1 \chapterauthor{Raphaël Couturier}{Femto-ST Institute, University of Franche-Comte, France}
4 \chapter{Presentation of the GPU architecture and of the CUDA environment}
7 \section{Introduction}\label{ch1:intro}
8 This chapter introduces the Graphics Processing Unit (GPU) architecture and all
9 the concepts needed to understand how GPUs work and can be used to speed up the
10 execution of some algorithms. First of all this chapter gives a brief history of
11 the development of the graphics cards up to the point when they started being
12 used in order to perform general purpose computations. Then the architecture of
13 a GPU is illustrated. There are many fundamental differences between a GPU and
14 a traditional processor. In order to benefit from the power of a GPU, a CUDA
15 programmer needs to use threads. They have some particularities which enable the
16 CUDA model to be efficient and scalable when some constraints are addressed.
19 \section{Brief history of the video card}
21 Video cards or graphics cards have been introduced in personal computers to
22 produce high quality graphics faster than classical Central Processing Units
23 (CPU) and to free the CPU from this task. In general, display tasks are very
24 repetitive and very specific. Hence, some manufacturers have produced more and
25 more sophisticated video cards, providing 2D accelerations, then 3D accelerations,
26 then some light transforms. Video cards own their own memory to perform their
27 computations. For at least two decades, every personal computer has had a video
28 card which is simple for desktop computers or which provides many accelerations
29 for game and/or graphic-oriented computers. In the latter case, graphics cards
30 may be more expensive than a CPU.
32 Since 2000, video cards have allowed users to apply arithmetic operations
33 simultaneously on a sequence of pixels, later called stream processing. In
34 this case, the information of the pixels (color, location and other information) is
35 combined in order to produce a pixel color that can be displayed on a screen.
36 Simultaneous computations are provided by shaders which calculate rendering
37 effects on graphics hardware with a high degree of flexibility. These shaders
38 handle the stream data with pipelines.
41 Some researchers tried to apply those operations on other data, representing
42 something different from pixels, and consequently this resulted in the first
43 uses of video cards for performing general purpose computations. The programming
44 model was not easy to use at all and was very dependent on the hardware
45 constraints. More precisely it consisted in using either DirectX of OpenGL
46 functions providing an interface to some classical operations for videos
47 operations (memory transfers, texture manipulation, etc.). Floating point
48 operations were most of the time unimaginable. Obviously when something went
49 wrong, programmers had no way (and no tools) to detect it.
53 In order to benefit from the computing power of more recent video cards, CUDA
54 was first proposed in 2007 by NVIDIA. It unifies the programming model for some
55 of their most efficient video cards. CUDA~\cite{ch1:cuda} has quickly been
56 considered by the scientific community as a great advance for general purpose
57 graphics processing unit (GPGPU) computing. Of course other programming models
58 have been proposed. The other well-known alternative is OpenCL which aims at
59 proposing an alternative to CUDA and which is multiplatform and portable. This
60 is a great advantage since it is even possible to execute OpenCL programs on
61 traditional CPUs. The main drawback is that it is less close to the hardware
62 and, consequently, it sometimes provides less efficient programs. Moreover, CUDA
63 benefits from more mature compilation and optimization procedures. Other less
64 known environments have been proposed, but most of them have been discontinued,
65 such as FireStream by ATI, which is not maintained anymore and has been replaced by
66 OpenCL and BrookGPU by Stanford University~\cite{ch1:Buck:2004:BGS}. Another
67 environment based on pragma (insertion of pragma directives inside the code to
68 help the compiler to generate efficient code) is called OpenACC. For a
69 comparison with OpenCL, interested readers may refer to~\cite{ch1:Dongarra}.
73 \section{Architecture of current GPUs}
75 The architecture \index{GPU!architecture of a} of current GPUs is constantly
76 evolving. Nevertheless some trends remain constant throughout this evolution.
77 Processing units composing a GPU are far simpler than a traditional CPU and
78 it is much easier to integrate many computing units inside a GPU card than to do
79 so with many cores inside a CPU. In 2012, the most powerful GPUs contained more than 500
80 cores and the most powerful CPUs had 8
81 cores. Figure~\ref{ch1:fig:comparison_cpu_gpu} shows the number of cores inside
82 a CPU and inside a GPU. In fact, in a current NVIDIA GPU, there are
83 multiprocessors which have 32 cores (for example, on Fermi cards). The core clock
84 of a CPU is generally around 3GHz and the one of a GPU is about 1.5GHz. Although
85 the core clock of GPU cores is slower, the number of cores inside a GPU provides
86 more computational power. This measure is commonly represented by the number of
87 floating point operation per seconds. Nowadays the most powerful GPUs provide more
88 than 1TFlops, i.e., $10^{12}$ floating point operations per second.
89 Nevertheless GPUs are very efficient at executing repetitive work in which
90 only the data change. It is important to keep in mind that multiprocessors
91 inside a GPU have 32 cores. Later we will see that these 32 cores need to do the
92 same work to get maximum performance.
95 \centerline{\includegraphics[]{Chapters/chapter1/figures/nb_cores_CPU_GPU.pdf}}
96 \caption{Comparison of number of cores in a CPU and in a GPU.}
97 %[Comparison of number of cores in a CPU and in a GPU]
98 \label{ch1:fig:comparison_cpu_gpu}
101 On the most powerful GPU cards, called Fermi, multiprocessors are called streaming
102 multiprocessors (SMs). Each SM contains 32 cores and is able to perform 32
103 floating points or integer operations per clock on 32-bit numbers or 16 floating
104 points per clock on 64-bit numbers. SMs have their own registers, execution
105 pipelines and caches. On Fermi architecture, there are 64Kb shared memory plus L1
106 cache and 32,536 32-bit registers per SM. More precisely the programmer can
107 decide what amounts of shared memory and L1 cache SM are to be used. The constraint is
108 that the sum of both amounts should be less than or equal to 64Kb.
110 Threads are used to benefit from the large number of cores of a GPU. These
111 threads are different from traditional threads for a CPU. In
112 Chapter~\ref{chapter2}, some examples of GPU programming will explain the
113 details of the GPU threads. Threads are gathered into blocks of 32
114 threads, called ``warps''. These warps are important when designing an algorithm
118 Another big difference between a CPU and a GPU is the latency of memory. In a CPU,
119 everything is optimized to obtain a low latency architecture. This is possible
120 through the use of cache memories. Moreover, nowadays CPUs carry out many
121 performance optimizations such as speculative execution which roughly speaking
122 consists of executing a small part of the code in advance even if later this work
123 reveals itself to be useless. GPUs do not have low latency
124 memory. In comparison GPUs have small cache memories; nevertheless the
125 architecture of GPUs is optimized for throughput computation and it takes into
126 account the memory latency.
132 Figure~\ref{ch1:fig:latency_throughput} illustrates the main difference of
133 memory latency between a CPU and a GPU. In a CPU, tasks ``ti'' are executed one
134 by one with a short memory latency to get the data to process. After some tasks,
135 there is a context switch that allows the CPU to run concurrent applications
136 and/or multi-threaded applications. Memory latencies are longer in a GPU. The
137 principle to obtain a high throughput is to have many tasks to
138 compute. Later we will see that these tasks are called threads with CUDA. With
139 this principle, as soon as a task is finished the next one is ready to be
140 executed while the wait for data for the previous task is overlapped by the
141 computation of other tasks.
146 \centerline{\includegraphics[scale=0.7]{Chapters/chapter1/figures/low_latency_vs_high_throughput.pdf}}
147 \caption{Comparison of low latency of a CPU and high throughput of a GPU.}
148 \label{ch1:fig:latency_throughput}
151 \section{Kinds of parallelism}
153 Many kinds of parallelism are available according to the type of hardware.
154 Roughly speaking, there are three classes of parallelism: instruction-level
155 parallelism, data parallelism, and task parallelism.
157 Instruction-level parallelism consists in reordering some instructions in order
158 to execute some of them in parallel without changing the result of the code.
159 In modern CPUs, instruction pipelines allow the processor to execute instructions
160 faster. With a pipeline a processor can execute multiple instructions
161 simultaneously because the output of a task is the input of the
164 Data parallelism consists in executing the same program with different data on
165 different computing units. Of course, no dependency should exist among the
166 data. For example, it is easy to parallelize loops without dependency using the
167 data parallelism paradigm. This paradigm is linked with the Single Instructions
168 Multiple Data (SIMD) architecture. This is the kind of parallelism provided by
171 Task parallelism is the common parallelism achieved on clusters and grids and
172 high performance architectures where different tasks are executed by different
175 \section{CUDA multithreading}
177 The data parallelism of CUDA is more precisely based on the Single Instruction
178 Multiple Thread (SIMT) model, because a programmer accesses
179 the cores by the intermediate of threads. In the CUDA model, all cores
180 execute the same set of instructions but with different data. This model has
181 similarities with the vector programming model proposed for vector machines through
182 the 1970s and into the 90s, notably the various Cray platforms. On the CUDA
183 architecture, the performance is led by the use of a huge number of threads
184 (from thousands up to millions). The particularity of the model is that there
185 is no context switching as in CPUs and each thread has its own registers. In
186 practice, threads are executed by SM and gathered into groups of 32
187 threads, called warps. Each SM alternatively executes
188 active warps and warps becoming temporarily inactive due to waiting of data
189 (as shown in Figure~\ref{ch1:fig:latency_throughput}).
193 The key to scalability in the CUDA model is the use of a huge number of threads.
194 In practice, threads are gathered not only in warps but also in thread blocks. A
195 thread block is executed by only one SM and it cannot migrate. The typical size of
196 a thread block is a power of two (for example, 64, 128, 256, or 512).
200 In this case, without changing anything inside a CUDA code, it is possible to
201 run code with a small CUDA device or the best performing Tesla CUDA cards.
202 Blocks are executed in any order depending on the number of SMs available. So
203 the programmer must conceive code having this issue in mind. This
204 independence between thread blocks provides the scalability of CUDA codes.
209 A kernel is a function which contains a block of instructions that are executed
210 by the threads of a GPU. When the problem considered is a two-dimensional or
211 three-dimensional problem, it is possible to group thread blocks into a grid.
212 In practice, the number of thread blocks and the size of thread blocks are given
213 as parameters to each kernel. Figure~\ref{ch1:fig:scalability} illustrates an
214 example of a kernel composed of 8 thread blocks. Then this kernel is executed on
215 a small device containing only 2 SMs. So in this case, blocks are executed 2 by
216 2 in any order. If the kernel is executed on a larger CUDA device containing 4
217 SMs, blocks are executed 4 by 4 simultaneously. The execution times should be
218 approximately twice as fast in the latter case. Of course, that depends on other
219 parameters that will be described later (in this chapter and other chapters).
223 \centerline{\includegraphics[scale=0.65]{Chapters/chapter1/figures/scalability.pdf}}
224 \caption{Scalability of GPU.}
225 \label{ch1:fig:scalability}
228 Thread blocks provide a way to cooperate in the sense that threads of the same
229 block cooperatively load and store blocks of memory they all
230 use. Synchronizations of threads in the same block are possible (but not between
231 threads of different blocks). Threads of the same block can also share results
232 in order to compute a single result. In Chapter~\ref{chapter2}, some examples
236 \section{Memory hierarchy}
238 The memory hierarchy of GPUs\index{memory hierarchy} is different from that of CPUs. In practice, there are registers\index{memory hierarchy!registers}, local
239 memory\index{memory hierarchy!local memory}, shared
240 memory\index{memory hierarchy!shared memory}, cache
241 memory\index{memory hierarchy!cache memory}, and global
242 memory\index{memory hierarchy!global memory}.
245 As previously mentioned each thread can access its own registers. It is
246 important to keep in mind that the number of registers per block is limited. On
247 recent cards, this number is limited to 64Kb per SM. Access to registers is
248 very fast, so it is a good idea to use them whenever possible.
250 Likewise each thread can access local memory which, in practice, is much slower
251 than registers. Local memory is automatically used by the compiler when all the
252 registers are occupied, so the best idea is to optimize the use of registers
253 even if this involves reducing the number of threads per block.
256 \centerline{\includegraphics[scale=0.60]{Chapters/chapter1/figures/memory_hierarchy.pdf}}
257 \caption{Memory hierarchy of a GPU.}
258 \label{ch1:fig:memory_hierarchy}
263 Shared memory allows cooperation between threads of the same block. This kind
264 of memory is fast but it needs to be manipulated manually and its size is
265 limited. It is accessible during the execution of a kernel. So the idea is
266 to fill the shared memory at the start of the kernel with global data that are
267 used very frequently, then threads can access it for their computation. Threads
268 can obviously change the content of this shared memory either with computation
269 or by loading other data and they can store its content in the global memory. So
270 shared memory can be seen as a cache memory, which is manually managed. This
271 obviously requires effort from the programmer.
273 On recent cards, the programmer may decide what amount of cache memory and
274 shared memory is attributed to a kernel. The cache memory is an L1 cache which is
275 directly managed by the GPU. Sometimes, this cache provides very efficient
276 result and sometimes the use of shared memory is a better solution.
281 Figure~\ref{ch1:fig:memory_hierarchy} illustrates the memory hierarchy of a
282 GPU. Threads are represented on the top of the figure. They can have access to their
283 own registers and their local memory. Threads of the same block can access
284 the shared memory of that block. The cache memory is not represented here but it
285 is local to a thread. Then each block can access the global memory of the
290 In this chapter, a brief presentation of the video card, which has later been
291 used to perform computation, has been given. The architecture of a GPU has been
292 illustrated focusing on the particularity of GPUs in terms of parallelism, memory
293 latency, and threads. In order to design an efficient algorithm for GPU, it is
294 essential to keep all these parameters in mind.
297 \putbib[Chapters/chapter1/biblio]
