前言
线程的组织形式对程序的性能影响是至关重要的,本篇博文主要以下面一种情况来介绍线程组织形式:
- 2D grid 2D block
一些基本的描述:
gridDim.x-线程网络X维度上线程块的数量
gridDim.y-线程网络Y维度上线程块的数量
blockDim.x-一个线程块X维度上的线程数量
blockDim.y-一个线程块Y维度上的线程数量
blockIdx.x-线程网络X维度上的线程块索引
blockIdx.y-线程网络Y维度上的线程块索引
threadIdx.x-线程块X维度上的线程索引
threadIdx.y-线程块Y维度上的线程索引
线程索引
一般,一个矩阵以线性存储在global memory中的,并以行来实现线性:
在kernel里,线程的唯一索引非常有用,为了确定一个线程的索引,我们以2D为例:
- 线程和block索引
- 矩阵中元素坐标
- 线性global memory 的偏移
首先可以将thread和block索引映射到矩阵坐标:
ix = threadIdx.x + blockIdx.x * blockDim.x
iy = threadIdx.y + blockIdx.y * blockDim.y
之后可以利用上述变量计算线性地址:
idx = iy * nx + ix
上图展示了block和thread索引,矩阵坐标以及线性地址之间的关系,谨记,相邻的thread拥有连续的threadIdx.x,也就是索引为(0,0)(1,0)(2,0)(3,0)...的thread连续,而不是(0,0)(0,1)(0,2)(0,3)...连续,跟我们线代里玩矩阵的时候不一样。
现在可以验证出下面的关系:
thread_id(2,1)block_id(1,0) coordinate(6,1) global index 14 ival 14
下图显示了三者之间的关系:
代码
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int main(int argc, char **argv) {
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printf("%s Starting...\n", argv[0]);
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// set up device
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int dev = 0;
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cudaDeviceProp deviceProp;
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CHECK(cudaGetDeviceProperties(&deviceProp, dev));
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printf("Using Device %d: %s\n", dev, deviceProp.name);
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CHECK(cudaSetDevice(dev));
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// set up date size of matrix
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int nx = 1<<14;
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int ny = 1<<14;
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int nxy = nx*ny;
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int nBytes = nxy * sizeof(float);
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printf("Matrix size: nx %d ny %d\n",nx, ny);
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// malloc host memory
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float *h_A, *h_B, *hostRef, *gpuRef;
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h_A = (float *)malloc(nBytes);
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h_B = (float *)malloc(nBytes);
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hostRef = (float *)malloc(nBytes);
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gpuRef = (float *)malloc(nBytes);
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// initialize data at host side
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double iStart = cpuSecond();
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initialData (h_A, nxy);
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initialData (h_B, nxy);
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double iElaps = cpuSecond() - iStart;
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memset(hostRef, 0, nBytes);
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memset(gpuRef, 0, nBytes);
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// add matrix at host side for result checks
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iStart = cpuSecond();
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sumMatrixOnHost (h_A, h_B, hostRef, nx,ny);
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iElaps = cpuSecond() - iStart;
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// malloc device global memory
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float *d_MatA, *d_MatB, *d_MatC;
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cudaMalloc((void **)&d_MatA, nBytes);
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cudaMalloc((void **)&d_MatB, nBytes);
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cudaMalloc((void **)&d_MatC, nBytes);
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// transfer data from host to device
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cudaMemcpy(d_MatA, h_A, nBytes, cudaMemcpyHostToDevice);
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cudaMemcpy(d_MatB, h_B, nBytes, cudaMemcpyHostToDevice);
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// invoke kernel at host side
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int dimx = 32;
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int dimy = 32;
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dim3 block(dimx, dimy);
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dim3 grid((nx+block.x-1)/block.x, (ny+block.y-1)/block.y);
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iStart = cpuSecond();
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sumMatrixOnGPU2D <<< grid, block >>>(d_MatA, d_MatB, d_MatC, nx, ny);
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cudaDeviceSynchronize();
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iElaps = cpuSecond() - iStart;
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printf("sumMatrixOnGPU2D <<<(%d,%d), (%d,%d)>>> elapsed %f sec\n", grid.x,
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grid.y, block.x, block.y, iElaps);
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// copy kernel result back to host side
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cudaMemcpy(gpuRef, d_MatC, nBytes, cudaMemcpyDeviceToHost);
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// check device results
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checkResult(hostRef, gpuRef, nxy);
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// free device global memory
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cudaFree(d_MatA);
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cudaFree(d_MatB);
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cudaFree(d_MatC);
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// free host memory
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free(h_A);
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free(h_B);
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free(hostRef);
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free(gpuRef);
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// reset device
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cudaDeviceReset();
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return (0);
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}
编译运行:
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$ nvcc -arch=sm_20 sumMatrixOnGPU-2D-grid-2D-block.cu -o matrix2D
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$ ./matrix2D
输出:
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./a.out Starting...
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Using Device 0: Tesla M2070
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Matrix size: nx 16384 ny 16384
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sumMatrixOnGPU2D <<<(512,512), (32,32)>>> elapsed 0.060323 sec
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Arrays match.
接下来,我们更改block配置为32x16,重新编译,输出为:
sumMatrixOnGPU2D <<<(512,1024), (32,16)>>> elapsed 0.038041 sec
可以看到,性能提升了一倍,直观的来看,我们会认为第二个配置比第一个多了一倍的block所以性能提升一倍,实际上也确实是因为block增加了。但是,如果你继续增加block的数量,则性能又会降低:
sumMatrixOnGPU2D <<< (1024,1024), (16,16) >>> elapsed 0.045535 sec
下图展示了不同配置的性能;
关于性能的分析将在之后的博文中总结,现在只是了解下,本文在于掌握线程组织的方法。