1 \chapterauthor{Raphaël Couturier}{Femto-ST Institute, University of Franche-Comte}
4 \chapter{Presentation of the GPU architecture and of the Cuda environment}
7 \section{Introduction}\label{ch1:intro}
9 This chapter introduces the Graphics Processing Unit (GPU) architecture and all
10 the concepts needed to understand how GPUs work and can be used to speed up the
11 execution of some algorithms. First of all this chapter gives a brief history of
12 the development of Graphics card until they have been used in order to make
13 general purpose computation. Then the architecture of a GPU is
14 illustrated. There are many fundamental differences between a GPU and a
15 tradition processor. In order to benefit from the power of a GPU, a Cuda
16 programmer needs to use threads. They have some particularities which enable the
17 Cuda model to be efficient and scalable when some constraints are addressed.
21 \section{Brief history of Video Card}
23 Video cards or Graphics cards have been introduced in personal computers to
24 produce high quality graphics faster than classical Central Processing Units
25 (CPU) and to alleviate CPU from this task. In general, display tasks are very
26 repetitive and very specific. Hence, some manufacturers have produced more and
27 more sophisticated video cards, providing 2D accelerations then 3D accelerations,
28 then some light transforms. Video cards own their own memory to perform their
29 computation. For at least two decades, every personal computer has had a video
30 card which is simple for desktop computers or which provides many accelerations
31 for game and/or graphic oriented computers. In the latter case, graphic cards
32 may be more expensive than a CPU.
34 Since 2000, video cards have allowed users to apply arithmetic operations
35 simultaneously on a sequence of pixels, also later called stream processing. In
36 this case, the information of the pixels (color, location and other information) are
37 combined in order to produce a pixel color that can be displayed on a screen.
38 Simultaneous computations are provided by shaders which calculate rendering
39 effects on graphics hardware with a high degree of flexibility. These shaders
40 handles the stream data with pipelines.
43 Some researchers tried to apply those operations on other data, representing
44 something different from pixels, and consequently this resulted in the first
45 uses of video cards for performing general purpose computation. The programming
46 model was not easy to use at all and was very dependent of the hardware
47 constraints. More precisely it consisted in using either DirectX of OpenGL
48 functions providing an interface to some classical operations for videos
49 operations (memory transfers, texture manipulation, ...). Floating point
50 operations were most of the time unimaginable. Obviously when something went
51 wrong, programmers had no way (and neither the tools) to detect it.
55 In order to benefit from the computing power of more recent video cards, Cuda
56 was first proposed in 2007 by NVidia. It unifies the programming model for some
57 of their most performant video cards. Cuda~\cite{ch1:cuda} has quickly been
58 considered by the scientific community as a great advance for general purpose
59 graphics processing unit (GPGPU) computing. Of course other programming models
60 have been proposed. The other well-known alternative is OpenCL which aims at
61 proposing an alternative to Cuda and which is multi-platform and portable. This
62 is a great advantage since it is even possible to execute OpenCL programs on
63 traditional CPUs. The main drawback is that it is less tight with the hardware
64 and consequently sometimes provides less efficient programs. Moreover, Cuda
65 benefits from more mature compilation and optimization procedures. Other less
66 known environments have been proposed, but most of them have been stopped, for
67 example we can cite: FireStream by ATI which is not maintained anymore and
68 replaced by OpenCL, BrookGPU by Standford University~\cite{ch1:Buck:2004:BGS}.
69 Another environment based on pragma (insertion of pragma directives inside the
70 code to help the compiler to generate efficient code) is call OpenACC. For a
71 comparison with OpenCL, interested readers may refer to~\cite{ch1:CMR:12}.
75 \section{Architecture of current GPUs}
77 The architecture \index{architecture of a GPU} of current GPUs is constantly
78 evolving. Nevertheless some trends remain constant throughout this evolution.
79 Processing units composing a GPU are far more simple than a traditional CPU but
80 it is much easier to integrate many computing units inside a GPU card than to do
81 so with many cores inside a CPU. This is due to the fact that the cores of a GPU are
82 simpler than the cores of a CPU. In 2012, the most powerful GPUs own more than 500
83 cores and the most powerful CPUs have 8
84 cores. Figure~\ref{ch1:fig:comparison_cpu_gpu} shows the number of cores inside
85 a CPU and inside a GPU. In fact, in a current NVidia GPU, there are
86 multiprocessors which have 32 cores (for example on Fermi cards). The core clock
87 of CPU is generally around 3GHz and the one of GPU is about 1.5GHz. Although
88 the core clock of GPU cores is slower, the amount of cores inside a GPU provides
89 more computational power. This measure is commonly represented by the number of
90 floating point operation per seconds. Nowadays the most powerful GPUs provide more
91 than 1TFlops, i.e. $10^{12}$ floating point operations per second.
92 Nevertheless GPUs are very efficient to perform some operations but not all
93 kinds of operations. They are very efficient to execute repetitive work in which
94 only the data change. It is important to keep in mind that multiprocessors
95 inside a GPU have 32 cores. Later we will see that these 32 cores need to do the
96 same work to get maximum performance.
99 \centerline{\includegraphics[]{Chapters/chapter1/figures/nb_cores_CPU_GPU.pdf}}
100 \caption{Comparison of number of cores in a CPU and in a GPU.}
101 %[Comparison of number of cores in a CPU and in a GPU]
102 \label{ch1:fig:comparison_cpu_gpu}
105 On the most powerful GPU cards, called Fermi, multiprocessors are called streaming
106 multiprocessors (SM). Each SM contains 32 cores and is able to perform 32
107 floating points or integer operations on 32 bits numbers per clock or 16 floating
108 points on 64 bits number per clock. SMs have their own registers, execution
109 pipelines and caches. On Fermi architecture, there are 64Kb shared memory + L1
110 cache and 32,536 32bits registers per SM. More precisely the programmer can
111 decide what amount of shared memory and L1 cache SM can use. The constraint is
112 that the sum of both amounts should be less or equal to 64Kb.
114 Threads are used to benefit from the important number of cores of a GPU. Those
115 threads are different from traditional threads for CPU. In
116 chapter~\ref{chapter2}, some examples of GPU programming will explicit the
117 details of the GPU threads. However, threads are gathered into blocks of 32
118 threads, called ``warps''. Those warps are important when designing an algorithm
122 Another big difference between CPU and GPU is the latency of memory. In CPU,
123 everything is optimized to obtain a low latency architecture. This is possible
124 through the use of cache memories. Moreover, nowadays CPUs perform many
125 performance optimizations such as speculative execution which roughly speaking
126 consists in executing a small part of code in advance even if later this work
127 reveals itself to be useless. On the contrary, GPUs do not have low latency
128 memory. In comparison GPUs have small cache memories. Nevertheless the
129 architecture of GPUs is optimized for throughput computation and it takes into
130 account the memory latency.
135 \centerline{\includegraphics[scale=0.7]{Chapters/chapter1/figures/low_latency_vs_high_throughput.pdf}}
136 \caption{Comparison of low latency of CPU and high throughput of GPU.}
137 \label{ch1:fig:latency_throughput}
140 Figure~\ref{ch1:fig:latency_throughput} illustrates the main difference of
141 memory latency between a CPU and a GPU. In a CPU, tasks ``ti'' are executed one
142 by one with a short memory latency to get the data to process. After some tasks,
143 there is a context switch that allows the CPU to run concurrent applications
144 and/or multi-threaded applications. Memory latencies are longer in a GPU, the
145 the principle to obtain a high throughput is to have many tasks to
146 compute. Later we will see that those tasks are called threads with Cuda. With
147 this principle, as soon as a task is finished the next one is ready to be
148 executed while the wait for data for the previous task is overlapped by
149 computation of other tasks.
153 \section{Kinds of parallelism}
155 Many kinds of parallelism are amiable according to the type of hardware.
156 Roughly speaking, there are three classes of parallelism: instruction-level
157 parallelism, data parallelism and task parallelism.
159 Instruction-level parallelism consists in re-ordering some instructions in order
160 to execute some of them in parallel without changing the result of the code.
161 In modern CPUs, instruction pipelines allow processor to execute instructions
162 faster. With a pipeline a processor can execute multiple instructions
163 simultaneously due to the fact that the output of a task is the input of the
166 Data parallelism consists in executing the same program with different data on
167 different computing units. Of course, no dependency should exist between the
168 data. For example, it is easy to parallelize loops without dependency using the
169 data parallelism paradigm. This paradigm is linked with the Single Instructions
170 Multiple Data (SIMD) architecture. This is the kind of parallelism provided by
173 Task parallelism is the common parallelism achieved out on clusters and grids and
174 high performance architectures where different tasks are executed by different
177 \section{Cuda Multithreading}
179 The data parallelism of Cuda is more precisely based on the Single Instruction
180 Multiple Thread (SIMT) model. This is due to the fact that a programmer accesses
181 to the cores by the intermediate of threads. In the Cuda model, all cores
182 execute the same set of instructions but with different data. This model has
183 similarities with the vector programming model proposed for vector machines through
184 the 1970s into the 90s, notably the various Cray platforms. On the Cuda
185 architecture, the performance is led by the use of a huge number of threads
186 (from thousands up to to millions). The particularity of the model is that there
187 is no context switching as in CPUs and each thread has its own registers. In
188 practice, threads are executed by SM and are gathered into groups of 32
189 threads. Those groups are called ``warps''. Each SM alternatively executes
190 ``active warps'' and warps becoming temporarily inactive due to waiting of data
191 (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 not only gathered 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 number 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 your code with a small Cuda device or the most performing Tesla Cuda cards.
202 Blocks are executed in any order depending on the number of SMs available. So
203 the programmer must conceive its code having this issue in mind. This
204 independence between thread blocks provides the scalability of Cuda codes.
207 \centerline{\includegraphics[scale=0.65]{Chapters/chapter1/figures/scalability.pdf}}
208 \caption{Scalability of GPU.}
209 \label{ch1:fig:scalability}
213 A kernel is a function which contains a block of instructions that are executed
214 by the threads of a GPU. When the problem considered is a two dimensional or three
215 dimensional problem, it is possible to group thread blocks into a grid. In
216 practice, the number of thread blocks and the size of thread blocks is given as
217 parameters to each kernel. Figure~\ref{ch1:fig:scalability} illustrates an
218 example of a kernel composed of 8 thread blocks. Then this kernel is executed on
219 a small device containing only 2 SMs. So in this case, blocks are executed 2
220 by 2 in any order. If the kernel is executed on a larger Cuda device containing
221 4 SMs, blocks are executed 4 by 4 simultaneously. The execution times should be
222 approximately twice faster in the latter case. Of course, that depends on other
223 parameters that will be described later.
225 Thread blocks provide a way to cooperation in the sense that threads of the same
226 block cooperatively load and store blocks of memory they all
227 use. Synchronizations of threads in the same block are possible (but not between
228 threads of different blocks). Threads of the same block can also share results
229 in order to compute a single result. In chapter~\ref{chapter2}, some examples
233 \section{Memory hierarchy}
235 The memory hierarchy of GPUs\index{memory~hierarchy} is different from the CPUs
236 one. In practice, there are registers\index{memory~hierarchy!registers}, local
237 memory\index{memory~hierarchy!local~memory}, shared
238 memory\index{memory~hierarchy!shared~memory}, cache
239 memory\index{memory~hierarchy!cache~memory} and global
240 memory\index{memory~hierarchy!global~memory}.
243 As previously mentioned each thread can access its own registers. It is
244 important to keep in mind that the number of registers per block is limited. On
245 recent cards, this number is limited to 64Kb per SM. Access to registers is
246 very fast, so it is a good idea to use them whenever possible.
248 Likewise each thread can access local memory which, in practice, is much slower
249 than registers. Local memory is automatically used by the compiler when all the
250 registers are occupied. So the best idea is to optimize the use of registers
251 even if this implies to reduce the number of threads per block.
253 \begin{figure}[hbtp!]
254 \centerline{\includegraphics[scale=0.60]{Chapters/chapter1/figures/memory_hierarchy.pdf}}
255 \caption{Memory hierarchy of a GPU.}
256 \label{ch1:fig:memory_hierarchy}
261 Shared memory allows cooperation between threads of the same block. This kind
262 of memory is fast because it requires to be manipulated manually and its size is
263 limited. It is accessible during the execution of a kernel. So the principle is
264 to fill the shared memory at the start of the kernel with global data that are
265 used very frequently, then threads can access it for their computation. They
266 can obviously change the content of this shared memory either with computation
267 or load of other data and they can store its content in the global memory. So
268 shared memory can be seen as a cache memory manageable manually. This requires
269 obviously an effort from the programmer.
271 On recent cards, the programmer may decide what amount of cache memory and
272 shared memory is attributed to a kernel. The cache memory is a L1 cache which is
273 directly managed by the GPU. Sometimes, this cache provides very efficient
274 result and sometimes the use of shared memory is a better solution.
279 Figure~\ref{ch1:fig:memory_hierarchy} illustrates the memory hierarchy of a
280 GPU. Threads are represented on the top of the figure. They can access to their
281 own registers and their local memory. Threads of the same block can access to
282 the shared memory of this block. The cache memory is not represented here but it
283 is local to a thread. Then each block can access to the global memory of the
288 In this chapter, a brief presentation of the video card, which has later been
289 used to perform computation, has been given. The architecture of a GPU has been
290 illustrated focusing on the particularity of GPUs in term of parallelism, memory
291 latency and threads. In order to design an efficient algorithm for GPU, it is
292 essential to have all these parameters in mind.
295 \putbib[Chapters/chapter1/biblio]
