cuda - 分析我的 CUDA 内核的内存访问合并

Question

我想通过 BS_x*BS_Y 线程将内容移动到共享内存来读取 (BS_X+1)*(BS_Y+1) 全局内存位置，并且我开发了以下代码。

int i       = threadIdx.x;
int j       = threadIdx.y;
int idx     = blockIdx.x*BLOCK_SIZE_X + threadIdx.x;
int idy     = blockIdx.y*BLOCK_SIZE_Y + threadIdx.y;

int index1  = j*BLOCK_SIZE_Y+i;

int i1      = (index1)%(BLOCK_SIZE_X+1);
int j1      = (index1)/(BLOCK_SIZE_Y+1);

int i2      = (BLOCK_SIZE_X*BLOCK_SIZE_Y+index1)%(BLOCK_SIZE_X+1);
int j2      = (BLOCK_SIZE_X*BLOCK_SIZE_Y+index1)/(BLOCK_SIZE_Y+1);

__shared__ double Ezx_h_shared_ext[BLOCK_SIZE_X+1][BLOCK_SIZE_Y+1];     

Ezx_h_shared_ext[i1][j1]=Ezx_h[(blockIdx.y*BLOCK_SIZE_Y+j1)*xdim+(blockIdx.x*BLOCK_SIZE_X+i1)];

if ((i2<(BLOCK_SIZE_X+1))&&(j2<(BLOCK_SIZE_Y+1))) 
Ezx_h_shared_ext[i2][j2]=Ezx_h[(blockIdx.y*BLOCK_SIZE_Y+j2)*xdim+(blockIdx.x*BLOCK_SIZE_X+i2)];

在我的理解中，合并是顺序处理的连续内存读取的并行等价物。我现在如何检测全局内存访问是否已合并？我注意到有一个从 (i1,j1) 到 (i2,j2) 的索引跳转。
提前致谢。

Answer 1

我已经使用手写合并分析器评估了您的代码的内存访问。评估显示代码较少利用合并。这是您可能会发现有用的合并分析器:

#include <stdio.h>
#include <malloc.h>

typedef struct dim3_t{
    int x;
    int y;
} dim3;


// KERNEL LAUNCH PARAMETERS
#define GRIDDIMX 4
#define GRIDDIMY 4
#define BLOCKDIMX 16
#define BLOCKDIMY 16


// ARCHITECTURE DEPENDENT
// number of threads aggregated for coalescing
#define COALESCINGWIDTH 32
// number of bytes in one coalesced transaction
#define CACHEBLOCKSIZE 128
#define CACHE_BLOCK_ADDR(addr,size)  (addr*size)&(~(CACHEBLOCKSIZE-1))


int main(){
    // fixed dim3 variables
    // grid and block size
    dim3 blockDim,gridDim;
    blockDim.x=BLOCKDIMX;
    blockDim.y=BLOCKDIMY;
    gridDim.x=GRIDDIMX;
    gridDim.y=GRIDDIMY;

    // counters
    int unq_accesses=0;
    int *unq_addr=(int*)malloc(sizeof(int)*COALESCINGWIDTH);
    int total_unq_accesses=0;

    // iter over total number of threads
    // and count the number of memory requests (the coalesced requests)
    int I, II, III;
    for(I=0; I<GRIDDIMX*GRIDDIMY; I++){
        dim3 blockIdx;
        blockIdx.x = I%GRIDDIMX;
        blockIdx.y = I/GRIDDIMX;
        for(II=0; II<BLOCKDIMX*BLOCKDIMY; II++){
            if(II%COALESCINGWIDTH==0){
                // new coalescing bunch
                total_unq_accesses+=unq_accesses;
                unq_accesses=0;
            }
            dim3 threadIdx;
            threadIdx.x=II%BLOCKDIMX;
            threadIdx.y=II/BLOCKDIMX;

            ////////////////////////////////////////////////////////
            // Change this section to evaluate different accesses //
            ////////////////////////////////////////////////////////
            // do your indexing here
            #define BLOCK_SIZE_X BLOCKDIMX
            #define BLOCK_SIZE_Y BLOCKDIMY
            #define xdim 32
            int i       = threadIdx.x;
            int j       = threadIdx.y;
            int idx     = blockIdx.x*BLOCK_SIZE_X + threadIdx.x;
            int idy     = blockIdx.y*BLOCK_SIZE_Y + threadIdx.y;

            int index1  = j*BLOCK_SIZE_Y+i;

            int i1      = (index1)%(BLOCK_SIZE_X+1);
            int j1      = (index1)/(BLOCK_SIZE_Y+1);

            int i2      = (BLOCK_SIZE_X*BLOCK_SIZE_Y+index1)%(BLOCK_SIZE_X+1);
            int j2      = (BLOCK_SIZE_X*BLOCK_SIZE_Y+index1)/(BLOCK_SIZE_Y+1);
            // calculate the accessed location and offset here
            // change the line "Ezx_h[(blockIdx.y*BLOCK_SIZE_Y+j1)*xdim+(blockIdx.x*BLOCK_SIZE_X+i1)];" to
            int addr = (blockIdx.y*BLOCK_SIZE_Y+j1)*xdim+(blockIdx.x*BLOCK_SIZE_X+i1);
            int size = sizeof(double);
            //////////////////////////
            // End of modifications //
            //////////////////////////

            printf("tid (%d,%d) from blockid (%d,%d) accessing to block %d\n",threadIdx.x,threadIdx.y,blockIdx.x,blockIdx.y,CACHE_BLOCK_ADDR(addr,size));
            // check whether it can be merged with existing requests or not
            short merged=0;
            for(III=0; III<unq_accesses; III++){
                if(CACHE_BLOCK_ADDR(addr,size)==CACHE_BLOCK_ADDR(unq_addr[III],size)){
                    merged=1;
                    break;
                }
            }
            if(!merged){
                // new cache block accessed over this coalescing width
                unq_addr[unq_accesses]=CACHE_BLOCK_ADDR(addr,size);
                unq_accesses++;
            }
        }
    }
    printf("%d threads make %d memory transactions\n",GRIDDIMX*GRIDDIMY*BLOCKDIMX*BLOCKDIMY, total_unq_accesses);
}

该代码将为网格的每个线程运行并计算合并请求的数量、内存访问合并的度量。

要使用分析器，请将代码的索引计算部分粘贴到指定区域，并将内存访问(数组)分解为“地址”和“大小”。我已经为你的代码做了这个，索引是:

int i       = threadIdx.x;
int j       = threadIdx.y;
int idx     = blockIdx.x*BLOCK_SIZE_X + threadIdx.x;
int idy     = blockIdx.y*BLOCK_SIZE_Y + threadIdx.y;

int index1  = j*BLOCK_SIZE_Y+i;

int i1      = (index1)%(BLOCK_SIZE_X+1);
int j1      = (index1)/(BLOCK_SIZE_Y+1);

int i2      = (BLOCK_SIZE_X*BLOCK_SIZE_Y+index1)%(BLOCK_SIZE_X+1);
int j2      = (BLOCK_SIZE_X*BLOCK_SIZE_Y+index1)/(BLOCK_SIZE_Y+1);

并且内存访问是:

Ezx_h_shared_ext[i1][j1]=Ezx_h[(blockIdx.y*BLOCK_SIZE_Y+j1)*xdim+(blockIdx.x*BLOCK_SIZE_X+i1)];

分析器报告 4096 个线程访问 4064 个缓存块。为您的实际网格和块大小运行代码并分析合并行为。