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# Graphics processing unit (GPU) programming strategies and trends in GPU computing

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## Abstract

Over the last decade, there has been a growing interest in the use of graphics processing units (GPUs) for non-graphics applications. From early academic proof-of-concept papers around the year 2000, the use of GPUs has now matured to a point where there are countless industrial applications. Together with the expanding use of GPUs, we have also seen a tremendous development in the programming languages and tools, and getting started programming GPUs has never been easier. However, whilst getting started with GPU programming can be simple, being able to fully utilize GPU hardware is an art that can take months or years to master. The aim of this article is to simplify this process, by giving an overview of current GPU programming strategies, profile-driven development, and an outlook to future trends.
GPU PROGRAMMING STRATEGIES AND TRENDS IN GPU COMPUTING
ANDRÉ R. BRODTKORB1, TROND R. HAGEN1,2, AND MARTIN L. SÆTRA2
This is a draft.
The full paper can be found in Journal of Parallel and Distributed Computing 73(1): 4–13:
http://dx.doi.org/10.1016/j.jpdc.2012.04.003
ABSTRACT. Over the last decade, there has been a growing interest in the use of graphics processing units (GPUs) for non-
graphics applications. From early academic proof-of-concept papers around the year 2000, the use of GPUs has now matured
to a point where there are countless industrial applications. Together with the expanding use of GPUs, we have also seen a
tremendous development in the programming languages and tools, and getting started programming GPUs has never been
easier. However, whilst getting started with GPU programming can be simple, being able to fully utilize GPU hardware is
current GPU programming strategies, proﬁle driven development, and an outlook to future trends.
Keywords: GPU Computing, Heterogeneous Computing, Proﬁling, Optimization, Debugging, Hardware, Future Trends.
1{Andre.Brodtkorb@sintef.no,TrondRunar.Hagen@sintef.no},
SINTEF ICT, Department of Applied Mathematics, P.O. Box 124, Blindern, NO-0314 Oslo, Norway.
2Martin.L.Satra@cma.uio.no,
Centre of Mathematics for Applications, University of Oslo, P.O. Box 1053 Blindern, NO-0316 Oslo, Norway.
1. INTRODUCTION
Graphics processing units (GPUs) have for well over a decade been used for general purpose computation, called
GPGPU [8]. When the ﬁrst GPU programs were written, the GPU was used much like a calculator: it had a set of ﬁxed
operations that were exploited to achieve some desired result. As the GPU is designed to output a 2D image from a 3D
FIGURE 1. History of programming languages for GPGPU. When GPUs were ﬁrst being used for
non-graphics applications, one had to rewrite the application in terms of operations on graphical
primitives using languages such as OpenGL or DirectX. As this was cumbersome and error prone,
several academic and third-party languages that abstracted away the graphics appeared. Since 2007,
however, vendors started releasing general purpose languages for programming the GPU, such as
AMD Close-to-Metal (CTM) and NVIDIA CUDA. Today, the predominant general purpose lan-
guages are NVIDIA CUDA, DirectCompute, and OpenCL.
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FIGURE 2. Historical comparison of theoretical peak performance in terms of gigaﬂops and band-
width for the fastest available NVIDIA GPUs and Intel CPUs. Today, the performance gap is cur-
rently roughly seven times for both metrics.
virtual world, the operations it could perform were fundamentally linked with graphics, and the ﬁrst GPU programs
were expressed as operations on graphical primitives such as triangles. These programs were difﬁcult to develop, debug
and optimize, and compiler bugs were frequently encountered. However, the proof-of-concept programs demonstrated
that the use of GPUs could give dramatic speedups over CPUs for certain algorithms [20, 4], and research on GPUs
soon led to the development of higher-level third-party languages that abstracted away the graphics. These languages,
however, were rapidly abandoned when the hardware vendors released dedicated non-graphics languages that enabled
the use of GPUs for general purpose computing (see Figure 1).
The key to the success of GPU computing has partly been its massive performance when compared to the CPU:
Today, there is a performance gap of roughly seven times between the two when comparing theoretical peak bandwidth
and gigaﬂops performance (see Figure 2). This performance gap has its roots in physical per-core restraints and
architectural differences between the two processors. The CPU is in essence a serial von Neumann processor, and
is highly optimized to execute a series of operations in order. One of the major performance factors of CPUs has
traditionally been its steadily increasing frequency. If you double the frequency, you double the performance, and
there has been a long withstanding trend of exponential frequency growth. In the 2000s, however, this increase came
to an abrupt stop as we hit the power wall [3]: Because the power consumption of a CPU is proportional to the
frequency cubed [4], the power density was approaching that of a nuclear reactor core [22]. Unable to cool such chips
sufﬁciently, the trend of exponential frequency growth stopped at just below 4.0 GHz. Coupled with the memory wall
and the ILP wall, serial computing had reached its zenith in performance [3], and CPUs started increasing performance
through multi-core and vector instructions instead.
At the same time as CPUs hit the serial performance ceiling, GPUs were growing exponentially in performance
due to massive parallelism: As computing the color of a pixel on screen can be performed independently of all
other pixels, parallelism is a natural way of increasing performance in GPUs. Parallelism appears to be a sustainable
way of increasing performance, and there are many applications that display embarrassingly parallel workloads that
are perfectly suited for GPUs. However, increased parallelism will only increase the performance of parallel code
sections, meaning that the serial part of the code soon becomes the bottleneck. This is often referred to as Amdahl’s
law [2]. Thus, most applications beneﬁt from the powerful combination of the massively parallel GPU and the fast
multi-core CPU. This combination is known as a heterogeneous computer: the combination of traditional CPU cores
and specialized accelerator cores [4].
GPUs are not the only type of accelerator core that has gained interest over the last decade. Other examples include
ﬁeld programmable gate arrays (FPGAs) and the Cell Broadband Engine (Cell BE), which have both been highly
successful in many application areas [4]. Today, however, these receive only a fraction of the attention of GPUs (see
Figure 3). One of the major reasons for this is that a very large percentage of desktop and laptop computers have a
dedicated GPU already, whilst FPGAs and the Cell BE are only found in specially ordered setups. Furthermore, the
future of the Cell BE is currently uncertain as the road map for the second version has not been followed through.
FPGAs, on the other hand, have a thriving community in the embedded markets, but are unfortunately too hard to
program for general purpose computing. The cumbersome and difﬁcult programming process that requires detailed
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FPGA
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FIGURE 3. Google search trends for GPU, FPGA and Cell BE. These terms were chosen as the
highest ranking keywords for each of the architectures. Whilst both FPGAs and the Cell BE today
receive a modest number of searches, the GPU has a large growing interest.
knowledge of low-level hardware has recently improved dramatically with the advent of C-like languages, but the
development cycle is still slow due to the time consuming place and route stage.
There are three major GPU vendors for the PC market today, Intel being the largest. However, Intel is only dominant
in the integrated and low-performance market. For high-performance and discrete graphics, AMD and NVIDIA are the
sole two suppliers. In academic and industrial environments, NVIDIA appears to be the clear predominant supplier,
and we thus focus on GPUs from NVIDIA in this article, even though most of the concepts and techniques are also
directly transferable to GPUs from AMD. For NVIDIA GPUs there are three languages suitable for general purpose
computing as shown in Figure 1. Of these, we focus on NVIDIA CUDA. Even though the three languages are
conceptually equivalent and offer more or less the same functionality, CUDA is the most mature technology with the
Much of the information in this article can be found in a variety of different sources, including books, documenta-
tion, manuals, conference presentations, and on Internet fora. Getting an overview of all this information is an arduous
programming techniques and proﬁle driven development, and to serve as a step-by-step guide for optimizing GPU
codes. The rest of the article is sectioned as follows: First, we give a short overview of current GPU hardware in
Section 2, followed by general programming strategies in Section 3. Then, we give a thorough overview of proﬁle
driven development for GPUs in Section 4, and a short overview of available debugging tools in Section 5. Finally, we
offer our view on the current and future trends in Section 6, and conclude with a short summary in Section 7.
2. FERM I GPU ARCHITECTURE
The multi-core CPU is composed of a handful of complex cores with large caches. The cores are optimized for
that a lot of transistor space is dedicated to complex instruction level parallelism such as instruction pipelining, branch
prediction, speculative execution, and out-of-order execution, leaving only a tiny fraction of the die area for integer and
ﬂoating point execution units. In contrast, a GPU is composed of hundreds of simpler cores that can handle thousands
of concurrent hardware threads. GPUs are designed to maximize ﬂoating-point throughput, whereby most transistors
within each core are dedicated to computation rather than complex instruction level parallelism and large caches. The
following part of this section gives a short overview of a modern GPU architecture.
Todays Fermi-based architecture [17] feature up to 512 accelerator cores called CUDA cores (see Figure 4a). Each
CUDA core has a fully pipelined integer arithmetic logic unit (ALU) and a ﬂoating point unit (FPU) that executes one
integer or ﬂoating point instruction per clock cycle. The CUDA cores are organized in 16 streaming multiprocessors,
each with 32 CUDA cores (see Figure 4b). Fermi also includes a coherent L2 cache of 768 KB that is shared across all
16 multiprocessors in the GPU, and the GPU has a 384-bit GDDR5 DRAM memory interface supporting up-to a total
of 6 GB of on-board memory. A host interface connects the GPU to the CPU via the PCI express bus. The GigaThread
global scheduler distributes thread blocks to multiprocessor thread schedulers (see Figure 4a). This scheduler handles
concurrent kernel1execution and out of order thread block execution.
1A kernel is a GPU program that typically executes in a data-parallel fashion.
3
(A)
Core Core
Core Core
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SFU
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Register File (32 768 x 32-bit)
Warp Scheduler
Dispatch Unit
Warp Scheduler
Dispatch Unit
Instruction Cache
Uniform Cache
64 KB Shared Memory / L1 Cache
Interconnect Network
(B)
FIGURE 4. Current Fermi-class GPU hardware. The GPU consisting of up-to 16 Streaming Multi-
processors (also known as SMs) is shown in (a), and (b) shows a single multiprocessor.
Each multiprocessor has 16 load/store units, allowing source and destination addresses to be calculated for 16
threads per clock cycle. Special Function Units (SFUs) execute intrinsic instructions such as sine, cosine, square root,
and interpolation. Each SFU executes one instruction per thread, per clock. The multiprocessor schedules threads
in groups of 32 parallel threads called warps. Each multiprocessor features two warp schedulers and two instruction
dispatch units, allowing two warps to be issued and executed concurrently. The Fermi dual warp scheduler selects two
warps, and issues one instruction from each warp to a group of 16 CUDA cores, 16 load/store units, or four SFUs. The
multiprocessor has 64 KB of on-chip memory that can be conﬁgured as 48 KB of shared memory with 16 KB of L1
cache or as 16 KB of shared memory with 48 KB of L1 cache.
A traditional critique of GPUs has been their lack of IEEE compliant ﬂoating point operations and error-correcting
code (ECC) memory. However, these shortcomings have been addressed by NVIDIA, and all of their recent GPUs
offer fully IEEE-754 compliant single and double precision ﬂoating point operations, in addition to ECC memory.
3. GPU PROGRAMMING STRATEGIES
Programming GPUs is unlike traditional CPU programming, because the hardware is dramatically different. It can
often be a relatively simple task to get started with GPU programming and get speedups over existing CPU codes, but
these ﬁrst attempts at GPU computing are often sub-optimal, and do not utilize the hardware to a satisfactory degree.
Achieving a scalable high-performance code that uses hardware resources efﬁciently is still a difﬁcult task that can
take months and years to master.
3.1. Guidelines for Latency Hiding and Thread Performance. The GPU execution model is based around the
concept of launching a kernel on a grid consisting of blocks (see Figure 5). Each block again consists of a set of
threads, and threads within the same block can synchronize and cooperate using fast shared memory. This maps to
the hardware so that a block runs on a single multiprocessor, and one multiprocessor can execute multiple blocks in a
time-sliced fashion. The grid and block dimensions can be one, two, and three dimensional, and determine the number
of threads that will be used. Each thread has a unique identiﬁer within its block, and each block has a unique global
identiﬁer. These are combined to create a unique global identiﬁer per thread.
The massively threaded architecture of the GPU is used to hide memory latencies. Even though the GPU has a
vastly superior memory bandwidth compared to CPUs, it still takes on the order of hundreds of clock cycles to start
4
Block (0,0)
Block (0,1)
Block (1,0)
Block (1,1)
Block (2,0)
Block (2,1)
Compute Grid
Block (1,0)
FIGURE 5. The CUDA concept concept of a grid of blocks. Each block consists of a set of threads
that can communicate and cooperate. Each thread uses its block index in combination with its thread
index to identify its position in the global grid.
1 8
(A)
1 8 16 24 32
/ / Co h e r e n t c od e
i f ( a > b ) {
a ++ ;
someFunction (a ) ;
}else {
b ++ ;
someFunction(b ) ;
someFunction (a ) ;
}
/ / Co h e r e n t c od e
Time
(B)
FIGURE 6. Bank conﬂicts and thread divergence. (a) shows conﬂict free column-wise access of
shared memory. The example shows how padding shared memory to be the number of banks
plus one gives conﬂict free access by columns (marked with circles for the ﬁrst column). Without
padding, all elements in the ﬁrst column would belong to the same bank, and thus give an eight-
way bank conﬂict. By padding the width by one, we ensure that the elements belong to different
banks. Please note that the constructed example shows eight banks, whilst current hardware has 32
banks. (b) shows branching on 32-wide SIMD GPU architectures. All threads perform the same
computations, but the result is masked out for the dashed boxes.
the fetch of a single element from main GPU memory. This latency is automatically hidden by the GPU through rapid
switching between threads. Once a thread stalls on a memory fetch, the GPU instantly switches to the next available
thread in a fashion similar to Hyper-Threading [14] on Intel CPUs. This strategy, however, is most efﬁcient when there
are enough available threads to completely hide the memory latency, meaning we need a lot of threads. As there is
a maximum number of concurrent threads a GPU can support, we can calculate how large a percentage of this ﬁgure
we are using. This number is referred to as the occupancy, and as a rule of thumb it is good to keep a relatively high
occupancy. However, a higher occupancy does not necessarily equate higher performance: Once all memory latencies
are hidden, a higher occupancy may actually degrade performance as it also affects other performance metrics.
from these CPU threads. One of the things that differs from traditional CPU programming is that the GPU executes
instructions in a 32-way SIMD2fashion, in which the same instruction is simultaneously executed for 32 different
data elements, called a warp. This is illustrated in Figure 6b, in which a branch is taken by only some of the threads
within a warp. This means that all threads within a warp must execute both parts of the branch, which in the utmost
2SIMD stands for single instruction multiple data.
5
consequence slows down the program by a factor 32. Conversely, this does not affect performance when all threads in
a warp take the same branch.
One technique used to avoid expensive branching within a kernel is to sort the elements according to the branch,
and thus make sure the threads within each warp all execute their code without branching. Another way of preventing
branching, is to perform the branch once on the CPU instead of for each warp on the GPU. This can be done for
example using templates: by replacing the branch variable with a template variable, we can generate two kernels: one
for condition true, and one for condition false, and let the CPU perform the branch and from this select the correct
kernel. The use of templates, however, is not particularly powerful in this example, as the overhead of running a
simple coherent if-statement in the kernel would be small. But when there are a lot of parameters, there can be a large
performance gain from using template kernels [7, 5]. Another prime example of the beneﬁt of template kernels, is
the ability to specify different shared memory sizes at compile time, thus allowing the compiler to issue warnings for
out-of-bounds access. The use of templates can also be used to perform compile-time loop unrolling, which has a great
performance impact. By using a switch-case statement, with a separate kernel being launched for different for-loop
sizes, performance can be greatly improved.
3.2. Memory Guidelines. CPUs have struggled with the memory wall for a long time. The memory wall, in which
transferring data to the processor is far more expensive than computing on that data, can also be a problem on GPUs.
This means that many algorithms will often be memory bound, making memory optimizations important. The ﬁrst
lesson in memory optimization is to reuse data and keep it in the fastest available memory. For GPUs, there are three
memory areas, listed in decreasing order by speed: registers, shared memory, and global memory.
Registers are the fastest memory units on a GPU, and each multiprocessor on the GPU has a large, but limited,
register ﬁle which is divided amongst threads residing on that multiprocessor. Registers are private for each thread,
and if the threads use more registers than are physically available, registers will also spill to the L1 cache and global
memory. This means that when you have a high number of threads, the number of registers available to each thread is
very restricted, which is is one of the reasons why a high occupancy may actually hurt performance. Thus, thread-level
parallelism is not the only way of increasing performance. It is also possible to increase performance by decreasing
the occupancy to increase the number of registers available per thread.
The second fastest memory type is shared memory, and this memory can be just as fast as registers if accessed
properly. Shared memory is a very powerful tool in GPU computing, and the main difference between registers and
shared memory is the ability for several threads to share data. Shared memory is accessible to all threads within one
block, thus enabling cooperation. It can be thought of as a kind of programmable cache, or scratchpad, in which the
programmer is responsible for placing often used data there explicitly. However, as with caches, its size is limited
(up-to 48 KB) and this can often be a limitation on the number of threads per block. Shared memory is physically
organized into 32 banks that serves one warp with data simultaneously. However, for full speed, each thread must
access a distinct bank. Failure to do so leads to more memory requests, one for each bank conﬂict. A classical way
to avoid bank conﬂicts is to use padding. In Figure 6a, for example, we can avoid bank conﬂicts for column-wise
access by padding the shared memory with an extra element, so that neighboring elements in the same column belong
to different banks.
The third, and slowest type of memory on the GPU is the global memory, which is the main memory of the GPU.
Even though it has an impressive bandwidth, it has a high latency, as discussed earlier. These latencies are preferably
hidden by a large number of threads, but there are still large pitfalls. First of all, just as with CPUs, the GPU transfers
full cache lines across the bus (called coalesced reads). As a rule of thumb, transferring a single element consumes
the same bandwidth as transferring a full cache line. Thus, to achieve full memory bandwidth, we should program the
kernel such that warps access continuous regions of memory. Furthermore we want to transfer full cache lines, which
is done by starting at a quad word boundary (the start address of a cache line), and transfer full quadwords (128 bytes)
as the smallest unit. This address alignment is typically achieved by padding arrays. Alternatively, for non-cached
loads, it is sufﬁcient to align to word boundaries and transfer words (32 bytes). To fully occupy the memory bus the
GPU also uses memory parallelism, in which a large number of outstanding memory requests are used to occupy the
bandwidth. This is both a reason for a high memory latency, and a reason for high bandwidth utilization.
Fermi also has hardware L1 and L2 caches that work in a similar fashion as traditional CPU caches. The L2 cache
size is ﬁxed and shared between all multiprocessors on the GPU, whilst the L1 cache is per multiprocessor. The L1
cache can be conﬁgured to be either 16 KB or 48 KB, at the expense of shared memory. The L2 cache, on the other
hand, can be turned on or off on at compile-time, or by using inline PTX assembly instructions in the kernel. The
6
beneﬁt of turning off the L2 cache is that the GPU is now allowed to transfer smaller amounts of data than a full cache
line, which will often improve performance for sparse and other random access algorithms.
In addition to the L1 and L2 caches, the GPU also has dedicated caches that are related to traditional graphics
functions. The constant memory cache is one example, which in CUDA is typically used for arguments sent to a GPU
kernel. It has its own dedicated cache tailored for broadcast, in which all threads in a block access the same data. The
GPU also has a texture cache that can be used to accelerate reading global memory. However, the L1 cache has a
higher bandwidth, so the texture cache is mostly useful if combined with texture functions such as linear interpolation
between elements.
3.3. Further Guidelines. The CPU and the GPU are different processors that operate asynchronously. This means
that we can let the CPU and the GPU perform different tasks simultaneously, which is a key ingredient of heteroge-
neous computing: the efﬁcient use of multiple different computational resources, in which each resource performs the
taks for which it is best suited. In the CUDA API, this is exposed as streams. Each stream is an in-order queue of
operations that will be performed by the GPU, including memory transfers and kernel launches. A typical use-case
is that the CPU schedules a memory copy from the CPU to the GPU, a kernel launch, and a copy of results from the
GPU to the CPU. The CPU then continues to perform CPU-side calculations simultaneously as the GPU processes its
operations, and only synchronizes with the GPU when its results are needed. There is also support for independent
streams, which can execute their operations simultaneously as long as they obey their own streams order. Current
GPUs support up-to 16 concurrent kernel launches [18], which means that we can both have data parallelism, in terms
of a computational grid of blocks, and task parallelism, in terms of different concurrent kernels. GPUs furthermore
support overlaping memory copies between the CPU and the GPU and kernel execution. This means that we can
simultaneously copy data from the CPU to the GPU, execute 16 different kernels, and copy data from the GPU back
to the CPU if all these operations are scheduled properly to different streams.
When transferring data between the CPU and the GPU over the PCI express bus, it is beneﬁcial to use so-called
page-locked memory. This essentially disables the operating system from paging memory, meaning that the memory
area is guaranteed to be continuous and in physical RAM (not swapped out to disk, for example). However, page-
locked memory is scarce and rapidly exhausted if used carelessly. A further optimization for page-locked memory is
to use write-combining allocation. This disables CPU caching of a memory area that the CPU will only write to, and
can increases the bandwidth utilization by up-to 40% [18]. It should also be noted that enabling ECC memory will
negatively affect both the bandwidth utilization and available memory, as ECC requires extra bits for error control.
CUDA now also supports a uniﬁed address space, in which the physical location of a pointer is automatically
determined. That is, data can be copied from the GPU to the CPU (or the other way round) without specifying the
direction of the copy. While this might not seem like a great beneﬁt at ﬁrst, it greatly simpliﬁes code needed to
copy data between CPU and GPU memories, and enables advanced memory accesses. The uniﬁed memory space is
particularly powerful when combined with mapped memory. A mapped memory area is a continuous block of memory
that is available directly from both the CPU and the GPU simultaneously. When using mapped memory, data transfers
between the CPU and the GPU are executed asynchronously with kernel execution automatically.
CUDA contexts (e.g., one for each physical GPU), and conversely multiple CPU threads can share control of one
CUDA context. The uniﬁed memory model together with the new thread safe context handling enables much faster
transfers between multiple GPUs. The CPU thread can simply issue a direct GPU-GPU copy, bypassing a superﬂuous
copy to CPU memory.
4. PRO FILE DRIVEN DEVELO PMENT3.
A famous quote attributed to Donald Knuth is that “premature optimization is the root of all evil” [11], or put
another way; make sure the code produces the correct results before trying to optimize it, and optimize only where
it will make an impact. The ﬁrst step in optimization is always to identify the major application bottlenecks, as
performance will increase the most when removing these. However, locating the bottleneck is hard enough on a CPU,
and can be even more difﬁcult on a GPU. Optimization should also be considered a cyclic process, meaning that after
having found and removed one bottleneck, we need to repeat the proﬁling process to ﬁnd the next bottleneck in the
application. This cyclic optimization can be repeated until the kernel operates close to the theoretical hardware limits
or all optimization techniques have been exhausted.
3Many of the optimization techniques presented in this section are from the excellent presentations by Paulius Micikevisius [16, 15]
7
1
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Math
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FIGURE 7. Normalized run-time of modiﬁed kernels which are used to identify bottlenecks: (top
left) a well balanced kernel, (top right) a latency bound kernel, (bottom left) a memory bound kernel,
and (bottom right) an arithmetic bound kernel. “Total” refers to the total kernel time, whilst “Mem-
ory” refers to a kernel stripped of arithmetic operations, and “Math” refers to a kernel stripped of
memory operations. It is important to note that latencies are part of the measured run-times for all
kernel versions.
To identify the performance bottleneck in a GPU application, it is important to chose the appropriate performance
metrics, and then compare the measured performance to the theoretical peak performance. There are several bottle-
necks one can encounter when programming GPUs. For a GPU kernel, there are three main bottlenecks; the kernel
may be limited by instruction throughput, memory throughput, or latencies. However, it might also be that CPU-GPU
communication is the bottleneck, or that application overheads dominate the run-time.
4.1. Locating Kernel Bottlenecks. There are two main approaches to locating the performance bottleneck of a
CUDA kernel, the ﬁrst and most obvious being to use the CUDA proﬁler. The proﬁler is a program that that samples
different hardware counters, and the correct interpretation of these numbers is required to identify bottlenecks. The
second option is to modify the source code, and compare the execution time of the differently modiﬁed kernels.
The proﬁler can be used to identify whether a kernel is limited by bandwidth or arithmetic operations. This is done
by simply looking at the instruction-to-byte ratio, or in other words ﬁnding out how many arithmetic operations your
kernel performs per byte it reads. The ratio can be found by comparing the instructions issued counter (multiplied
with the warp size, 32) to the sum of global store transactions and L1 global load miss counters (both multiplied
with the cache line size, 128 bytes), or directly through the instruction/byte counter. Then we compare this
ratio to the theoretical ratio for the speciﬁc hardware the kernel is running on, which is available in the proﬁler as the
Ideal Instruction/Byte ratio counter.
The proﬁler does not always report accurate ﬁgures because the number of load and store instructions may be lower
than the actual number of memory transactions, depending on address patterns and individual transfer sizes. To get the
most accurate ﬁgures, we can follow another strategy which is to compare the run-time of three modiﬁed versions of
the kernel: The original kernel, one Math version in which all memory loads and stores are removed, and one Memory
version in which all arithmetic operations are removed (see Figure 7). If the Math version is signiﬁcantly faster than
the original and Memory kernels, we know that the kernel is memory bound, and conversely for arithmetics. This
method has the added beneﬁt of showing how well memory operations and arithmetic operations overlap.
To create the Math kernel, we simply comment out all load operations, and move every store operation inside
conditionals that will always evaluate to false. We do this to fool the compiler so that it does not optimize away
the parts we want to proﬁle, since the compiler will strip away all code not contributing to the ﬁnal output to global
memory. However, to make sure that the compiler does not move the computations inside the conditional as well, the
result of the computations must also be used in the condition as shown in Listing 1. Creating the Memory kernel, on
the other hand, is much simpler. Here, we can simply comment out all arithmetic operations, and instead add all data
used by the kernel, and write out the sum as the result.
8
__global__ void main(..., int flag) {
float result = ...;
if(1.0f == result *flag)
output[i] = value;
}
LISTING 1. Compiler trick for arithmetic only kernel. By adding the kernel argument ﬂag (which
we always set to 0), we disable the compiler from optimizing away the if-statement, and simultane-
ously disable the global store operation.
Note that if control ﬂow or addressing is dependent on data in memory the method becomes less straightforward
and requires special care. A further issue with modifying the source code is that register count can change, which
again can increase the occupancy and thereby invalidate the measured run-time. This, however, can be is solved by
increasing the shared memory parameter in the launch conﬁguration of the kernel,
someKernel<<<grid_size, block_size, shared_mem_size, ...>>>(...),
until the occupancy of the unmodiﬁed version is matched. The occupancy can easily be examined using the proﬁler or
the CUDA Occupancy Calculator.
If a kernel appears to be well balanced (i.e., neither memory nor arithmetics appear to be the bottleneck), we must
still check whether or not our kernel operates close to the theoretical performance numbers since it can suffer from
latencies. These latencies are typically caused by problematic data dependencies or the inherent latencies of arithmetic
operations. Thus, if your kernel is well balanced, but operates at only a fraction of the theoretical peak, it is probably
bound by latencies. In this case, a reorganization of memory requests and arithmetic operations is often required. The
goal should be to have many outstanding memory requests that can overlap with arithmetic operations.
4.2. Proﬁling and Optimizing Memory. Let us assume that we have identiﬁed the major bottleneck of the kernel to
be memory transactions. The ﬁrst and least time-consuming thing to try for memory-bound kernels is to experiment
with the settings for caching and non-caching loads and the size of the L1 cache to ﬁnd the best settings. This can have
a large impact in cases of register spilling and for strided or scattered memory access patterns, and it requires no code
changes. Outside these short experiments, however, there are two major factors that we want to examine, and that is
the access pattern and the number of concurrent memory requests.
To determine if the access pattern is the problem, we compare the number of memory instructions with the number
of transferred bytes. For example, for global load we should compare the number of bytes requested (gld_request
multiplied by bytes per request, 128 bytes) to the number of transferred bytes (the sum of l1_global_load_miss
and l1_global_load_hit multiplied by the cache line size, 128 bytes). If the number of instructions per byte is far
larger than one, we have a clear indication that global memory loads have a problematic access pattern. In that case,
we should try reorganizing the the memory access pattern to better ﬁt with the rules in Section 3. For global store, the
counters we should compare are gst_request and global_store_transactions.
If we are memory bound, but the access patterns are good, we might be suffering from having too few outstanding
memory requests: according to Little’s Law [13], we need (memory latency ×bandwidth) bytes in ﬂight to saturate
the bus. To determine if the number of concurrent memory accesses is too low, we can compare the achieved memory
throughput (glob_mem_read_throughput and glob_mem_write_throughput in the proﬁler) against the theo-
retical peak. If the hardware throughput is dramatically lower, the memory bus is not saturated, and we should try
increasing the number of concurrent memory transactions by increasing the occupancy. This can be done through
adjustment of block dimensions or reduction of register count, or we can modify the kernel to process more data ele-
ments per thread. A further optimization path is to move indexing calculations and memory transactions in an attempt
to achieve better overlap of memory transactions and arithmetic operations.
An important note when it comes to optimization of memory on the GPU, is that traditional CPU cache blocking
techniques typically do not work. This is because the GPUs L1 and L2 caches are not aimed at temporal reuse like
CPU caches usually are, which means that attempts at cache blocking can even be counter-productive. The rule of
thumb is therefore that when optimizing for memory throughput on GPUs, do not think of caches at all. However,
fetching data from textures can alleviate pressure on the memory system since these fetches go through a different
cache in smaller transactions. Nevertheless, the L1 cache is superior in performance, and only in rare cases will the
texture cache increase performance [18].
9
4.3. Proﬁling and Optimizing Latencies and Instruction Throughput. If a kernel is bound by instruction through-
put, there may be several underlying causes. Warp serialization (see Section 3.1) may be a major bottleneck, or
we may similarly have that bank conﬂicts cause shared memory serialization. The third option is that we have data
dependencies that inhibit performance.
Instruction serialization means that some threads in a warp “replay” the same instruction as opposed to all threads
issuing the same instruction only once (see Figure 6b). The proﬁler can be used to determine the level of instruc-
tion serialization by comparing the instructions_executed counter to the instructions_issued counter, in
which the difference is due to serialization. Note that even if there is a difference between instructions executed and
instructions issued, this is only a problem if it constitutes a signiﬁcant percentage.
One of the causes for instruction replays is divergent branches, identiﬁed by comparing the divergent_branch
counter to the branch counter. We can also proﬁle it by modifying the source code so that all threads take the same
branch, and compare the run-times. The remedy is to remove as many branches as possible, for example by sorting
the input data or splitting the kernel into two separate kernels (see Section 3.1).
Another cause for replays is bank conﬂicts, which can be the case if the l1_shared_bank_conflict counter is
a signiﬁcant percentage of the sum of the shared_loads and shared_stores counters. Another way of proﬁling
bank conﬂicts is to modify the kernel source code by removing the bank conﬂicts. This is done by changing the
indexing so that all accesses are either broadcasts (all threads access the same element) or conﬂict free (each thread
uses the index threadIdx.y*blockDim.x+threadIdx.x). The shared memory variables also need to be declared
as volatile to prevent the compiler from storing them in registers in the modiﬁed kernel. Padding is one way of
removing these bank conﬂicts (see Section 3.2), and one can also try rearranging the shared memory layout (e.g., by
storing by columns instead of by rows).
If we have ruled out the above causes, our kernel might be suffering from arithmetic operation latencies and data
dependencies. We can ﬁnd out if this is the case by comparing the kernel performance to hardware limits. This is done
by examining the IPC - Instructions/Cycle counter in the proﬁler, which gives the ratio of executed instructions
per clock cycle. For compute capability 2.0, this ﬁgure should be close to 2, whilst for compute capability 2.1 it should
approach 4 instructions per cycle. If the achieved instructions per cycle count is very low, this is a clear indication
that there are data dependencies and arithmetic latencies that affect the performance. In this case, we can try storing
intermediate calculations in separate registers to minimize arithmetic latencies due to register dependencies.
4.4. Further Optimization Parameters. Grid- and block-size are important optimization parameters, and they are
usually not easy to set. Both the grid size and the block size must be chosen according to the size and structure of
the input data, but they must also be tuned to ﬁt the GPU’s architecture in order to yield a high performance. Most
importantly we need enough total threads to keep the GPU fully occupied in order to hide memory and other latencies.
Since each multiprocessor can execute up-to eight blocks simultaneously, choosing too small blocks prevents a high
occupancy. Simultaneously, we do not want too large blocks since this may cause register spilling if the kernel uses a
lot of registers. The number of threads per block should also, if possible, be a multiple of 32, since each multiprocessor
executes full warps. When writing a kernel for the GPU, one also often encounters a situation in which the number
of data elements is not a multiple of the block size. In this case, it is recommended to launch a grid larger than the
number of elements and use an out-of-bounds test to discard the unnecessary computations.
In many cases, it is acceptable to trade accuracy for performance, either because we simply need a rough estimate,
or that modeling or data errors shadow the inevitable ﬂoating point rounding errors. The double-precision to single-
precision ratio for GPUs is 2:1 4, which means that a double precision operation takes twice as long as a single precision
operation (just as for CPUs). This makes it well worth investigating whether or not single-precision is sufﬁciently
accurate for the application. For many applications, double precision is required for calculating results, but the results
themselves can be stored in single precision without loss of accuracy. In these cases, all data transfers will execute
twice as fast, simultaneously as we will only occupy half the space in memory. For other cases, single precision is
sufﬁciently accurate for arithmetics as well, meaning we can also perform the ﬂoating point operations twice as fast.
Remember also that all ﬂoating-point literals without the fsufﬁx are interpreted as 64-bit according to the C standard,
and that when one operand in an expression is 64-bit, all operations must be performed in 64-bit precision. Some math
functions can be compiled directly into faster, albeit less accurate, versions. This is enabled using double underscore
version of the function, for example __sin() instead of sin(). By using the --use_fast_math compiler ﬂag, all
fast hardware math functions that are available will be used. It should also be noted that this compiler ﬂag also treats
4For the GeForce products this ratio is 8:1.
10
denormalized numbers as zero, and faster (but less accurate) approximations are used for divisions, reciprocals, and
square roots.
Even if an application does none of its computing on the CPU, there still must be some CPU code for setting up and
controlling CUDA contexts, launching kernels, and transferring data between the CPU and the GPU. In most cases
it is also desirable to have some computations performed on the CPU. There is almost always some serial code in an
algorithm that cannot be parallelized and therefore will execute faster on the CPU. Another reason is that there is no
point in letting the CPU idle while the GPU does computations: use both processors when possible.
There are considerable overheads connected with data transfers between the CPU and the GPU. To hide a mem-
ory transfer from the CPU to the GPU before a kernel is launched one can use streams, issue the memory transfer
asynchronously, and do work on the CPU while the data is being transferred to the GPU (see Section 3.3). Since data
transfers between the CPU and the GPU goes through the PCI Express bus both ways, these transfers will often be a
bottleneck. By using streams and asynchronous memory transfers, and by trying to keep the data on the GPU as much
as possible, this bottleneck can be reduced to a minimum.
4.5. Auto-tuning of GPU Kernels. The above mentioned performance guidelines are often conﬂicting, one example
being that you want to optimize for occupancy to hide memory latencies simultaneously as you want to increase the
number of per-thread registers for more per-thread storage. The ﬁrst of these criteria requires more threads per block,
the second requires fewer threads per block, and it is not given which conﬁguration will give the best performance.
With the sheer number of conﬂicting optimization parameters, it rapidly becomes difﬁcult to ﬁnd out what to optimize.
Experienced developers are somewhat guided by educated guesses together with a trial and error approach, but ﬁnding
the global optimum is often too difﬁcult to be performed manually.
Auto-tuning strategies have been known for a long time on CPUs, and are used to optimize the performance using
cache blocking and many other techniques. On GPUs, we can similarly create auto-tuning codes that execute a kernel
for each set of optimization parameters, and select the best performing conﬁguration. However, the search space is
large, and brute force techniques are thus not a viable solution. Pruning of this search space is still an open research
question, but several papers on the subject have been published (see for example [12, 6]).
Even if auto-tuning is outside the scope of a project, preparing for it can still be an important part of proﬁler guided
development. The ﬁrst part of auto-tuning is often to use template arguments for different kernel parameters such as
shared memory, block size, etc. This gives you many different varieties of the same kernel so that you can easily
switch between different implementations. A beneﬁt of having many different implementations of the same kernel, is
that you can perform run-time auto-tuning of your code. Consider the following example: For a dam break simulation
on the GPU, you might have one kernel that is optimized for scenarios in which most of the domain is dry. However,
as the dam breaks, water spreads throughout the domain, making this kernel inefﬁcient. You then create a second
kernel that is efﬁcient when most of the domain contains water, and switch between these two kernels at run-time.
One strategy here is to perform a simple check of which kernel is the fastest after given number of iterations, and use
this kernel. Performing this check every hundredth time-step, for example, gives a very small overhead to the total
computational time, and ensures that you are using the most efﬁcient kernel throughout the simulation.
4.6. Reporting Performance. One of the key points made in early GPU papers was that one could obtain high
speedups over the CPU for a variety of different algorithms. The tendency has since been to report ever increasing
speedups, and today papers report that their codes run anything from tens to hundreds and thousands times faster than
CPU “equivalents”. However, when examining the theoretical performance of the architectures, the performance gap
is roughly seven times between state-of-the-art CPUs and GPUs (see Figure 2). Thus, reporting a speedup of hundreds
of times or more holds no scientiﬁc value without further explanations supported by detailed benchmarks and proﬁling
results.
The sad truth about many papers reporting dramatic speedup ﬁgures is that the speedup is misleading, at best.
Often, a GPU code can be compared to an inefﬁcient CPU code, or a state-of-the-art desktop GPU can be compared to
a laptop CPU several years old. Some claims of SSE and other optimizations of the CPU code are often made, giving
the impression that the CPU code is efﬁcient. However, for many implementations, this still might not be the case:
if you optimize using SSE instructions, but the bottleneck is memory latency your optimizations are worthless in a
performance perspective.
Reporting performance is a difﬁcult subject, and the correct way of reporting performance will vary from case to
case. There is also a balance between the simplicity of the benchmark and its ease of interpretation. For example, the
Top500 list [23], which rates the fastest supercomputers in the world, is often criticized for being too simplistic as it
11
gives a single rating (gigaﬂops) from a single benchmark (solving a linear system of equations). For GPUs, we want
to see an end to the escalating and misleading speedup-race, and rather see detailed proﬁling results. This will give a
much better view of how well the algorithm exploits the hardware, both on the CPU and on the GPU. Furthermore,
reporting how well your implementation utilizes the hardware, and what the bottlenecks are, will give insight into how
well it will perform on similar and even future hardware. Another beneﬁt here, is that it becomes transparent what the
bottleneck of the algorithm is, meaning it will become clear what hardware vendors and researchers should focus on
for improving performance.
5. DEBUGG ING
As an ever increasing portion of the C++ standard is supported by CUDA and more advanced debugging tools
emerge, debugging GPU codes becomes more and more like debugging CPU codes. Many CUDA programmers have
encountered the “unspeciﬁed launch failure”, which could be notoriously hard to debug. Such errors were typically
only found by either modiﬁcation and experimenting, or by careful examination of the source code. Today, however,
there are powerful CUDA debugging tools for all the most commonly used operating systems.
CUDA-GDB, available for Linux and Mac, can step through a kernel line by line at the granularity of a warp,
e.g., identifying where an out-of-bounds memory access occurs, in a similar fashion to debugging a CPU program
with GDB. In addition to stepping, CUDA-GDB also supports breakpoints, variable watches, and switching between
blocks and threads. Other useful features include reports on the currently active CUDA threads on the GPU, reports
on current hardware and memory utilization, and in-place substitution of changed code in running CUDA application.
The tool enables debugging on hardware in real-time, and the only requirement for using CUDA-GDB is that the kernel
is compiled with the -g -G ﬂags. These ﬂags make the compiler add debugging information into the executable, and
the executable to spill all variables to memory.
On Microsoft Windows, Parallel NSight is a plug-in for Microsoft Visual Studio which offers conditional break-
points, assembly level debugging, and memory checking directly in the Visual Studio IDE. It furthermore offers an
excellent proﬁling tool, and is freely available to developers. However, debugging requires two distinct GPUs; one for
display, and one for running the actual code to be debugged.
6. TRENDS IN GPU COMPUTING
GPUs have truly been a disruptive technology. Starting out as academic examples, they were shunned in scientiﬁc
and high-performance communities: they were inﬂexible, inaccurate, and required a complete redesign of existing
software. However, with time, there has been a low-end disruption, in which GPUs have slowly conquered a large
portion of the high-performance computing segment, and three of ﬁve of todays fastest supercomputers are powered
mainly by GPUs [23]. GPUs were never designed for high-performance computing in mind, but have nevertheless
evolved into powerful processors that ﬁt this market perfectly in less than ten years. The software and the hardware
has developed in harmony, both due to the hardware vendors seeing new possibilities and due to researchers and
industry identifying points of improvement. It is impossible to predict the future of GPUs, but by examining its
history and current state, we might be able to identify some trends that are worth noting.
The success of GPUs has partly been due to its price: GPUs are ridiculously inexpensive in terms of performance
per dollar. This again comes from the mass production and target market. Essentially, GPUs are inexpensive because
NVIDIA and AMD sell a lot of GPUs to the entertainment market. After researchers started to exploit GPUs, these
hardware vendors have eventually developed functionality that is tailored for general-purpose computing and started
selling GPUs intended for computing alone. Backed by the mass market, this means that the cost for the vendors to
target this emerging market is very low, and the proﬁts high.
There are great similarities between the vector machines of the 1990ies and todays use of GPUs. The interest in
vector machines eventually died out, as the x86 market took over. Will the same happen to GPUs once we conquer
the power wall? In the short term, the answer is no for several reasons: First of all, conquering the power wall is not
even on the horizon, meaning that for the foreseeable future, parallelism will be the key ingredient that will increase
performance. Also, while vector machines were reserved for supercomputers alone, todays GPUs are available in
everything from cell phones to supercomputers. A large number of software companies now use GPUs for computing
in their products due to this mass market adaption, and this is one of the key reasons why we will have GPUs or GPU-
like accelerator cores in the future. Just as x86 is difﬁcult to replace, it is now becoming difﬁcult to replace GPUs, as
the software and hardware investments in this technology are large and increasing.
12
NVIDIA have just released their most recent architecture, called Kepler [19], which has several updates compared
to the Fermi architecture described in this paper. First of all, it has changed the organization of the multiprocessors:
whilst the Fermi architecture has up-to sixteen streaming multiprocessors, each with 32 CUDA cores, the new Kepler
architecture has only four multiprocessors, but each with 192 CUDA cores. This totals to 1536 CUDA cores for one
chip, compared to 512 for the Fermi architecture. A second change is that the clock frequency has been decreased from
roughly 1.5 GHz to just over 1 GHz. These two changes are essentially a continuation of the existing trend of increasing
performance through more cores running at a decreased clock frequency. Combined with a new production process
of 28 nm (compared to 40 nm for Fermi), the effect is that the new architecture has a lower power consumption, yet
roughly doubles the gigaﬂops performance. The bandwidth to the L2 cache has been increased giving a performance
boost for applications that can utilize the L2 cache well, yet the main memory bandwidth is the same. NVIDIA has
also announced that it plans to build high-performance processors with integrated CPU cores and GPU cores based on
the low-power ARM architecture and the future Maxwell GPU architecture.
Competition between CPUs and GPUs is likely to be intense for conquering the high performance and scientiﬁc
community, and heterogeneous CPU-GPU systems are already on the market. The AMD Fusion architecture [1]
incorporates multiple CPU and GPU cores into a single die, and Intel have released their Sandy Bridge architecture
based on the same concept. Intel have also developed other highly parallel architectures, including the Larrabee [21]
based on simple and power efﬁcient x86 cores, the Single-chip Cloud Computer (SCC) [10], and the 80-core Tera-scale
research chip Polaris [24]. These architectures have never been released as commercial products, but have culminated
into the Knights Corner co-processor with up-to 1 teraﬂops performance [9]. This co-processor resides on an adapter
connected through the PCI express bus, just as todays graphics adapters, but exposes a traditional C, C++ and Fortran
programming environment, in which existing legacy code can simply be recompiled for the new architecture.
Today, we see that the processors converge towards incorporating traditional CPU cores in addition to highly par-
allel accelerator cores on the same die, such as the aforementioned AMD Fusion and Intel Sandy Bridge. These
architectures do not target the high-performance segment. However, NVIDIA have plans for the high performance
segment with their work on combining ARM CPU cores and Maxwell GPU cores on the same chip. We thus see it as
likely that we will see a combination of CPU and GPU cores on the same chip, sharing the same memory space, in the
near future. This will have a dramatic effect on the memory bottleneck that exists between these architectures today,
and open for tighter cooperation between fast serial execution and massive parallel execution to tackle Amdahl’s law.
7. SUMMARY
In this article, we have given an overview of hardware and traditional optimization techniques for the GPU. We have
furthermore given a step-by-step guide to proﬁle driven development, in which bottlenecks and possible solutions
are outlined. The focus is on state-of-the-art hardware with accompanying tools, and we have addressed the most
prominent bottlenecks: memory, arithmetics, and latencies.
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