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How to measure energy-efficiency of software: Metrics and measurement results

Authors:
  • University of Applied Sciences Trier, Environmental Campus Birkenfeld
  • Leuphana University Lüneburg | Environmental Campus Birkenfeld

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In the field of information and computer technology (ICT), saving energy has its focus set on energy efficient hardware and its operation. Recently, efforts have also been made in the area of computer software. However, the development of energy efficient software requires metrics, which measure the software's energy consumption as well as models to monitor and minimize it. In software and software development processes they hardly exist. In this work we present a generic metric to measure software and a method to apply it in a software engineering process.
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How to Measure Energy-Efficiency of Software: Metrics and Measurement Results
Timo Johann, Markus Dick, Stefan Naumann, Eva Kern
Trier University of Applied Sciences, Germany
(t.johann, m.dick, s.naumann, e.kern)@umwelt-campus.de
Abstract—In the field of information and computer tech-
nology (ICT), saving energy has its focus set on energy
efficient hardware and its operation. Recently, efforts have
also been made in the area of computer software. However,
the development of energy efficient software requires metrics,
which measure the software’s energy consumption as well as
models to monitor and minimize it. In software and software
development processes they hardly exist. In this work we
present a generic metric to measure software and a method to
apply it in a software engineering process.
Keywords-Energy Efficiency, Green Software, Metrics
I. INTRODUCTION
Activities in the field of Green IT set new standards for the
requirements of information and communication technology.
Generally, these requirements primary concern the energy
consumption of ICT. The developments in Green IT, which
primarily focus on computer hardware, led to the under-
standing that “the most strategic aspect of energy efficient
computing will be the evolution of application software
to facilitate system-wide energy efficiency” [1, p.58]. This
paper presents a method to develop and apply metrics and
measurement methods to measure the energy consumption
that is directly related to the software. The method presented
here should enable software developers to continuously
measure and monitor the energy consumption of the software
during the development process. It seamlessly integrates into
the GREENSOFT Model (cf. figure. 1), a reference model for
Green and Sustainable Software [2].
II. DEVE LOPMENT OF CUSTOM METRICS
Metrics for energy efficient software rely on its useful
work done [3]. Since, modern software consists of manifold
modules that all have a special purpose, there can be more
than just one metric. The software parts can be measured
individually or combined. For a proper comparison of soft-
ware the measured modules should be as similar as possible.
Basically a generic metric can be defined as
EnergyEff iciency =
UsefulWorkDone
UsedEnergy
.
III. MEAS URING THE ENERGY CONSUMPTION
There already exist approaches for measuring software en-
ergy consumption, mostly as black box measurement. White
box measurements, relying on source code instrumentation,
are hardly used. Figure 2 intends to classify the known
Figure 1. The Greensoft Model [2, p. 270]
methods of energy efficiency measurement. Benchmarking
methods are able to measure a system as a black box
and can generate a statement on how the entire system
(software and hardware) performs on the whole. When it
comes to measuring a given software, one cannot apply
these benchmarks because each of them is customized for
one specific group of tasks (database benchmarks, graphic
benchmarks, etc.). The approach of Dick et al. [4] defines
individual scenarios for a specific group of software (e.g.
Browsers) and then measures concrete occurrences with the
same scenario, which can give a better statement about the
software’s energy consumption. These approaches can be
categorized as black box measurements because they do not
allow to take a look inside, which is their main disadvantage.
That is why we propose a kind of white box measurement
to tell in which part of software there is potential for energy
savings. A white box method is better suited to find resource
intensive parts of programs and to improve them. To do so,
we will use a known technique: source code instrumentation,
which is often used in the context of software profiling, e.g.
for runtime analysis.
The experimental set-up to perform a white box
measurement consists of a set of components (cf. figure
3). The system under test runs an instrumented application
and is connected to a power meter. The power meter
is connected to the measuring system, where an Energy
978-1-4673-1832-7/12/$31.00
c
2012 IEEE
GREENS 2012, Zurich, Switzerland
51
Figure 2. Methods to Measure Energy Efficiency
Server converts the values to performance counters. The
instrumented application also writes performance counters.
These can be read, e.g. by logging or monitoring tools
to analyze the software regarding its energy consumption.
To accomplish the instrumentation the set-up builds on
an API provided by Intel to perform the source code
instrumentation [3]. Using this set-up, existing source code
can be expanded by counters. These numerical values can
be used e.g. for counting loop cycles, marking entry and
exit points of code fragments, counting the amount of how
often parts of software are frequented, etc.
Figure 3. Measurement Environment
IV. EXAMPLE ENERGY EFFICIENCY METRICS
A. Sorting Application
Since sorting is a common task in computer science, and
is easy enough to quickly find the useful work, it is good
as a first example. Here it is obvious that the useful work is
sorting items and it follows that the metric is
SortedItems
UsedEnergy
.To
make it a little bit more exciting we will use two different
implementations of sorting algorithms that basically differ
in their complexity. Thus, we have something to compare.
Therefore, we picked Bubble Sort with a complexity of
O(n
2
) and Heap Sort with a complexity of O ( n · log(n)).
The sorting application is implemented to generate an array
with n randomly generated double values. Then the array
should be sorted using Bubble Sort; make a break and remix
the array values, and then use Heap Sort to perform the same
task. For the instrumentation, we declared two counters; one
counter counts the items that are already sorted, the other
counter is used as a phase counter and is only meant to
generate a positive edge when sorting starts and a negative
edge when sorting ends. Bubble Sort uses double nested
for-loops. The outer loop goes over the array from 0 to
n once. Every time the loop reenters the counter increases
and an item is sorted. Counting the sorted items in Heap
Sort is similar. After transforming the array into a binary
heap, a for-loop goes over it and after every pass the counter
increases and an item is sorted.
Since the system performs several background processes and
services, which cause some peaks in the processor time (red)
and in the energy consumption (green), we measured the
average idle consumption which is about 65 Watt.
Then the actual measurement of the algorithms with
bubble sort is started, which is shown in figure 4. The
algorithms sort 200,000 double values. The phase counter
(blue) indicates the start of sorting, which instantly stresses
the CPU (red) and increases the consumed energy (green).
When all items are sorted (b lack), the phase counter gener-
ates the negative edge which is the end of the measurement.
The sorting time was 110 seconds and the average energy
consumption was 100 Watts.
100W 110s = 11000Ws =3Wh = 11000J
The whole algorithm consumed 10,800 Joule for the
sorting of 200,000 double values, which results in
200, 000
10, 800J
= 18
SortedItems
Joule
The Heap Sort algorithm also sorted 200,000 double
values and the result is shown in figure 4, where it is obvious
that the algorithm worked - as expected - much faster. The
values were sorted in five seconds. Due to the resolution
(1 second) the power consumption line (green) did not
immediately follow the phase counter, but here the average
power consumption during processing was 100 Watts, which
results in 100W 5s = 500Ws =0.14Wh = 500J.
For a better comparison it is rational to give the algorithms
the same time frame and calculate the power consumption
of the idle time of the faster algorithm and add it to the total
power consumption. Since the idle time is 110 5 = 105,
and the idle power consumption is 65, the power consump-
tion is 65W 105s = 6825Ws =1.89Wh = 6825J.
Adding the energy consumed during sorting the result is
7325 J.
200, 000
500J + 6825J
= 27.3
SortedItems
Joule
The algorithm with the better asymptotic run time seems
to also have the better energy efficiency, which corroborates
the assumption that an algorithm with a higher performance
also has a higher energy efficiency.
52
Figure 4. Energy Consumption of Heap Sort (Top) and Bubble Sort
(Bottom)
B. Multi User Web Application
As a second example we chose a web-based information
system. Usually there are parts such as browser, web-server
and database involved. Therefore, we chose a client/server
application, which is used in productive operations at the
Environmental Campus Birkenfeld. Its purpose is to collect,
process and visualize consumption data of the campus
buildings (like: energy, water, etc). The server stores the
data in a database that is managed by a web interface. For
demonstration purposes only a cut-out of the software is
used. One of its capabilities is to visualize the consumption
data as a line graph over a time period. The charts are
dynamically generated for each request.
When looking at the description of the chart feature,
a useful metric is:
Joules
DeliveredChart
. This metric can be
expanded to other parts of the chart generation, e.g.
database queries, the image generation, and the complete
request/response cycle. We assume that this is a good way
to represent the energy consumption of this part of the
software. It must be kept in mind that there is no ’one size
fits all’ metric and applications can be split up into more
or less fine-grained modules, which have their own metrics.
The load for the measurement will be simulated and is
generated by a fixed number of users, which send multiple
requests to the server. Therefore, the source code must be
instrumented at the point where the request comes in and
where the response goes out. The counters must increase
with an incoming request and decrease when the response
is sent. Thus, we know how many requests are currently in
progress. In addition to the two systems described in the
Table I
ENERGY PER USER
User Count 15 Users 25 Users
Energy
Consumption
7, 3
J oules
DeliveredChart
5.5
J oules
DeliveredChart
Average
Response Time
0.5 seconds 0.8 seconds
set-up, one can use a third system that generates the load.
Here, we will try to demonstrate how the energy efficiency
changes when serving different amount of users.
In a first testrun a load of 15 users was generated.
The result is shown in figure 5. After the ramp-up period
the users that are waiting for their response (blue) even
out at approximately six users. In this phase the average
power rating of the server is about 110 Watts (green),
while simultaneously serving 15 users with a waiting time
way under one second. In one hour the system with 15
simultaneous users consumes 110 Wh = 396kJ.
The same measurement is performed with 25 simultane-
ous served users (cf. figure 5), whereas the average queued
users is approximately 17, which means that users have to
wait a little bit longer than in the measurement with 15 users,
but still get their response under one second. The energy
consumed is also 110 Watts and users can still be served at a
satisfactory time. As one can see, the claimed response time
plays a key role: the higher the demand for fast response,
the lower is the energy efficiency. Table I shows the average
consumption after one hour in both cases.
Figure 5. Energy Consumption Serving 15 (Top) and 25 (Bottom) Users
53
V. C ONCLUSION AND OUTLOOK
The procedure that we discussed and presented in this
paper works and produces beneficial results. This means
that energy consumption of specific parts of software can
be measured with a white box method and programmers are
able to find resource intensive code, which is an advantage
compared to black box methods. From the programmer’s
point of view, there is yet a lot of additional work.
Not only the exemplary set-up, also the development of
adequate metrics and the instrumentation cause a significant
overhead when the application becomes more complex.
There is by now no ready-to-use software or library for
source code instrumentation in context of energy efficiency
measurement. Furthermore, a professional power meter is
needed, which can be very expensive depending on the
desired application. In larger software projects maybe more
than one power meter will be necessary which causes
additional costs. After all the overhead for setting up a
measure environment and instrumenting source code, the
measurement itself is circumstantial. To conclude, we
evaluate the procedure as feasible for research but yet too
complex for every-day use in software projects.
The measurements and the metrics presented in this paper
should take place in a software development project as
early as possible. In principle, this means that even early
executable software artifacts should be examined regarding
energy efficiency issues. Due to the high affinity of the
method regarding performance measurements, its workload
can be re-used for this purpose and vice versa. In this way,
it is possible to combine performance and energy efficiency
measurement efforts. In accordance with continuously inte-
gration paradigms, we propose that the measurement runs
are automatically executed by build tools e.g. during nightly
builds. This leads to a database of measures that can be
used for an ongoing optimization of the software product.
Regarding metrics, an initial or preceding measure is used
as a reference value for succeeding measures, which enables
those responsible to calculate delta values, which can be
used as a distance metric for the optimization or to visualize
the outcomes of the energy optimization efforts in charts.
The amount of publications in the fields of green and
sustainable software, and the resulting demand for metrics
to measure sustainability aspects, show the importance of
the development of tools and guidelines for developers and
users of software. The small number of sophisticated in-
dustrial solutions demands an increased effort in developing
tools. In industry some pioneer projects, such as Intel’s
Energy Checker SDK, already exist, but it is still not a
widespread topic. Manual source code instrumentation is a
way to measure software, but is an additional expenditure
for developers. Automated instrumentation or at least semi-
automated instrumentation would be a large enhancement.
When instrumenting the code, the possibility to use the IDE
to set counters just like adding a breakpoint for debugging
would have been an enormous benefit.
Modern open source IDEs have the ability to have add-ons or
plug-ins developed. To our knowledge, such an extension for
energy measurement of desktop and server CPUs does not
yet exist. Besides the additional effort, another disadvantage
of the method is the need for hardware devices that measure
energy consumption. In the future, measurement should
be centralized and the results of measurements should be
aggregated to have more efficient power models that can be
used by others. Profilers would be able to use these models
to estimate energy consumption of software. A specific
function on a specific system would use approximately
the same amount of energy. Therefore, an instrumentation
on compiler level is needed where the compilers add in-
formation for energy debugging into the code. The more
systems are measured, the better the power models will
become. In combination with the power models, the profiler
should then be able to give an accurate prediction of energy
consumption. It has yet to be proven, if these energy models
supersede hardware-based measurement of software. Based
on robust metrics and measuring methods, the development
of seals of quality will be an important step for sustainable
software. As known from the application of other seals, they
create user awareness and encourage software vendors to
develop energy saving software products.
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54
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Intel Energy Checker Software Development Kit User Guide
  • J Tayeb
  • K Bross
  • B S Chang
  • L Cong
  • S Rogers
J. Tayeb, K. Bross, B. S. Chang, L. Cong, and S. Rogers, "Intel Energy Checker Software Development Kit User Guide," 2010-12-15. [Online]. Available: http://software.intel. com/file/32957/