﬀectiveness of
ROOT’s training material. These ”ambassadors” are community connections, ex-
pected to give multiple trainings per year, and collect feedback on ROOT and the
training material, to commonly advance it. Keeping the material with the project
allows ROOT to evolve the material together with ROOT, addressing new demands
quickly and sharing new features early. Nobody knows ROOT as well as its develop-
ers; both the community and the ROOT project beneﬁt from these close ties to the
trainers and the trained communities. Even once the ambassadors take over, ROOT
expects to continue to give introductory ROOT courses, for instance to summer
students (traditionally three per year) and as part of the experiments’ introductory
courses for PhD students.
ROOT has engaged with the Masterclasses project, for instance with ALICE and
tries to support them both on a technical level and by making HEP more visible out-
side the CERN member states: following ROOT’s workshop in Sarajevo, including
a public lecture to local students, the Sarajevo University joined the Masterclasses
– 10 –
Figure 2. Distribution of monthly ROOT forum postings in recent history.
Figure 3. Distribution of consolidated ROOT forum pageviews in recent history.
The ROOT forum sees vivid interaction from young physicists to senior statistics
experts, with monthly numbers that are usually signiﬁcantly higher than 300 thou-
sand page views and thousand messages posted (see Fig. 2and 3). Over the last 12
months, the average response time to questions is below one hour (3 FTE/y shared
among the project members). The forum is a knowledge base for ROOT and HEP
analysis in general, and a fantastic indicator of where the community’s problems are,
and where the ROOT project needs to invest.
With the creation of a new generation of ROOT interfaces, the ROOT team ex-
pects to asymptotically reduce the investment in support. Robust, simple interfaces
with good documentation, excellent tutorials, and good defaults deploying expert
optimizations ”behind the scenes” are expected to reduce boilerplate code that gets
”inherited” over generations of PhD students. This will result in analysis code that
is simpler to understand, more robust, and can serve as a better starting point for
future PhD generations.
– 11 –
4 Analysis Data
4.1 Data Models Optimized for Analysis
Today’s CPUs and GPUs, such as available through exascale systems, beneﬁt greatly
from large amounts of data, allocated consecutively in memory, that an algorithm can
iterate over. This favors ”structs of arrays” (SoA) rather than the more traditional
object-oriented ”arrays of structs” (AoS). Modern ROOT features enable SoA as
input, handle memory as SoA internally, and pass memory as SoA to GPUs.
This is further facilitated by ROOT’s I/O format: TTree and RNTuple store
data in a columnar way. Reading this into memory provides, by default, SoA layout.
With TTree this was a lucky coincidence that became relevant only after the interface
design of TTree. RNTuple on the other hand can immediately beneﬁt from this, and
makes columnar data access as SoA a ﬁrst-class interface.
While for many programs, AoS-to-SoA transformation is a costly operation,
HEP does it as part of ROOT I/O serialization. This means HEP and ROOT have
decades of experience with it, with the experiments’ analysis data formats migrating
to columns (TTree ”branches”) of simple data types. These ”simple aods / ntuples”
simplify data discovery and reading, as they can rely purely on ROOT without the
need for extra libraries, reading data into the beneﬁcial SoA in-memory layout.
SoA layout of simple data types has the additional advantage of being language
agnostic (or compatible): virtually all languages can handle arrays of ints or dou-
bles. This improves very eﬃcient language interoperability. The language boundary
is crossed not once per value, but once per array, making interoperability signiﬁ-
cantly faster as can be seen by ROOT’s benchmarks with handing data into NumPy,
machine learning libraries, or GPU processing.
4.2 Machine Learning
As described in section 2.1, ROOT plays a role as a data source for training and
inference of machine learning models. Currently, ML (whether run on GPU or not)
beneﬁts from large arrays as input data. These data ”structures” are simple enough
that ROOT - and here especially RNTuple - will be able to provide them with
virtually no overhead. With the advent of graph neural networks, data must be
”reformatted” into more complex memory data structures. Most state-of-the-art
ML algorithms are fairly young, also in relation to when HL-LHC is supposed to
start. The whole sector of quantum-computing inspired ML has not seen production
use yet. Given these expected fundamental changes, it is essential for ROOT to
have the capability to adjust in-memory data structures, more than to optimize
to current requirements. Here, LLAMA can be a fundamental ingredient, together
with RNTuple’s ability to handle custom data layouts and ROOT’s C++ interpreter
and just-in-time compiler (JIT) cling enabling optimized transformation of memory
– 12 –
ROOT is expected to contribute to ML advances indirectly, with its C++ inter-
preter and JIT: As an example, trained models can be read and converted to C++
code at runtime, with state-of-the-art optimizers (provided by the compiler tech-
nology community) signiﬁcantly accelerating and simplifying inference . Similar
technology advances can be expected also in the future. Leveraging those, ROOT can
increase its visibility and relevance also outside HEP, and provide beneﬁts to HEP
with little additional investment, by building on ROOT’s existing core components.
Many of these tasks require expertise in ROOT’s key enabling technologies that
are part of and used by ROOT, such as clang, ROOT’s Python binding, or the I/O
system. They also require an excellent understanding of the ML application side and
its requirements, as well as credibility and community trust. This enables conception,
design and implementation of solutions with suﬃcient lead time ahead of demand,
and gradual (”agile”) adoption. Given the required expertise and development ef-
fort, ROOT can signiﬁcantly impact eﬃcient use of compute resources, including
GPUs, for analyses and other workﬂows employing ML (reconstruction, simulation).
This happens directly through software technology advances relevant to HEP using
ROOT’s technologies, and by encouraging and guiding the use of eﬃcient analysis
RDataFrame-based training on GPUs is completed by 2023 and ML-optimized
RNTuple-to-GPU transfer by 2025. Much of the evolution cannot be planned ahead
and needs to adapt to the evolving requirements of physics analyses and ML research.
The inherent risk is the lack of predictability of ML’s future evolution. We hope that
a versatile setup like ROOT’s (and here especially that of RNTuple and RDataFrame)
allows us to adjust quickly to ML’s upcoming requirements. Lack of delivering said
adjustments in a timely manner will drive ML research, and as a consequence also
analyses involving state-of-the-art ML, into other ecosystems, causing segregation by
use of diﬀerent tools that are potentially less suited, optimized, or adapted for HEP.
4.3 Data Movement to GPU
GPUs are ideal for instance for ML training; ROOT is working to accelerate data
movement from storage to GPUs. R&D areas include memory layout particularly
suitable for GPU algorithms ; direct transfer from storage to GPU, bypassing
of CPU; use of compression algorithms optimized for GPUs; and total throughput
optimization of these diﬀerent options, possibly combining them. Much of this sees
very recent and ongoing technology evolution such as nvCOMP and DirectStorage;
ROOT is following these developments, making sure that they can be captured for
production use in the context of RNTuple, ROOT’s future I/O library. Here, RNTu-
ple is expected to initiate and schedule the data transfer to GPUs, making use of its
abilities to provide high-bandwidth I/O in a ﬂexible way including, most importantly
here, its scheduling.
– 13 –
As this is a highly evolving area, risks are mostly associated with a design that
prevents HEP from beneﬁting from these performance improvements. Even after
considering current R&D in the design of RNTuple, residual risks are associated with
much of the GPU ecosystem being closed source. As this is in the high performance
area of computing, ROOT expects that C++ will continue to play an important
role. This should allow ROOT to beneﬁt from its C++ core, possibly after adapting
RNTuple, its supported compression algorithms, and its scheduler to requirements
of future high-bandwidth storage-to-GPU interfaces.
4.4 Data Location, Transfer, Caching
On the other extreme of data transfer is remote I/O, where data is transmitted from
a data lake or any other non-local, medium to high latency storage service. This
plays a role in centralized storage, possibly separating storage and computer centers
geographically, as well as in (on- and oﬀ-premises) cloud computing.
ROOT’s approach is to make use of application-side knowledge to drive I/O
before the data is needed: after an initial latency cost, subsequent data should arrive
before it is needed by the processing code. This was already exercised by TTree’s
asynchronous prefetching which was not commonly used in production, likely due
to insuﬃcient investment in the feature’s robustness. With asynchronous I/O being
at the heart of RNTuple’s design, this is supported from the get-go and the default
mode of operation.
To reduce the impact of remote I/O latency, ROOT combines transfer requests
across an entry range. This was exercised with TTree’s TTreeCache for many years,
enabled by default since 2015. RNTuple utilizes a similar mechanism for grouping
I/O requests; its performance is being tuned in the context of HPC ﬁle systems
such as Ceph/Lustre and remote I/O through Davix. Remote I/O through Xrootd
is expected to be implemented by 2023, in close cooperation with CERN’s Xrootd
Determining which columns need to be read can be based on past usage or can be
conﬁgured by the application. With eﬃcient ”read what you need” and asynchronous
I/O, pre-placement of jobs and data ﬁles should be superseded by ROOT reading
only those bytes that the application needs, ahead of time, reducing I/O requirements
and increasing CPU eﬃciency.
ROOT allows for gradual production of derived data columns, such as calibrated
jet pTs derived from uncalibrated jet pTs, among others, and extending an original
dataset. These additional columns can be stored in dedicated ﬁles, extending the
original tree (”friend trees”). This together with columns existing only for certain
entries allows a storage eﬃcient creation of sub-samples, where overlaps between
samples are not duplicated.
Analyses see repeated runs on identical input data. ROOT is working on a
transparent, multi-tier caching layer to be deployed on batch systems such as Spark
– 14 –
clusters: ROOT’s upcoming Distributed RDataFrame will favor the ”sticky” distri-
bution of jobs’ input data to enable node-local data caches. This has been shown
to accelerate processing . Given suﬃcient resources, ROOT hopes to extend
this to an automatic data cache of intermediary analysis results, to short-cut and
dramatically accelerate the re-running of analyses. Such a mechanism is considered
for deployment in analysis facilities (”ServiceX”) that come with their very distinct
Given the relevance of object stores in next-generation data centers (such as
Aurora) as an alternative to conventional ﬁlesystems, there are ongoing eﬀorts to
support alternative backends in RNTuple, such as Intel DAOS  or Amazon S3.
The use of object stores as a cache for accessed data is also being investigated .
The risks associated with remote I/O are suboptimal usage of network and CPU
resources - corresponding to the key resources of HEP computing. For the ROOT
project it is of paramount importance that its I/O layer is capable of making adequate
use of these resources; benchmarks are TTree’s behavior as well as raw network speed
and CPU usage. Given the pace of technology evolution in the network and remote
storage environments, ROOT does not expect technology evolution to be a signiﬁcant
risk on the HL-LHC timeframe.
”Black-box” systems optimized for re-processing of analyses can be a motivation
for analysis physicists to migrate to alternative ecosystems. This is a residual risk
that ROOT tries to reduce through technology advances and acceleration of repeated
analysis. From ROOT’s experience, these optimizations are rarely applicable to
multiple analyses, for instance on the level of an analysis facility: reuse of data
and especially intermediary analysis results are expected to be speciﬁc for a given
5 Analysis Design
The ”old” ROOT forced physicists to deal with the data source, reading, and the
event loop. This prevented many optimizations, for instance multithreading. PROOF
tried to formalize analyses, enabling ROOT to process the analysis concurrently.
Still, for optimal eﬃciency, the analysis was commonly interacting directly with the
I/O layer, and handling the values read from storage, and their types, explicitly.
In 2018, RDataFrame became ROOT’s modern analysis interface, revolutionizing
how physicists write analyses today. As can be seen from ROOT’s user forum,
RDataFrame is now a topic as popular as ”tree” or ”histogram”. This can be seen as
a recent major success and a crucial contribution that the community seems to have
been longing for. The signiﬁcance for the community is certainly similar to RooFit.
Unlike alternative approaches, ROOT believes that analyses should be allowed
to be conceived and written ”event-centric”: the minimum number of jets per event,
maximum missing ET, number of jets associated with a muon - all these parameters
– 15 –
are traditionally per event. ROOT expected it to be much easier for physicists to
not give this up when writing their analysis, but instead to handle arrays of events
(and arrays of arrays of jets and muons) ”behind the scenes”, where concurrent and
batch processing can run on the user declared analysis, without physicists having to
think about the array-ness of certain values such as jet pT.
5.1 Declarative Analysis
RDataFrame’s main beneﬁt is its declarative interface style, where physicists declare
what needs to be done, allowing ROOT to take care of how to do this optimally. As
an example, to consider only events with more than two jets reads Filter(”njets >
2”). This is done with minimal overhead, yielding a very eﬃcient, multi-threaded
analysis, which evaluates a complex analysis graph in a single pass through all input
The community has built several ”mini-frameworks” on top of RDataFrame, for
instance bamboo,CROWN , and TIMBER. This by itself is a good sign, showing
that ROOT has successfully provided another fundamental building block signiﬁ-
cant enough to be picked up. ROOT is following closely what these frameworks
provide, and what they have diﬃculties in providing, to ”ﬁll the gaps” and - where
it is adequate - to provide centrally maintained facilities. Examples of current de-
velopments include the variation of event weights and other analysis parameters, to
determine the eﬀect of uncertainties. This will be possible without re-running the
whole analysis for each variation. This, too, is expected to have signiﬁcant eﬀects on
how HL-LHC analyses will be written.
While RDataFrame supports multithreaded analyses from the start, multi-node
support is introduced by Distributed RDataFrame. The latter works together with
schedulers and job submission backends (Spark, Dask, etc) for an optimal distribution
of input data and compute tasks to clusters of machines. Such clusters are often
institute clusters, beneﬁting from a much higher data-reuse and code-rerun rate
than for instance the grid. This motivates ROOT’s work on backend-speciﬁc work
placement, which will allow for analysis acceleration through node-local caches. In
order to enable support for a variety of use cases, distributed RDataFrame features
a modular backend design. In the future, users will be able to distribute the same
computation graph over a set of diﬀerent cluster frameworks by changing a single
line of code. Usability is a key ingredient for such a distributed analysis feature,
and conﬁguration of underlying scheduler backends, authentication, data placement,
and writable disk space and result collection is generally complex. ROOT plans to
address this with a community-wide, community-maintained database of available
conﬁgurations, such that physicists can use Distributed RDataFrame with their local
resources simply by providing the cluster identiﬁcation.
As with any new ROOT interface, RDataFrame is written with Python in mind;
Distributed RDataFrame is only possible because of this. Several optimizations are
– 16 –
pending, for instance for optimized just-in-time compile code; compiled code gen-
erated from Python code to be run in the ”hot” event loop; more eﬃcient bulk
processing. Much of this requires understanding of RDataFrame, ROOT’s C++
interpreter and just-in-time compiler called cling, PyROOT, and Python in general.
ROOT : : En abl eI mp li ci tM T () ;
R OO T : : R D at a F r am e d f ( d at a s et );
auto d f2 = df . Fi l te r ( "x > 0")
.Define(" r 2 " ,"x*x + y*y");
auto r Hi s t = d f2 . H i s to 1 D ( "r 2 " );
d f2 . S na p sh ot ( "newtree"," o u t . ro o t ") ;
Listing 1. RDataFrame C++ code example.
df = R OO T . R Da t a Fr a me ( d a ta s et )
d f2 = df . F i lt e r ( "x > 0")
.Define(" r 2 " ,"x*x + y*y")
r Hi st = d f2 . H i st o1 D ( " r2 " )
d f2 . S na p sh ot ( "newtree"," o u t . ro o t ")
Listing 2. RDataFrame Python code example.
As can be seen from the uncertainty variation of parameters, investment can
have a signiﬁcant performance impact for analyses (O(10) speed-up for regular anal-
ysis, compared to TTree, with an investment of 0.5 FTE for 2 years), running the
analysis once instead of once per variation. ROOT plans to invest in RDataFrame to
make it signiﬁcantly easier to use with ML, RooFit, and GPUs (O(10) speed-up for
average analysis, with an investment of 2 FTE for 4 years). Together with snapshot-
ting of intermediary results, optimizations of the Python and just-in-time interfaces,
and internal bulk processing of RNTuple data, common analyses will be accelerated
by at least an order of magnitude. We expect that by the time of HL-LHC, the
vast majority of analyses will be using RDataFrame - especially as already now,
RDataFrame is as popular on ROOT’s user forum as ROOT’s histograms or TTree.
Such investment thus scales out to a very large number of analyses, with a signiﬁcant
community impact. It also fosters ecosystems to be built around RDataFrame - tools
that can be shared and integrated into ROOT if they are of general use.
This acceleration can bring analyses that are too time consuming into reach;
it can improve analyses’ ability to optimize their parameters. This is beneﬁcial
for traditional analyses with iterative analysis optimization, and a prerequisite for
diﬀerentiable analysis  approaches.
– 17 –
At the HL-LHC timescale, ROOT expects GPUs to be the commonly available accel-
erators. For analysis, their main use will certainly continue to be ML training. Also
because of the expected ubiquitous use of ML for HL-LHC analyses, ROOT expects
GPUs to be ubiquitous for analyses, and is working on R&D to integrate GPU accel-
eration in RooFit which builds upon prior work on architecture-speciﬁc vectorization
. This shows very promising results. To be of general use, such acceleration must
be enabled by default, when available and advantageous. Much of the R&D goes
into determining mechanisms that transparently turn on such acceleration.
ROOT’s future I/O subsystem RNTuple is designed to utilize (de-)compression
accelerators, which can play a crucial role in analysis throughput. These have already
been available in past Xeon generations.
6 Analysis Algorithms
ROOT’s histogramming package has been the core ingredient for most analyses,
for decades. Hardware performance characteristics have changed since its original
conception, and much of the design is limiting today’s and tomorrow’s use. The
interfaces of ROOT’s histograms are not designed for their usage from Python, nor
their interoperability with the Python ecosystem such as NumPy. Alternatives, such
as Boost histograms, have been created. They lack commodity features expected
from a ROOT-provided histogram package, and are optimized more towards raw
performance than usability. ROOT’s histograms must provide graphics, serialization,
and ﬁtting facilities.
The creation of alternative packages and the ”impedance mismatch” of modern
code with ROOT’s histograms (including ownership, lack of generalization, more
than 300 member functions, no support for atomic bins) clearly signal that this code
part needs to be revisited. Thanks to a partial FTE contribution from LAL, ROOT
made much progress with a new histogram library. Nonetheless, without sustained
eﬀort for two years, a new ROOT histogram library cannot be advanced to the
minimal feature level required for adoption. The community risks continued work
around the ownership discrepancies of ROOT’s histograms, paying a performance
price for analyses, and additional fragmentation in the experiments’ online and of-
ﬂine framework, where even today, experiment speciﬁc histogramming facilities have
been reintroduced to not use ROOT’s histogram library . CMS has provided a
review of issues of ROOT histograms for DQM. Despite these issues, ROOT his-
tograms currently remain the most commonly used histogramming library. Without
investment, usage will further segregate, with no obvious place to invest for the whole
– 18 –
This new generation of histograms will allow ROOT to implement long requested
features, such as concurrent ﬁlling, consistent availability of arbitrary dimensions, or
special axes such as circular axes or logarithmic axes with a custom base, eﬃciency
axes (pass versus all), or axes for multiplicity studies. While some of these features
merely simplify writing an analysis, some other features counter commonly seen
correctness issues with histograms, such as ﬂoating point precision issues with mul-
tiplicity axes, or can help reduce memory use in concurrent analyses such as analysis
6.2 Modeling and Fitting
RooFit is high energy physics’s standard tool for modeling statistical distributions
and building likelihood functions. It is used in most LHC analyses for estimating
physical parameters, conﬁdence intervals and discovery signiﬁcances. The minimiza-
tion of a likelihood function deﬁned in the RooFit framework is central in most
LHC analyses. With the HL-LHC, the number of parameters and observables in the
likelihood function is expected to increase. Usually, minimization time grows su-
perlinear with the number of parameters, meaning technical innovation in likelihood
minimization is necessary.
Thanks to modern deep-learning frameworks such as TensorFlow or PyTorch, it
is increasingly easy to minimize a function expressible as a chain of linear algebra
operations. However, the mathematical operations in a typical LHC likelihood ﬁt are
often much more complex, requiring for instance the numerical integration of prob-
ability densities for normalization purposes or looking up the result of an auxiliary
measurement in a histogram. The RooFit library was designed with such general
likelihood functions in mind, supporting unbinned minimization. As RooFit is writ-
ten in C++, there is still ample room for performance optimization at all the levels
of the likelihood evaluation and minimization, making it ready to face the challenges
of the HL-LHC data volume.
– 19 –
2 4 6 8 10 12 14 16
2nd order Polynomial
5th order Bernstein
2nd order Chebychev
Sum of 2 Gaus and Exp
ChiSquarePdf clang9 i7-7820X AVX512
clang8 i7-4790 AVX2
clang9 i7-7820X AVX2
gcc9 i7-7820X AVX2
gcc9 i7-7820X AVX512
clang9 i7-7820X AVX512
clang8 i7-4790 AVX2
clang9 i7-7820X AVX2
gcc9 i7-7820X AVX2
gcc9 i7-7820X AVX512
Speed up using vectorisation
Figure 4. Acceleration from vectorized functions in RooFit.
The ROOT developers pursue several paths to optimize RooFit’s performance:
speeding up the likelihood gradient calculation, accelerating speciﬁc computations
with GPUs, writing more vectorizable code (see Fig. 4), general optimization of ex-
pensive operations, and improving the interoperability with other libraries that can
handle large datasets. The larger the number of parameters in the likelihood func-
tion, the more expensive it is to numerically determine the gradient by varying one
parameter at a time. In late 2021, RooFit will introduce functionality to parallelize
the gradient calculation over multiple CPU cores. In the following year, R&D on
auto-diﬀerentiation for gradient computation in constant time will move into the fo-
cus (see Minimization section). To increase the throughput of computations, much
of RooFit was recently rewritten to support auto-vectorization. This path will also
be followed in the future, alongside the continuous optimization of CPU code in gen-
eral. In 2021, the foundational work to oﬄoad computations to a GPU was done.
Next, the remaining likelihood-building blocks that are frequently used for HL-LHC
analyses have to be implemented.
With the increasing dataset sizes, it is crucial to improve RooFit’s interoper-
ability with other libraries meant to deal with big data. This concerns other ROOT
components (RDataFrame and RNTuple) and libraries from the Python ecosystem
(e.g. NumPy and Pandas).
A redesign of the RooFit interfaces based on value semantics instead of RooFit’s
current reference semantics would make RooFit signiﬁcantly easier to use, simplify
ownership management, and enable the same straight-forward interfaces from C++
– 20 –
and Python. This will attract physicists who would otherwise use Python-AST based
interfaces with reduced performance and feature characteristics.
0 20 40 60 80 100 120 140 160 180 200
Fitting time (ms)
Figure 5. Speed-up of multi-parameter minimization due to automatic diﬀerentiation.
Minimization is another area that can beneﬁt greatly from the use of GPUs. The
evaluation of minimizer functions can be accelerated on GPUs, especially when op-
erating on large amounts of data. Use of analytical gradients, or automatic gradient
calculation for instance with clad can provide further acceleration, see Fig. 5. Some
of the algorithmic challenges remain hard, such as normalization of probability den-
sity functions when evaluating likelihood functions as seen with RooFit models; we
are conﬁdent that continued R&D investment will pay oﬀ.
Advances in minimizers will provide immediate beneﬁts to RooFit. Even the
side-eﬀects from these R&D eﬀorts are useful to the community and outside, with
the C++ automatic diﬀerentiation tool clad being a perfect example.
ROOT continues to try state-of-the-art minimizers, benchmarking them against
MINUIT and other production minimizers. Production-grade, general purpose min-
imizers are rare; ROOT does not expect that major development eﬀort will need to
be invested here, except for the above-mentioned GPU minimization.
7 Visualization and Graphical Interfaces
ROOT’s graphics style and abilities have deﬁned the visual language of HEP. With
the ever increasing complexity of architecture speciﬁc graphics and the web taking
– 21 –
over GUI and graphics, ROOT has opted for a redesign of its graphics and GUI
7.1 Scientiﬁc Graphics
ROOT’s graphics system allows extreme conﬁgurability while creating graphics in
publication quality. Several of the defaults would be chosen diﬀerently today, for in-
stance the font size being relative to the window size, or the line widths of histograms,
or histograms’ default colors.
The complexity of maintaining highly eﬃcient graphics on all platforms has
increased enormously: Linux is migrating from X11 to Wayland (and alternatives);
macOS oﬀers Cocoa and Metal; Windows oﬀers GDI+ or DirectX. Luckily, web
browser engines have established themselves as a platform independent abstraction
layer. The Chrome / Chromium Embedded Framework for instance is used by many
programs such as Mattermost, the Atom editor, and Spotify. This means ROOT’s
architecture speciﬁc libraries can be succeeded by libraries maintained by the open
Any scientiﬁc graphics library is built around primitives such as axes, lines, greek
letters and formulae, and the ability to zoom. Luckily, these features are available
instance histograms in a browser window, with virtually the same conﬁgurability
and performance as the previous architecture-speciﬁc implementations. Hundreds of
histograms can be shown interactively, with smooth interaction.
The move to web graphics enables embedding of ROOT graphics in custom GUI
applications such as Qt applications. Before, embedding of ROOT graphics depended
to a large extent on platform-speciﬁc features (and, as a consequence, stopped work-
ing on macOS unless using legacy X11 implementations). Today, virtually all GUI
systems allow embedding of browser windows, or are written themselves as browser
With this new technology in its hands, ROOT is now working on making the user
facing implementation of ROOT’s graphics much simpler to use. New interfaces allow
ROOT to deﬁne new defaults and to streamline the graphics interfaces, simplifying
for instance ownership management and separating data structure from graphics
abilities. This work is ongoing and expected to deliver a robust, simpler graphics
interface in time for HL-LHC. Prototyping new graphics programming models is
expected to conclude in 2022. Implementation of the new default style and CSS-
based style customization is expected to be available by 2024. Grid deployment (for
instance with the above mentioned Chrome Embedded Framework available through
LCG) is expected to be achieved by 2022.
– 22 –
7.2 Graphical User Interfaces
With the graphics system moving to web technology, moving ROOT’s custom GUI
system along was an obvious next step. ROOT has decided to use the feature-
rich, open source Web GUI library named OpenUI5. Communication of the browser
window with the ROOT process happens through an open source web server library
called civetweb, which is developed independently of ROOT. The ROOT process
communicates through its interpreter / JIT compiler cling, passing data both ways,
subsystem is used to transform C++ objects into JSON and back, to transport state
between the ROOT process and the browser.
This shows that ROOT is able to re-use its own core components to generate a
wealth of fundamental tools speciﬁc for HEP. While ROOT’s GUI itself is important
(for instance ROOT’s new RBrowser, or the new ﬁt panel), it is equally important
that ROOT and its interactivity can be embedded in ”foreign” GUI systems, and
retain its functionality and interactivity. The current setup based on web technology
allows just that, as demonstrated for instance by the ROOT masterclasses or Eve7.
7.3 Jupyter Notebooks
For ”exploratory analysis”, i.e. the ﬁrst steps into discovering data, fundamental
distributions, or even for training of machine learning models and the evaluation of
their quality, Jupyter has established itself as one of the interfaces. Being based on
web technology itself, it integrates nicely with ROOT’s web graphics already today,
ROOT’s C++ interpreter / JIT compiler cling is the engine behind the oﬃcial
C++ Jupyter kernel. ROOT provides an enhanced version of this, making many
ROOT features more accessible through its own, dedicated kernel.
Work is ongoing to integrate ROOT’s web GUI system with the new Jupyter
GUI system. Given suﬃcient demand from physicists, this feature is expected to be
available in 2024.
7.4 Event Display
ROOT’s event display EVE was in production use in many experiments, for instance
CMS, ALICE, Belle II, T2K, HyperK, ILC, NA62 and several smaller experiments
in neutrino, nuclear, and medical physics. Being exposed to the same problems and
limitations as ROOT’s legacy graphics and GUI libraries, Eve has been redesigned
as Eve7, to work with ROOT’s new, web-based graphics and GUI system, too; see
Fig. 6. This has increased its versatility dramatically: Eve7 works on all platforms,
can present events remotely, and is thus a perfect implementation for everything
from online monitoring to outreach.
Apart from JSROOT and ROOT’s GUI system, Eve7 also uses ROOT’s C++
interpreter / JIT compiler internally for ﬁlter expressions, for instance to display
– 23 –
Figure 6. ROOT event display with object selection.
only jets satisfying certain cuts. As these expressions can use the full power of C++,
the event display is highly customizable and extremely versatile, useful for instance
to understanding eﬀects of detectors or detector algorithms.
Event displays play a role in analyses, too: they allow a visual inspection of
selected events, for sanity-checking and to determine for instance diﬀerent topologies
of samples. As such they are inherently an analysis feature, where usability and
performance is a key requirement.
Being developed in close collaboration with CMS and currently deployed as a
prototype in CMS, the feature set and performance of Eve7 has already impressed
several users of event displays. A ﬁrst experiment, Mu2e at Fermilab, has oﬃcially
announced their interest in using Eve7 in production in the coming years. ROOT
has every reason to expect community adoption of Eve7 to be at least on par with
the adoption of the legacy Eve.
We expect that the majority of eﬀort will be invested into Eve7 satisfying the
requirements of CMS, which can serve at the same time as a proof of being feature
complete. This work is expected to conclude in time for HL-LHC, and driven mostly
by CMS itself.
– 24 –
8 Project Requirements
The ROOT project is in its core a group of contributors to a common open source
project. Decisions are taken publicly at a weekly team meeting, upon discussion
and by consensus. Consensus building happens through good arguments; ROOT is
thus in practice a meritocracy, where new, capable contributors can quickly acquire
ROOT development was traditionally split in several loosely coupled parts - both
regarding libraries and regarding development subgroups, for instance for the I/O
or the statistical part of ROOT. In recent years, these delineations have reduced,
to encourage both sharing of expertise, and exposing of problems and solutions to a
wider audience, with a more diverse background of technical expertise.
ROOT continues to be a successful integrated ”hub” for R&D; several grants
were attributed to work in the context of ROOT. Examples include the R&D on the
hrefhttps://reviews.llvm.org/D96033integration of ROOT’s C++ interpreter / JIT
compiler cling into llvm / clang (”upstreaming”), compression studies , PhD stu-
dents’ work on topics such as LLAMA and data caching for distributed computing
. Some R&D - such as the core RNTuple work, or RDataFrame - requires sus-
tainability that goes beyond the time frame of PhD student or grant requests, and
needs to be taken up by long-term contributors.
Besides this R&D aspect, ROOT has a strong responsibility for sustainability and
support. ROOT’s interfaces need to be backward compatible, to facilitate sharing of
analysis code across ROOT versions. This requires an ”interface vision”: designing
interfaces now that allow optimizations behind the scenes in ten or 20 years from
now is an expertise that is required by the core ROOT developers.
Support is a signiﬁcant workload for ROOT’s core developers, with hundreds
of messages needing to be processed per day; issue reports and code contributions
requiring attention; infrastructure work (conﬁguration, continuous integration and
benchmarking systems); and the ROOT team presenting ROOT’s advances at physics
conferences, for instance iCHEP 2020,EIC,CEPC 2020,EPS-HEP 2021, and during
training sessions. This load can generally not be shouldered by contributors who have
dedicated R&D goals, but is carried by those developers with long-term contracts.
ROOT has virtually constant interaction with the experiments, from early dis-
cussions on ROOT’s development plans to high-priority issue processing. The ROOT
team engages with the experiments’ software and computing experts at a dedicated
ROOT / Experiments meeting series; at the Librarian and Integrators Meeting; and
at the Architects’ Forum. Oﬃce visits between core experiments’ developers and
ROOT are common (or rather, were common before the pandemic), as was pair de-
bugging. Requirements for the experiments’ core software is collected through the
discussion of the plan of work and through issues opened by the experiments.
– 25 –
This constant interaction leaves traces with all parties involved. ROOT has con-
tributed to the deﬁnition of the ALICE analysis model of Run3, to the interplay of
ATLAS XAODs and RDataFrame, and to optimizing CMS nanoaods. Multithread-
ing bottlenecks are addressed in cooperation; I/O performance improvements are
guided by measurements from the experiments, often with tools provided by ROOT.
With for instance RNTuple, ROOT further increases its investment in benchmarking
campaigns with the experiments, to guide design and development by realistic, early
Requirements for the experiments’ analyses are generally much harder to deﬁne.
ROOT invites to workshops (e.g. in 2018 and 2015) to solicit combined feedback,
makes use of the forum and GitHub issues as feedback platforms, and presents where
physicists are (see the list of recent contributions at physics conferences above, regular
invitations at experiments’ meetings, engagement / participation for introductory
ROOT courses) to engage in discussions.
ROOT owes much of its stability to the experiments’ investment: all experiments
test all main ROOT releases, or even their branches; some even report with low
latency (about one week) on issues found in ROOT’s main development branch
(”master”). This allows ROOT to carry out major changes, in a coordinated way
with the experiments. Recent examples include the migration to a new PyROOT
implementation; or updates of LLVM. Another example of ROOT’s engagement with
experiments is the integration of RNTuple in CMSSW, a combined eﬀort of CMS
and ROOT. The experiments trust ROOT to maintain Vc, VecCore, and VecMath,
packages used by the experiments directly or indirectly, for instance through Geant4.
8.2 Development Team
ROOT’s development is driven by R&D contributions, where short term contracts
dominate by number. ROOT currently hosts one technical student, one PhD student
paid by CERN; one PhD student funded externally; 2.5 FTE CERN fellows; one
CERN EP R&D fellow; and one externally funded fellow, i.e. in total 7.5 FTEs.
Sustainability, guidance, accretion and integration of expertise is guaranteed by
developers with long-term positions, totalling 8.25 FTEs, see Table 2.
A fair fraction of these developers (marked ”R&D” in above table) cannot con-
tribute to the project’s baseline load of supporting users on the ROOT forum, in-
frastructure work (such as maintenance of ROOT’s continuous integration system
or build system), or issue processing. Much of the recent ROOT developments and
expertise associated with those developments - notably RDataFrame and RooFit -
rely on fellows. RNTuple is an exception from this, where the main developer was
rewarded with an IC and expects to migrate to ROOT for the vast majority of his
The age proﬁle of ROOT’s development team versus its recent massive renovation
is one issue; the lack of gender diversity is another. ROOT has succeeded in capturing
– 26 –
FTE Expertise Funding
I/O, statistics, ML, C++ interpreter / JIT compiler
cling, ROOT’s type description system, PyROOT,
build system, documentation, platform support and
continuous integration infrastructure, support
0.5 I/O Fermilab
1 R&D and support: event display UCSD
0.5 R&D: C++ interpreter / JIT compiler cling; C++
0.5 R&D: compression, I/O UNLincoln
0.25 R&D: web-based GUI and graphics GSI
Table 2. Current long-term contributions to ROOT.
physicists and computer scientists; developers from diverse national backgrounds.
But with currently zero female team members paid by CERN it has failed to achieve
a reasonable gender balance. This is an issue that ROOT expects to address in the
coming years, by investing in recruitment, reaching out to universities and job fairs.
Improving this requires a noticeable eﬀort from the ROOT project. This eﬀort can
only be eﬀective in a sustainable way as appropriate positions become available.
For ROOT as a long-term software project, sustainability is a key requirement.
Apart from technical sustainability such as I/O and interface backward compati-
bility, separation of stable user interfaces from evolving optimizations behind the
scenes, and multi-platform support, ROOT needs to also guarantee sustainability on
a non-technical, ”cultural” level. Worth mentioning are ROOT’s long-term common
vision for a ROOT evolution that matches HEP’s requirements. ROOT’s expertise
is the basis for the community’s trust in ROOT and its continuing evolution. The
successful and established mode of working of ROOT’s development team guarantees
productivity and integration of many kinds of contributions, sources of contributors,
and an active, vivid, and continuously challenging ecosystem with constant commu-
nication with ROOT’s stakeholders.
ROOT’s signiﬁcant investment in support is addressed with a new generation of
coherently designed, well integrated, robust interfaces that are simple to use correctly,
and diﬃcult to use incorrectly. This should asymptotically lower the support load
– 27 –
Over the past years, ROOT has deprecated and removed interfaces, and even
large parts of ROOT. They either have seen insuﬃcient use to warrant the project’s
continuous investment; or they have not seen general adoption from the community,
thus beneﬁting from a lifecycle independent of ROOT. Some interfaces (such as
TLorentzVector) see continuing wide-spread adoption but are nonetheless seen as
problematic; the ROOT team is working through documentation and training to
migrate usage to recommended interfaces (such as PtEtaPhiMVector).
At the same time, ROOT targets development and investment in a very focused
way. With about 5 million source lines, ROOT needs to choose what to evolve, and
what to ”freeze”. Many parts of ROOT have existing usage, nonetheless much of their
motivation is not applicable anymore today. Examples include ROOT’s collection
classes which were needed because no general C++ standard library support existed
20 years ago; or parts written to bridge between ROOT’s previous, limited C++
interpreter and non-ROOT libraries; or internal tools that have been replaced by
other, open-source tools such as the transition from THtml to doxygen, or TThread
to tbb. Much of this legacy code has been written with a past understanding of
”software development’s best practices”, making any more fundamental investment
a challenge and of dubious value. Instead of causing friction and additional work for
the community, ROOT maintains these parts with low cost, by moving their code
to new C++ standard versions, platforms, and compiler versions as needed. Where
sensible, ROOT marks these parts as legacy, to clearly communicate that now new
adoption of these features should happen, and what the recommended alternatives
Lines of code changed per week, 2020 2800
Number of contributors, 2020 Approx. 100
Number of architectures and package managers per ROOT
New issues (bug reports) in 2020 690
Closed issues (bug reports) in 2020 660
Table 3. Key ROOT development metrics.
8.4 Generational Handover
For ROOT to be successful in perpetuating its ﬂourishing developments and convert-
ing them into sustainable, trusted software building blocks for HEP, the upcoming
generational handover must succeed. There is a strong risk that with the retirement
– 28 –
of a large fraction of very visible ROOT developers, expertise or at least visibility and
community trust get retired with them. To reduce this imminent risk considerably,
a small number of currently well-known young experts of ROOT should be retained
until they have a chance to apply for long-term contracts, succeeding the retired core
8.5 Cooperation & Innovation
Cooperation is essential for ROOT’s evolution and a crucial source of contributions.
Fostering cooperation, i.e. an open ROOT project that newcomers feel invited and
welcome to contribute to, is paramount for this. Being open source and with open
discussions and project meetings is a prerequisite, but in and by itself is not suﬃcient.
We see how innovation serves as an incentive to contribute. It creates atten-
tion, causes alternatives to be tried by the community, which is an incentive to
benchmark, compare, and improve. The better solutions generally get integrated
into ROOT, where it is of high relevance to the community and where it matches
ROOT’s evolution and responsibilities. Even where innovation does not get inte-
grated because it is outside of ROOT’s scope, it generally backfeeds requirements
and ideas, and by doing so triggers innovation. Cooperation can thus exist on many
levels, from formal ones such as ROOT’s current and past CERN knowledge transfer
projects, to informal ones where physicists give presentations to the ROOT team to
share ideas or propose solutions (e.g. Bamboo,TTreeIterator,CROWN).
ROOT is working towards making cooperation easier, also on a more formal level.
While this is ongoing, a major milestone in this regard is expected to be reached in
8.6 Creating Opportunities for Externally Funded Projects
With ROOT’s responsibility towards the community and the ongoing need to com-
pete with alternative solutions, core development is generally shouldered by long-
term contributors. Many of the peripheral developments that are still crucial but
less time critical can be owned by speciﬁc R&D projects.
ROOT tries to increase the availability and visibility of sizable, open R&D
projects that are not on ROOT’s critical path. The HEP community interested
in software does not have established mechanisms to facilitate this; ROOT’s past
eﬀorts to engage with this community have had very little successes. ROOT does
not yet see the HEP Software Foundation or CERN’s SIDIS eﬀort serving as a forum
for laboratories or university software R&D groups to engage. While investment in
software exists, the HEP community might beneﬁt more from synergies and a more
In its yearly published plan of work, ROOT will continue to include tasks that
are not foreseen to be covered by existing contributions. The hope is to trigger
discussions on these topics, to identify interested parties, and to engage in more
– 29 –
coordinated R&D. Having a forum or a set of entities to address this to would
8.7 Summary of Risks, Functionality Gaps, Dependencies
2018 2019 2020 2021 2022 2023 2024 2025 2026 2027
R&D prototype, beneﬁt estimate
Implementation MVP, perf validation
CMS NanoAOD framework integration
Object store DAOS, S3
Binary Format Spec
RDataFrame bulk processing
Tools conversion, inspection
Validation of feature coverage
Training of experiments’ devs
Integration experiments’ use
PB scale validation
Release for production use
ML training direct GPU transfer
Training of physicists / physics groups
Figure 7. Milestones and associated eﬀort: ROOT I/O
Part of ROOT’s core responsibilities is the I/O system, as covered in detail by the
Foundation part of the ROOT input. Its milestones and associated eﬀort are shown
in Fig. 7. Other areas and associated risks are as follows.
– 30 –
Analysis interfaces: eﬃcient, robust and obvious
2018 2019 2020 2021 2022 2023 2024 2025 2026 2027
Release for production use
RNTuple integration, optimization
Dataset tools weights, metadata
RNTuple bulk processing
GPU ML training
Python expression acceleration
Cache of intermediary RDF results R&D
Use of GPUs for compute kernels
Zero-copy I/O for GPU processing
Figure 8. Milestones and associated eﬀort: RDataFrame
2018 2019 2020 2021 2022 2023 2024 2025 2026 2027
PyROOT, C++ interpreter / JIT cling
LLVM releases, C++ std continuous adoption
NumPy data transfer
C++ Reﬂection system overhaul
Cling re-implementation based on clang-repl
Integration of cppyy based on clang-repl
Figure 9. Milestones and associated eﬀort: PyROOT, cling C++ Interpreter / JIT
– 31 –
2018 2019 2020 2021 2022 2023 2024 2025 2026 2027
Backend support Dask, HTCondor, etc
Site-speciﬁc automatic conﬁguration
Release for production use
Cache of intermediary dRDF results R&D
Figure 10. Milestones and associated eﬀort: Distributed RDataFrame
2018 2019 2020 2021 2022 2023 2024 2025 2026 2027
Prototype, R&D of new histogram library
Drawing support in web graphics
Histogram range views
Operations on views
Fitting interfacing with existing minimizers
Implementation of remaining axis types
Support for multiple uncertainties
Advanced operations (projection, proﬁle, etc.)
Concurrent buffered ﬁlling
Release for production use
Figure 11. Milestones and associated eﬀort: Histogramming
– 32 –
If evolved properly, ROOT’s prime analysis interface, RDataFrame (see Fig. 8), can
make use of the impressive uptake from physicists, by guiding them towards auto-
matically eﬃcient analyses that use all available resources (network, CPU, GPU).
This will enable physicists to work within a common ecosystem, together build fea-
tures and tools on top of RDataFrame, and thus increase and share the beneﬁts of an
analysis environment tailored for HEP. It is paramount to delimit the R&D surface:
where the open source data analysis community provides eﬃcient tools that are easy
to use, ROOT must invest in interoperability, with a focus on smooth, eﬃcient usage
and highest possible data transfer bandwidth.
ROOT must invest in interoperability of the expected HL-LHC data format
RNTuple with tools from the open source data analysis ecosystem, speciﬁcally for
machine learning (training), NumPy and future (likely Python-based) data exchange
RDataFrame needs to be extended to handle systematic variations within the
same event loop, providing a signiﬁcant speed-up for the average analysis. Bulk
processing of data must be enabled for RDataFrame, to beneﬁt from RNTuple’s new
data layout and to beneﬁt from signiﬁcantly higher throughput on CPUs and GPUs.
PyROOT will play an even increasing role in HEP’s data analysis environment.
It critically depends on ROOT’s C++ interpreter / JIT compiler cling, and ROOT’s
type description system, see Fig. 9. Future C++ standards, performance and feature
bottlenecks must be addressed in ROOT’s type description system. Python-speciﬁc
adapters to widely used ROOT interfaces must be implemented to ease their usage
also from Python.
With RDataFrame, ROOT has mostly addressed the issue of ”how to write an
analysis accelerated by multithreading”. This needs to be extended to a multi-node
environment, currently developed as Distributed RDataFrame, see Fig. 10. This
will reduce the need for the community to develop adapters for running analyses on
clusters such as Spark, for instance by reading ROOT ﬁles in Java. It will make
university clusters accessible to interactive analysis, signiﬁcantly reducing the turn-
around time for analyses.
ROOT’s new histogram library will address usability and performance issues,
while providing the feature set expected by HEP analyses, see Fig. 11.
– 33 –
Machine learning models and likelihood functions with ROOT data
2018 2019 2020 2021 2022 2023 2024 2025 2026 2027
Optimization for state-of-the-art ML
Training GUI based on Web GUI
Fast inference for ONNX models
Memory layout R&D (LLAMA / RNTuple)
Fast inference for BDTs
RNTuple direct GPU transfer
Figure 12. Milestones and associated eﬀort: Machine Learning
2018 2019 2020 2021 2022 2023 2024 2025 2026 2027
Multiprocess gradient calc
Core GPU ofﬂoading
Python usability improvements
GPU likelihood implementation
Auto-diff for gradient computation
Bulk processing with RNTuple, Pandas, etc
Value semantics interface
Figure 13. Milestones and associated eﬀort: RooFit
Many analyses can beneﬁt signiﬁcantly from direct RNTuple to GPU data transfer,
for instance for machine learning, see Fig. 12. This requires work on data layout and
GPU-compatible compression algorithms. Inference from machine learning models
must be simple to use from RDataFrame; results from RDataFrame must be easily
and eﬃciently usable as ML training input.
RooFit (see Fig. 13) needs to continue its renovation for increased eﬃciency,
for instance by processing arrays of input data also on GPUs. A signiﬁcant hurdle
– 34 –
of RooFit is the pointer-based interface with implicit ownership rules; a redesign
based on value semantics and thus similar for Python and C++ is needed for future
evolution of RooFit.
2018 2019 2020 2021 2022 2023 2024 2025 2026 2027
Web Graphics / GUI
Communication implementation, optimization
Programming model prototype
Drawing support for TH1, RHist etc
Embedding in GUI apps, e.g. DQM
Deployment for batch graphics
Feature coverage wrt TCanvas
JupyterLab GUI integration
Higher-level interfaces (”draw efﬁciency”)
Web graphics release for production use
Eve7 prototype implementation
Eve7 experiments’ prototype
Eve7 release for production use
Eve7 experiments’ adoption
Figure 14. Milestones and associated eﬀort: UI and Visualization
Further investment (see Fig. 14) will ensure smooth transition from the legacy graph-
ics and GUI interfaces to the new architecture independent, web-based graphics and
GUI implementations . For this to succeed, the new libraries must provide the
minimal feature set needed by analyses, before the legacy libraries will cease to func-
tion on commodity analysis systems, due to these systems deprecating and penalizing
X11, GL, Cocoa, GDI, etc, a process that is currently ongoing.
– 35 –
ROOT must invest in a productivity layer of its very conﬁgurable and currently
complex graphics interfaces. This allows graphics to be well integrated with the rest
of ROOT, still highly conﬁgurable, but with defaults that correspond to physicists’
expectations. It will guarantee that the community keeps and commonly evolves its
”visual language”, for instance regarding plots showing multiple uncertainty bands,
eﬃciencies, or higher-dimensional distributions.
ROOT’s developing event display seems to satisfy a real community need, with its
high-performance, highly customizable web graphics interface. ROOT is not aware
of any viable alternative with similar functionality and usability. ROOT expects
that this event display will see wide-spread adoption, if development continues, and
is backed by investment in its constituents such as ROOT’s HTTP data transfer
(based on ROOT’s I/O) and ROOT’s C++ interpreter / JIT compiler cling, and
ROOT’s web-based graphics and GUI system.
– 36 –
Description Beneﬁts FTE1Milestones Risks, dependencies
RNTuple as LH-
LHC data for-
>10% storage reduction;
*5 read throughput; reli-
able error detection; ro-
bust user interfaces; GPU-
oriented data layout
2.5 2022: bulk I/O
2022: conversion tools
2023: Xrootd support
2023: PB scale validation
2026: direct GPU I/O
R: delayed features prevent adoption due to lack of
R: ineﬃciencies and trust erosion due to lack of re-
tention of expertise
R: missed performance improvements due to lack of
expertise (software, storage, network; ROOT and ex-
R: inability to follow C++ evolution due to lack of
resources evolving I/O and type system support
D: remote I/O libraries (Xrootd, Davix)
*10 higher physicists’
productivity with obvious
analysis interface; O(10)
acceleration by multi-
threaded analysis; 2*
speedup from transparent
migration from TTree to
2 2023: dataset weights
2023: systematics varia-
2023: GPU ML training
2024: bulk processing
2026: Intermediary result
2027: 0-copy GPU pro-
D: signiﬁcant beneﬁts depend on RNTuple becoming
standard analysis format
D: performance and usability of PyROOT aﬀects
D: cling-CUDA implementation for runtime-
generated GPU kernels
R: interoperability of RDataFrame with big data
analysis ecosystems aﬀects adoption
R: limited adoption would require physicists’ invest-
ment to transition analyses from TTree to RNTuple
R: lack of investment means slower time-to-result for
analyses, triggering migration away from HEP’s eﬃ-
cient analysis ecosystem
1FTE is average per year from 2022 to feature completion; about 50% senior / 50% junior developer
– 37 –
O(10..100) acceleration of
(within seconds, ”interac-
tive”); smooth migration
from processing analysis
locally or on cluster
1.5 2024: node-local caching
2025: backend support for
key schedulers / submis-
2025: site-speciﬁc auto-
D: requires PyROOT support for distributing anal-
D: requires analysis written as RDataFrame
R: use of job HEP’s main schedulers / submission
systems through Python continues to be supported
R: adoption depends on availability of RDataFrame’s
with Python ecosystem;
Support of modern
C++ standards; eﬃcient
language binding accel-
erating Python calls into
ROOT by factor 2
2 Continuous: integration
of new LLVM releases
2022: cling optimizations
2023: NumPy data trans-
fer from C++ objects
2025: C++ type descrip-
2026: reimplementation of
cling based on clang-repl
2027: integration of cppyy
based on clang-repl
D: cppyy as the layer between ROOT’s type descrip-
tion and PyROOT
D: PyROOT depends on cling
D: cling depends on LLVM
R: friction (usability or performance) of using ROOT
from Python can reduce interoperability and usage
R: ROOT’s type description system cannot represent
current C++ standards
R: lack of support of ROOT’s type system for cppyy
can cause separation of cppyy from ROOT
Histogramming Avoids common sources of
errors; higher productiv-
ity through simpler, more
robust, more eﬃcient in-
terfaces; better interoper-
ability with modern C++
and Python code
1.5 2022: common operations
2022: bin ranges
2023: completion of axis
2024: multiple uncertain-
2025: concurrent ﬁling
2026: sparse storage
R: adoption requires near feature complete imple-
mentation; little incremental ”roll-out” to produc-
tion use; can cause design decisions not acceptable
R: lack of I/O support for modern C++ features
causes extra complexity in new histograms library
R: lack of interoperable histogram library can cause
dispersion over other libraries / reimplementations
– 38 –
*10 faster likelihood eval-
1.5 2023: Python interoper-
2023: GPU likelihood
2024: gradient com-
putation with auto-
2026: value semantics
D: PyROOT provides Python interoperability and
eﬃcient data transfer
D: CUDA / GPU programming model
D: ciad for automatic diﬀerentiation
R: Limited physics reach of HL-LHC data due to
performance-induced limitation of model complexity
*10 faster ML training
data throughput from
2 Continuous: support for
using state-of-the-art ML
models with ROOT data
2022: fast inference of
2023: fast inference for
2025: direct GPU transfer
for ML training of RNTu-
D: ML ecosystem
D: CUDA / GPU programming model
R: lack of eﬃcient interoperability causes data con-
version, additional storage use, and potentially re-
duction in physics reach (memory-limited input data
for model training)
visualization code with
reduced need for platform
UI experts; integration
in GUI applications;
ities; physicists produce
graphics more eﬀectively
1.5 2023: embedding in GUI
2024: style deﬁnition
2024: higher-level graph-
2025: feature equality
D: OpenUI5 for Web-GUI
in 2D, 3D
D: Chrome Embedded Framework (directly or
through Qt); browser availability and conﬁgurabil-
R: security implications of web-based graphics can
– 39 –
Event Display Improved analysis
inspection of event selec-
tion; improved outreach
/ communication; acces-
sible detector design /
1 2024: CMS (and likely
others such as Mu2e)
event display prototype
D: Web-based Graphics, GUI
Table 4: Summary of beneﬁts, priorities, eﬀorts, and risks of main
– 40 –
For ROOT, RNTuple has the highest priority and is most time critical, as adop-
tion depends on production readiness well before the start of HL-LHC. Analysis
interfaces (RDataFrame, RooFit, machine learning and Python interoperability, his-
togramming) are the second most important development, deﬁning the future of
ROOT-based HEP analysis, with a signiﬁcant beneﬁt for the eﬃciency of physicists
and the impact of delivered luminosity. Progress is deﬁned by a combination of these
priorities and available expertise and developer eﬀort. In general, ROOT must pro-
vide continuous eﬀort for support and maintenance of all these ﬁelds, which in turn
guarantees a sustained development progress in all these ﬁelds.
Several years ago, ROOT had identiﬁed the HL-LHC as a welcome timeline to rein-
vent itself. Today’s ROOT can still function as it did ten or twenty years ago, thanks
to its backward compatibility. But many physics analyses have moved away from
that, embracing ROOT’s new features that come with a much increased usability and
eﬃciency. This endeavor has attracted many R&D contributions, helping with funda-
mental innovation and fast progress in ROOT’s evolution. All of ROOT’s core areas
have beneﬁted from this investment: the I/O system, likelihood evaluation, machine
learning, data analysis interfaces including distributed analysis, Python bindings and
ROOT’s ”Python personality”, ease of installation and deployment.
Part of ROOT’s evolution speed is due to technology advances being accessible
to other parts of ROOT: the web graphics system and the event display beneﬁt
RooFit’s vectorization eﬀorts beneﬁt from experience in vectorizing ROOT’s ﬁtting
algorithms; the development teams shares commonly accrued expertise on writing
high-performance, highly concurrent code; ROOT’s Python-speciﬁc interfaces beneﬁt
from in-house experience of data transfer into Python other features of PyROOT and
the C++ interpreter / JIT compiler. ROOT is itself an ecosystem, re-using its own
innovations to multiply their eﬀect.
At the same time, ROOT is embedded in an ecosystem of tools built on top of
ROOT, or bridging into other data science ecosystems. ROOT works on providing
these bridges itself, arguing that eﬃciency and separation of concerns are best taken
care of by experts who can guide the analysis physicists in their use of for instance
ROOT data with machine learning tools. ROOT’s technologies and the continuing
relevance of C++ will likely allow ROOT to also satisfy interoperability needs for
the next 20 years.
Yet, ROOT cannot reinvent itself without providing sustainability for those new
features, which in turn requires long-term commitment of developer resources, and
the ability of these developers to also invest signiﬁcantly in work not related to
R&D. The currently ongoing peak of innovation will need to transition into an era
– 41 –
of optimizations and support, during the HL-LHC data taking years. To guarantee
this, retaining expertise and community trust is paramount.
ROOT is not only a software project: it is a team of technical experts and innovators,
communicators with the HEP community, people that this community trusts and
knows to rely on. At age 28, ROOT is currently reinventing itself, to beneﬁt from
the team’s and the community’s experience and support, and to adjust to tomorrow’s
challenges and requirements, in time for the HL-LHC.
ROOT’s evolution is driven by a rich bouquet of innovation with steady progress
over the last couple of years. The goal is to bring ROOT to the level of usability,
eﬃciency, robustness, and community trust required for HL-LHC analyses. Eﬃciency
improvements in orders of magnitude have been seen across the board; adoption by
physicists has been impressively rapid, likely because of the features’ quality, but
also due to increased eﬀorts invested in communication of these new features, and
involvement of the analysis community already during the development of the new
features, following ROOT’s tradition.
Even though ROOT is working on ﬁlling several feature, performance, and pro-
ductivity gaps, it needs to be able to do so in a sustainable way: today’s new features
are tomorrow’s source of bugs. ROOT relies on continued assistance from the com-
munity and contributors to provide user support and maintenance for HL-LHC. It
especially needs this assistance to transition from the current active R&D phase into
a support phase, with retained expertise.
 HL-LHC Analysis Mini-Workshop. https://indico.cern.ch/event/1028381/,
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Guilherme Amadio, Axel Naumann, and Enric Tejedor. RDataFrame: Easy Parallel
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Conf. Ser., 1085:032020, 2018.
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ROOT Tree I/O. EPJ Web Conf., 245:02030, 2020.
 Dan Graur, Ingo M¨uller, Mason Proﬃtt, Ghislain Fourny, Gordon T. Watts, and
Gustavo Alonso. Evaluating Query Languages and Systems for High-Energy Physics
– 42 –
 Kim Albertsson, Sitong An, Lorenzo Moneta, Stefan Wunsch, and Luca Zampieri.
Fast Inference for Machine Learning in ROOT/TMVA. EPJ Web Conf., 245:06008,
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Interoperable and More Pythonic. EPJ Web Conf., 245:06004, 2020.
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