本文为CUDA系列学习第四讲，首先介绍了Parallel communication patterns的几种形式（map, gather, scatter, stencil, transpose）, 然后复习了cuda memory model并从high level上分析怎样写出高效代码，最后学习了流程控制（control flow）以及其中一个重要部分——原子操作。参考资料：udacity cs344.
(一). Parallel communication Patterns
在上一章CUDA系列学习（二）CUDA memory & variables中我们介绍了memory和variable的不同类型，本章中根据不同的memory映射方式，我们将task分为以下几种类型：Map, Gather, Scatter, Stencil, transpose.
1.1 Map, Gather, Scatter
- Map: one input - one output
- Gather: several input - one output
e.g image blur by average - Scatter: one input - several output
e.g add a value to its neighbors
(因为每个thread 将结果scatter到各个memory，所以叫scatter)
图为Map, Gather & Scatter示意图:
1.2 Stencil, Transpose
stencil: 对input中的每一个位置，
stencil input：该点的neighborhood
stencil output：该点value
e.g image blur by average
这样也可以看出，stencil和gather很像，其实stencil是gather的一种，只不过stencil要求input必须是neighborhood而且对input的每一个元素都要操作
图示：
- 2D stencil: (示例为两种形式)
- 3D stencil:
transpose
input：matrix M
output: M^T
图示：
Matrix transpose
Transpose represents in vector
Exercise
Q:
看这个quiz图，给每个蓝线画着的句子标注map/gather/scatter/stencil/transpose:
A:四个位置分别选AECB。
这里我最后一个选错成B&D, 为什么不选D呢？看stencil的定义：如果是average，也应该对每一个位置都要进行average，而题目中有if(i%2)这个condition。
那么对于不同的Parallel communication Patterns需要关注哪些点呢？
1. threads怎样高效访问memory？- 怎样重用数据？
2. threads怎样相互交互部分结果？（通过sharing memory）这样安全吗？
我们将在下一节中首先回顾讲过的memory model，然后结合具体问题分析阐述how to program。
(二). Programming model and Memroy model
第一讲和第三讲中我们讲过SM与grid, block, thead的关系：各个grid, block的thread组织（gridDim，blockDim，grid shape, block shape）可以不同，分别用于执行不同kernel。
如我们第一章所讲，不同GPU有不同数量的硬件SM（streaming Multiprocessors），GPU负责将这些block分配到SMs，所有SM独立，并行地跑。
2.1 Memory model
第二讲中我们讲了memroy的几种形式，这里我们先来回顾一下memory model.
每个thread都可以访问：
1. 该thread独占的local memory
2. block内threads共享的shared memory
3. GPU中所有threads（包括不同SM的所有threads）共享的global memory
下面复习一下，做两个quiz。
Quiz -1 :
Ans：选择A,B,D
解读：根据定义，一个block只能run在一个SM；SM中不同blocks的threads不能cooperate
Quiz - 2 :
Ans: 都不选~~~
解读：block执行时间及顺序不可控；block分配到哪个SM是GPU做的事情，并非programmer能指定的；
2.2 Memory in Program
How to write Efficient Programs from high level
maximize arithmetic intensity
arithmetic intensity = calculation/memory
即要maximize calculation per thread 并 minimize memory per thread(其实目的是minimize memory access的时间)
方法：经常访问的数据放在可快速访问的memory（GPU中不同memory在硬件层的介绍参考第二章），对于刚才讲的local, shared and global memory的访问速度, 有
local > shared >> global >> CPU memory
所以，比如我想经常访问一个global memory，那可以在kernel中先将该global memory variable赋值给一个shared memory variable, 然后频繁访问那个shared memory variable.
minimize memory access stride
如coalesce memory access图所示:
如果GPU的threads访问相邻memory，我们称为coalesced，如果threads间访问memory有固定步长(蹦着走)，我们称stripped，完全没规律的memory访问称为random。访问速度，有
coalesced > strided > random
avoid thread divergence
这个我们在前两讲中有过相应说明。
Exercise:
给下面这段代码中5,6,7,8行的几句话执行速度排序（1最快，4最慢）：
<code class="hljs r has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> __global__ void f(float* x, float* y, float* z){<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> float s,t,u;<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span> __shared__ float a,b,c;<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span><span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span> s = *x;<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span> t = s;<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span> a = b;<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span> *y = *z;<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span> }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>Ans: 5,6,7,8行执行速度为：3,1,2,4。
下面一节我们来看具体programming问题中的流程控制与同步。
(三). Control flow and synchronisation
3.1 program 运行顺序
在讲流程控制之前我们首先看一个例子，用来测试不同block的运行顺序。
Demo code:
<code class="hljs vala has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include <stdio.h></span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#define Num_block 16</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#define Num_thread 1</span>__global__ <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> print(){ printf(“Num: %d\n”,blockIdx.x);}<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main(){ <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//launch the kernel</span> print<<<Num_block, Num_thread>>>(); cudaDeviceSynchronize();<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// what is the function of this sentence? - force the printf()s to flush， 不然运行时显示不出来</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul>编译命令：
nvcc -arch=sm_21 -I ~/NVIDIA_GPU_Computing_SDK/C/common/inc print.cu
运行两次结果：
可见程序执行每一次的结果都不同，也就是不同block之间的执行顺序是不可控的，正如刚才quiz的ans。那么如果我们希望同步各个threads呢？
3.2 同步机制
第二章中我们在一个例子中引入并使用了同步函数syncthreads(), 即设置一个barrier，使所有threads运行到同步函数的时候stop and wait, 直到所有threads运行到此处，那么问题来了。
Exercise:
考虑一个程序，将每个位置i的元素移到i-1的位置，需要多少个syncthreads()？
e.g kernel中声明如下：
<code class="hljs ocaml has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">…<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">int</span> idx = threadIdx.x;__shared__ <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">int</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span>[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">128</span>];<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span>[idx] = idx;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (idx<<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">127</span>){ <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span>[idx + <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>] = <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span>[idx];}…</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>Ans: 3个~
<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">…<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> idx = threadIdx.x;__shared__ <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span>[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">128</span>];<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span>[idx] = idx;__syncthreads(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//如果不加将导致array还没赋值就被操作</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (idx<<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">127</span>){ <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> tmp = <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span>[idx]; __syncthreads();<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//如不加导致先读后写，数据相关</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span>[idx] = tmp; __syncthreads(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//如不加不能确保下面的程序访问到正确数据</span>}…</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul>
Quiz: 看下面这个程序会不会出现collision，哪里会出现collision?
<code class="hljs perl has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1__</span>global_<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span> void f(){<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> __shared_<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">_</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">s</span>[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1024</span>];<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = threadIdx.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">x</span>;<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span> __syncthreads();<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">s</span>[i] = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">s</span>[i-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>];<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span> __syncthreads();<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span>(i<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%2</span>) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">s</span>[i] = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">s</span>[i-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>];<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span> __syncthreads();<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">s</span>[i] = (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">s</span>[i-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>]+<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">s</span>[i+<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>])/<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>;<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">printf</span>(“<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%d</span>\n”,<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">s</span>[i]);<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">11</span> }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul>Ans: Collision在
1. 第5行，如上题，应为int tmp = s[i-1]; __syncthread(); s[i] = tmp;
2. 第9行，同理
PS： 第7行是没问题的，模拟一下就知道
3.3 Atomic Memory Operation
这一节中我们将要接触到原子操作。
首先考虑一个问题：用1000000个threads给一个长为10个元素的array做加法，希望每个thread加100000，这个代码大家先写写看，很简单，依照我们之前的方法有下面的code:
注：这里的gputimer.h请去我的资源页面自行下载。
<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include <stdio.h></span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#include "gputimer.h"</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">using</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">namespace</span> Gadgetron;<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#define NUM_THREADS 1000000</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#define ARRAY_SIZE 10</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#define BLOCK_WIDTH 1000</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> print_array(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> *<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span>, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> size){ <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">printf</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"{ "</span>); <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i<size; i++) { <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">printf</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"%d "</span>, <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">array</span>[i]); } <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">printf</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"}\n"</span>);}__global__ <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> increment_naive(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> *g){ <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// which thread is this?</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = blockIdx.x * blockDim.x + threadIdx.x; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// each thread to increment consecutive elements, wrapping at ARRAY_SIZE</span> i = i % ARRAY_SIZE; g[i] = g[i] + <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;}__global__ <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> increment_atomic(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> *g){ <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// which thread is this?</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = blockIdx.x * blockDim.x + threadIdx.x; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// each thread to increment consecutive elements, wrapping at ARRAY_SIZE</span> i = i % ARRAY_SIZE; atomicAdd(&g[i], <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>);}<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> main(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> argc,<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> **argv){ GPUTimer timer; <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">printf</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"%d total threads in %d blocks writing into %d array elements\n"</span>, NUM_THREADS, NUM_THREADS / BLOCK_WIDTH, ARRAY_SIZE); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// declare and allocate host memory</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> h_array[ARRAY_SIZE]; <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> ARRAY_BYTES = ARRAY_SIZE * <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">sizeof</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// declare, allocate, and zero out GPU memory</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> * d_array; cudaMalloc((<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> **) &d_array, ARRAY_BYTES); cudaMemset((<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> *) d_array, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, ARRAY_BYTES); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// launch the kernel - comment out one of these</span> timer.start(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//increment_atomic<<<NUM_THREADS/BLOCK_WIDTH, BLOCK_WIDTH>>>(d_array);</span> increment_naive<<<NUM_THREADS/BLOCK_WIDTH, BLOCK_WIDTH>>>(d_array); timer.stop(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// copy back the array of sums from GPU and print</span> cudaMemcpy(h_array, d_array, ARRAY_BYTES, cudaMemcpyDeviceToHost); print_array(h_array, ARRAY_SIZE); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// free GPU memory allocation and exit</span> cudaFree(d_array); <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li><li style="box-sizing: border-box; padding: 0px 5px;">58</li></ul>执行两次的结果：
可见结果里每个元素都是648/647，不符合预期100000。这是为什么呢？
看我们的kernel部分代码，每次执行g[i] = g[i] + 1, 一个read-modify-write操作，这样会导致许多线程读到g[i]的value，然后慢的线程将快的线程写结果覆盖掉了。如何解决呢？我们引入原子操作（atomic operation）, 更改上面的kernel部分为：
<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">__global__ <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> increment_atomic(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> *g){ <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// which thread is this?</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = blockIdx.x * blockDim.x + threadIdx.x; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// each thread to increment consecutive elements, wrapping at ARRAY_SIZE</span> i = i % ARRAY_SIZE; atomicAdd(&g[i], <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>);}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>我们可以得到结果：
可见，结果正确。那么原子操作atomicAdd用了怎样的机制呢？——原子操作用了GPU built-in的特殊硬件，用以保证原子操作（同一时刻只能有一个thread做read-modify-write操作）
这里来看一下原子操作的limitations:
1. only certain operations, data type（功能有限）
2. still no ordering constraints（还是无序执行）
3. serializes access to memory（所以慢）
(四). 总结
本节课介绍了以下内容：
OK~ 第三课就结束了，过两天我把exercise上上来~ 敬请关注~.~
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