使用二维NDRange workgroup

作为初学者一直，经过多次的上网搜索你一定会看到迈克老狼的向量加法的示例，不知道你是否和我一样，刚开始并不是很准确的知道他的add.cl写的代码的意思，源码如下：

#pragma OPENCL EXTENSION cl_amd_printf : enable

__kernel void vecadd(__global const float* a, __global const float* b, __global float* c)

{

int x = get_global_id();

int y = get_global_id();

int width = get_global_size();

int height = get_global_size();

if(x ==  && y ==)

printf("%d, %d,%d,%d,%d,%d\n",get_local_size(),get_local_size(),get_local_id(),

get_local_id(),get_group_id(),get_group_id());

c[x + y * width] = a[x + y * width] + b[x + y * width];

}

我们在k

这里面我把二维的这些值全部罗列出来如下：

int idx = get_global_id(0);
int idy = get_global_id(1);

uint wiWidth = get_global_size(0);
uint wiHeight = get_global_size(1);

uint gix_t = get_group_id(0);
uint giy_t = get_group_id(1);

uint num_of_blocks_x = get_num_groups(0);
uint num_of_blocks_y = get_num_groups(1);

uint lix = get_local_id(0);
uint liy = get_local_id(1);

你会奇怪为什么他的add.cl优化会这么写：

c[x + y * width] = a[x + y * width] + b[x + y * width];

下面进行分析：

add.cpp关键的代码部分如下（只罗列出部分代码）：

#define width 8//256

#define height 16//256

#define NWITEMS  width*height//262144

size_t globalThreads[] = {width, height};

	size_t localx, localy;

	localx = 2;

	localy = 4;

	size_t localThreads[] = {localx, localy};

	clEnqueueNDRangeKernel( queue,

	kernel,

	2,

	NULL,

	globalThreads,

	localThreads, 0, NULL, &ev);

我自己的add.cl代码如下（里面添加啦打印，以便我们可以看出结果）：

 　 int idx = get_global_id();

    int idy = get_global_id();

    uint wiWidth  = get_global_size();

    uint wiHeight  = get_global_size();

    printf("Magnum Global idx = %d, idy = %d, sizeX =%d,sizeY =%d\n",idx,idy,wiWidth,wiHeight);

    uint gix_t = get_group_id();

    uint giy_t = get_group_id();

    uint num_of_blocks_x = get_num_groups();

    uint num_of_blocks_y = get_num_groups(); 

    printf("Magnum Group idx = %d, idy = %d, blockX=%d,blockY=%d\n",gix_t,gix_t,num_of_blocks_x,num_of_blocks_y);

    uint lix = get_local_id();

    uint liy = get_local_id();

    uint LocalX = get_local_size();

    uint LocalY = get_local_size();

    printf("Magnum Local idx = %d, idy = %d, localX=%d,localY=%d\n\n",lix,liy,LocalX,LocalY);

输出的结果如下：

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

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Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

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Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

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Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

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Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

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Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

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Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

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Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

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Magnum Group idx = , idy = , blockX=,blockY=

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Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

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Magnum Group idx = , idy = , blockX=,blockY=

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Magnum Group idx = , idy = , blockX=,blockY=

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Magnum Group idx = , idy = , blockX=,blockY=

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Magnum Group idx = , idy = , blockX=,blockY=

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Magnum Group idx = , idy = , blockX=,blockY=

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Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

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Magnum Group idx = , idy = , blockX=,blockY=

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Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

Magnum Global idx = , idy = , sizeX =,sizeY =

Magnum Group idx = , idy = , blockX=,blockY=

Magnum Local idx = , idy = , localX=,localY=

从从这个结果首先能说明几个问题：

首先我们在：

clEnqueueNDRangeKernel( queue, kernel, , NULL, globalThreads, localThreads, 0, NULL, &ev); 设置维度是2，否则add.cl中所有类似get_local_size(1)将只会返回一。
设置GlobaThreads 和 LocalThreads 不一定是 GlobalThreads= {8, 8}; 也可以不相等，LocalThreads也是一样。
理解opencl中的内存分布可以理解为二维数组，而且内存寻址的方向就像是：左上角（0，0）为原点，一行一行的扫描下去。
WorkGroup的大小，目前我认为X，Y可以分开来理解（后面若发现错误我会及时改正） WorkGroup number(x) = GlobalThread(x)/LocalThread(x).
最小的单位是LocalX * LocalY, 然后用总的GlobalThreads按照这个单位进行分割成一个一个的组。每个二维的小单位还是一行一行的扫描过去的。目前我们的这个是8*16，你会发现总的Thread也是8*16，对应的每一个线程执行一次加法操作，有人会问可不可以不用这么多呢,用4*16个Threads,每个线程里面做两次加法动作，其实是可以的，这个原则是根据你的具体的Device来说的，当前我们说的是Device是GPU，比如你的GPU当前支持最大的线程数是1024*1024，这种情况，你不用多的，而是用少的线程来做，会降低你的效率，你需要尽可能的让Device满负荷工作这样才能达到提高运算速度。

继续分析为什么 NDRange改成二维的之后,add.cl变成：

c[x + y * width] = a[x + y * width] + b[x + y * width];

因为在CPU端每个 C[8*16],通过打印可以在这里C[x+y*width]，x 最大是7，width是8,y最大是17，只有这样才能做完所有的向量相加。

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