问题
What I am attempting to do is Multiply Matrix A & Matrix B and then from the product matrix I get the index of the maximum value per column. But unfortunately, only the first 128*128 values of the matrix multiplication are correct while others are just garbage. I do not quite understand how this works. I request you to kindly guide me with this ..
#include<stdio.h>
#include "cuda.h"
#include<stdlib.h>
#define blockD 32
const int wA = 128;
const int hA = 4096;
const int wB = 4096;
const int hB = wA;
main(void){
void MatrixMultiplication(float *, float *, float *, float *);
int size_A = wA * hA * sizeof(float);
int size_B = wB * hB * sizeof(float);
int size_C = wB * hA * sizeof(float);
int size_max = 2 * wB * sizeof(float);
float *M, *N, *P, *C;
// allocate memory on the CPU
M = (float*)malloc(size_A);
N = (float*)malloc(size_B);
P = (float*)malloc(size_max);
C = (float*)malloc(size_C);
// initialize the matrices
for (int y=0; y < hA; y++) {
for (int x=0; x < wA; x++){
M[y*wA + x] = 32; //x + y*wA;
}
}
for (int y=0; y<hB; y++) {
for (int x=0; x<wB; x++){
N[y*wB + x] = 21; //x + y*wB;
}
}
MatrixMultiplication(M, N, P, C);
//Write
FILE *f1;
int i,j;
f1 = fopen("C.txt","w");
for(i = hA - 2 ; i < hA; i ++){
for(j = 0; j < wB; j++){
fprintf(f1,"%d\t",int(C[i*wB + j]));
}
fprintf(f1,"\n");
}
fclose(f1);
// free the memory allocated on the CPU
free( M );
free( N );
free( P );
free( C );
cudaDeviceReset();
return 0;
}
__device__ void MaxFunction(float* Pd, float* max)
{
int x = (threadIdx.x + blockIdx.x * blockDim.x);
int y = (threadIdx.y + blockIdx.y * blockDim.y);
int k = 0;
int temp = 0; int temp_idx = 0;
for (k = 0; k < wB; ++k) {
if(Pd[x*wB + k] > temp){
temp = Pd[x*wB + k];
temp_idx = x*wB + k;
}
}
max[y*2 + 0] = temp;
max[y*2 + 1] = temp_idx;
}
__global__ void MatrixMulKernel(float* Md, float* Nd, float* Pd, float* max)
{
// declare cache in the shared memory
__shared__ float Mds[blockD][blockD];
__shared__ float Nds[blockD][blockD];
float Pvalue = 0;
// Loop over the Md and Nd block dimension required to compute the Pd element
for (int m = (wA * blockD * blockIdx.y), n = (blockD * blockIdx.x);
m < ((wA * blockD * blockIdx.y)+wA-1);
m += blockD, n += (blockD*hB)){
// collaboratively loading of Md and Nd blocks into shared memory
Mds[threadIdx.y][threadIdx.x] = Md[m + wA * threadIdx.y + threadIdx.x];
Nds[threadIdx.y][threadIdx.x] = Nd[n + wA * threadIdx.y + threadIdx.x];
__syncthreads();
// keep track of the running sum
for (int k = 0; k < blockD; k++)
Pvalue += Mds[threadIdx.y][k] * Nds[k][threadIdx.x];
__syncthreads();
}
// write back to the global memory
int p = hB * blockD * blockIdx.y + blockD * blockIdx.x;
Pd[p + hB * threadIdx.y + threadIdx.x] = Pvalue;
__syncthreads();
MaxFunction(Pd, max);
}
void MatrixMultiplication(float *M, float *N, float *P, float *C) {
int size_A = wA * hA * sizeof(float);
int size_B = wB * hB * sizeof(float);
int size_C = wB * hA * sizeof(float);
int size_max = 2 * wB * sizeof(float);
float *Md, *Nd, *Pd, *max;
// allocate memory on the GPU
cudaMalloc((void**)&Md, size_A);
cudaMalloc((void**)&Nd, size_B);
cudaMalloc((void**)&Pd, size_C);
cudaMalloc((void**)&max, size_max);
// transfer M and N to device memory
cudaMemcpy(Md, M, size_A, cudaMemcpyHostToDevice);
cudaMemcpy(Nd, N, size_B, cudaMemcpyHostToDevice);
// kernel invocation code
dim3 dimBlock(blockD, blockD);
dim3 dimGrid(wA/blockD, hB/blockD);
//Execute Kernel
MatrixMulKernel<<<dimGrid, dimBlock>>>( Md, Nd, Pd, max);
// transfer P from device
cudaMemcpy(P, max, size_max, cudaMemcpyDeviceToHost);
cudaMemcpy(C, Pd, size_C, cudaMemcpyDeviceToHost);
// free the memory allocated on the GPU
cudaFree(Md);
cudaFree(Nd);
cudaFree(Pd);
cudaFree(max);
}
回答1:
In your code you seem to have more than one problem. One of the problems is, in place of this:
dim3 dimGrid(wA/blockD, hB/blockD);
You should have this:
dim3 dimGrid(wB/blockD, hA/blockD);
Ultimately you need one thread in your grid for each output point. Your formulation was giving you a grid of 4 blocks by 4 blocks, whereas you need a grid of 128 blocks by 128 blocks.
The other problem I found with your code was in these lines in the kernel:
int p = hB * blockD * blockIdx.y + blockD * blockIdx.x;
Pd[p + hB * threadIdx.y + threadIdx.x] = Pvalue;
They are not indexing properly through the output array. Rather than try to sort it out using your scheme, I used this instead:
Pd[(threadIdx.x + (blockIdx.x * blockDim.x)) + ((threadIdx.y + (blockIdx.y * blockDim.y))*(gridDim.x*blockDim.x))] = Pvalue;
When I made the above two changes to your code, I got what I believe are correct results throughout the array. And it took about 32 seconds on my machine to run it. (Note that I haven't tried fixing your original max-finding code -- see below for a better approach.)
Based on your previous question, you seemed to be concerned about speed. If you want to do fast matrix multiply, you should use cublas. The following code shows how to use cublas to multiply two ordinary C-style matrices (they don't have to be square). I've also included a column-max finding kernel that will be fast when the number of columns is large (say, over 500 or so. You have 4096 columns in your example). For small numbers of columns, there may be quicker ways to perform this function, but small numbers of columns also suggests that the overall problem size may be small and so speed (of this piece of code) will not really be an issue.
Here's the code:
#include <stdio.h>
#include <cublas_v2.h>
#define VERBOSE 1
#define nTPB 64
#define ROW_A 4
#define COL_A 4
#define ROW_B COL_A
#define COL_B 4
#define ROW_C ROW_A
#define COL_C COL_B
#define SIZ_A (ROW_A*COL_A)
#define SIZ_B (ROW_B*COL_B)
#define SIZ_C (ROW_C*COL_C)
// error check macros
#define cudaCheckErrors(msg) \
do { \
cudaError_t __err = cudaGetLastError(); \
if (__err != cudaSuccess) { \
fprintf(stderr, "Fatal error: %s (%s at %s:%d)\n", \
msg, cudaGetErrorString(__err), \
__FILE__, __LINE__); \
fprintf(stderr, "*** FAILED - ABORTING\n"); \
exit(1); \
} \
} while (0)
// for CUBLAS V2 API
#define cublasCheckErrors(fn) \
do { \
cublasStatus_t __err = fn; \
if (__err != CUBLAS_STATUS_SUCCESS) { \
fprintf(stderr, "Fatal cublas error: %d (at %s:%d)\n", \
(int)(__err), \
__FILE__, __LINE__); \
fprintf(stderr, "*** FAILED - ABORTING\n"); \
exit(1); \
} \
} while (0)
__global__ void col_max(float *mat, float *max, unsigned int *midx, unsigned int rows, unsigned int cols){
int idx = threadIdx.x + blockDim.x*blockIdx.x;
if (idx < cols){
float tempmax = mat[idx];
unsigned int tempmidx = 0;
for (int i = 1; i< rows; i++)
if (mat[idx + (i*cols)] > tempmax){
tempmax = mat[idx + (i*cols)];
tempmidx = i;}
max[idx] = tempmax;
midx[idx] = tempmidx;
}
}
int main(){
float *h_A, *h_B, *h_C, *d_A, *d_B, *d_C, *h_max, *d_max;
unsigned int *h_idx, *d_idx;
h_A = (float *)malloc(SIZ_A*sizeof(float));
if (h_A==0) {printf("malloc fail\n"); return -1;}
h_B = (float *)malloc(SIZ_B*sizeof(float));
if (h_B==0) {printf("malloc fail\n"); return -1;}
h_C = (float *)malloc(SIZ_C*sizeof(float));
if (h_C==0) {printf("malloc fail\n"); return -1;}
h_max = (float *)malloc(COL_C*sizeof(float));
if (h_max==0) {printf("malloc fail\n"); return -1;}
h_idx = (unsigned int*)malloc(COL_C*sizeof(unsigned int));
if (h_idx==0) {printf("malloc fail\n"); return -1;}
cudaMalloc((void **)&d_A, SIZ_A*sizeof(float));
cudaMalloc((void **)&d_B, SIZ_B*sizeof(float));
cudaMalloc((void **)&d_C, SIZ_C*sizeof(float));
cudaMalloc((void **)&d_max, COL_C*sizeof(float));
cudaMalloc((void **)&d_idx, COL_C*sizeof(unsigned int));
cudaCheckErrors("cuda malloc fail");
// initialize data
for (int i=0; i< SIZ_A; i++) h_A[i] = (float)(i+1);
for (int i=0; i< SIZ_B; i++) h_B[i] = (float)(i+2);
cudaMemcpy(d_A, h_A, SIZ_A*sizeof(float), cudaMemcpyHostToDevice);
cudaMemcpy(d_B, h_B, SIZ_B*sizeof(float), cudaMemcpyHostToDevice);
cudaCheckErrors("cuda memcpy 1 fail");
const float alpha = 1.0f;
const float beta = 0.0f;
cublasHandle_t handle;
cublasCheckErrors(cublasCreate(&handle));
// C = A*B
// due to cublas expecting column-major storage, parameters
// are scrambled
cublasCheckErrors(cublasSgemm(handle, CUBLAS_OP_N, CUBLAS_OP_N, COL_B, ROW_A, COL_A, &alpha, d_B, COL_B, d_A, COL_A, &beta, d_C, COL_C));
cudaMemcpy(h_C, d_C, SIZ_C*sizeof(float), cudaMemcpyDeviceToHost);
cudaCheckErrors("cuda memcpy 2 fail");
col_max<<<(COL_C + nTPB - 1)/nTPB, nTPB>>>(d_C, d_max, d_idx, ROW_C, COL_C);
cudaCheckErrors("kernel launch fail");
cudaMemcpy(h_max, d_max, COL_C*sizeof(float), cudaMemcpyDeviceToHost);
cudaMemcpy(h_idx, d_idx, COL_C*sizeof(unsigned int), cudaMemcpyDeviceToHost);
cudaCheckErrors("cuda memcpy 3 fail/kernel fail");
if (VERBOSE){
printf("A: \n");
for (int i=0; i< ROW_A; i++){
for (int j=0; j< COL_A; j++)
printf("%7.5G", h_A[j+(i*COL_A)]);
printf("\n");}
printf("B: \n");
for (int i=0; i< ROW_B; i++){
for (int j=0; j< COL_B; j++)
printf("%7.5G", h_B[j+(i*COL_B)]);
printf("\n");}
printf("C = A*B: \n");
for (int i=0; i< ROW_C; i++){
for (int j=0; j< COL_C; j++)
printf("%7.5G", h_C[j+(i*COL_C)]);
printf("\n");}
printf("COLUMN MAX:\n");
for (int i=0; i< COL_C; i++)
printf("%7.5G", h_max[i]);
printf("\nCOLUMN MAX IDX:\n");
for (int i=0; i< COL_C; i++)
printf("%7d", h_idx[i]);
}
printf("\n finished!\n");
return 0;
}
Here's what I used to compile:
$ nvcc -arch=sm_20 -O3 -o t221 t221.cu -lcublas
And here's the sample output:
$ cuda-memcheck ./t221
========= CUDA-MEMCHECK
A:
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
B:
2 3 4 5
6 7 8 9
10 11 12 13
14 15 16 17
C = A*B:
100 110 120 130
228 254 280 306
356 398 440 482
484 542 600 658
COLUMN MAX:
484 542 600 658
COLUMN MAX IDX:
3 3 3 3
finished!
========= ERROR SUMMARY: 0 errors
$
When I extended my code to handle the same sizes you indicated, (A = 4096x128, B=128x4096) it took about 1 second on my machine. So it's much faster than your code. However, when I take your code and comment out your call to MaxFunction in the kernel, it also only takes about 1 second to compute the matrix multiply result. So if you wanted to keep your matrix multiply code (i.e. not use cublas) you could break the code into 2 kernels, and use your multiply routine in the first kernel with my max-finding routine (col_max) in the second kernel, and also probably get a pretty fast result.
As @talonmies indicated, if you are running on a windows machine, be sure you are aware of the ramifications of windows TDR. (search that in the upper right corner search box if needed)
来源：https://stackoverflow.com/questions/18252526/matrix-multiplication-giving-wrong-output