这是一个小实验，在于验证GPU上使用零拷贝内存和页锁定内存的性能差别。使用的是点积计算，数据量在100M左右。实验步骤很简单，分别在主机上开辟普通内存，页锁定内存以及进行零拷贝内存的操作，看三者哪个完成的时间比较快，具体的代码在最后，这里是实验结果：
但是，页锁定内存相比于零拷贝内存到底慢在哪里呢，当然是慢在从主机内存拷贝到显存的时间了，注释掉页锁定的拷贝语句之后，可以得到以下结果：
可以看出来，由于没有拷贝内存，得出的结果是错误的，但是时间确实别零拷贝内存少了。 ps:但是，奇怪的是，如果只将a,b内存拷贝的语句注释掉，页锁定内存仍旧可以得到正确的结果，暂时想不明白是为什么 这时就要问了，看起来零拷贝比页锁定要快啊，那还要这个页锁定干嘛呢，当然是有用的，因为，零拷贝的内容不能缓存在显存里，如果内容要被反复使用，零拷贝就要不停地从内存里取值，增加总线压力，这样相比页锁定也就处于劣势。至于多大的数据量和使用率才能使零拷贝的效率低于页锁定呢，这等以后再做实验来验证吧～
附代码：
代码语言：javascript复制#include<stdio.h>
#define imin(a,b) (a<b?a:b)
const int N = 100 * 1024 * 1024;
const int threadsPerBlock = 256;
const int blocksPerGrid = imin(32, (N threadsPerBlock - 1) / threadsPerBlock);
__global__ void dot(int size, float *a, float *b, float *c) {
__shared__ float cache[threadsPerBlock];
int tid = threadIdx.x blockIdx.x * blockDim.x;
int cacheIndex = threadIdx.x;
float temp = 0;
while (tid < size) {
temp = a[tid] * b[tid];
tid = blockDim.x * gridDim.x;
}
// set the cache values
cache[cacheIndex] = temp;
// synchronize threads in this block
__syncthreads();
// for reductions, threadsPerBlock must be a power of 2
// because of the following code
int i = blockDim.x / 2;
while (i != 0) {
if (cacheIndex < i)
cache[cacheIndex] = cache[cacheIndex i];
__syncthreads();
i /= 2;
}
if (cacheIndex == 0)
c[blockIdx.x] = cache[0];
}
float malloc_test(int size) {
cudaEvent_t start, stop;
float *a, *b, c, *partial_c;
float *dev_a, *dev_b, *dev_partial_c;
float elapsedTime;
cudaEventCreate(&start);
cudaEventCreate(&stop);
// allocate memory on the CPU side
a = (float*) malloc(size * sizeof(float));
b = (float*) malloc(size * sizeof(float));
partial_c = (float*) malloc(blocksPerGrid * sizeof(float));
// allocate the memory on the GPU
cudaMalloc((void**) &dev_a, size * sizeof(float));
cudaMalloc((void**) &dev_b, size * sizeof(float));
cudaMalloc((void**) &dev_partial_c, blocksPerGrid * sizeof(float));
// fill in the host memory with data
for (int i = 0; i < size; i ) {
a[i] = i;
b[i] = i * 2;
}
cudaEventRecord(start, 0);
// copy the arrays 'a' and 'b' to the GPU
cudaMemcpy(dev_a, a, size * sizeof(float), cudaMemcpyHostToDevice);
cudaMemcpy(dev_b, b, size * sizeof(float), cudaMemcpyHostToDevice);
dot<<<blocksPerGrid, threadsPerBlock>>>(size, dev_a, dev_b, dev_partial_c);
// copy the array 'c' back from the GPU to the CPU
cudaMemcpy(partial_c, dev_partial_c, blocksPerGrid * sizeof(float),
cudaMemcpyDeviceToHost);
cudaEventRecord(stop, 0);
cudaEventSynchronize(stop);
cudaEventElapsedTime(&elapsedTime, start, stop);
// finish up on the CPU side
c = 0;
for (int i = 0; i < blocksPerGrid; i ) {
c = partial_c[i];
}
cudaFree(dev_a);
cudaFree(dev_b);
cudaFree(dev_partial_c);
// free memory on the CPU side
free(a);
free(b);
free(partial_c);
// free events
cudaEventDestroy(start);
cudaEventDestroy(stop);
printf("计算结果: %fn", c);
return elapsedTime;
}
float cuda_pinned_alloc_test(int size) {
cudaEvent_t start, stop;
float *a, *b, c, *partial_c;
float *dev_a, *dev_b, *dev_partial_c;
float elapsedTime;
cudaEventCreate(&start);
cudaEventCreate(&stop);
// allocate the memory on the CPU
cudaHostAlloc((void**) &a, size * sizeof(float),
cudaHostAllocWriteCombined | cudaHostAllocMapped);
cudaHostAlloc((void**) &b, size * sizeof(float),
cudaHostAllocWriteCombined | cudaHostAllocMapped);
cudaHostAlloc((void**) &partial_c, blocksPerGrid * sizeof(float),
cudaHostAllocMapped);
// find out the GPU pointers
cudaHostGetDevicePointer(&dev_a, a, 0);
cudaHostGetDevicePointer(&dev_b, b, 0);
cudaHostGetDevicePointer(&dev_partial_c, partial_c, 0);
// fill in the host memory with data
for (int i = 0; i < size; i ) {
a[i] = i;
b[i] = i * 2;
}
cudaEventRecord(start, 0);
dot<<<blocksPerGrid, threadsPerBlock>>>(size, dev_a, dev_b, dev_partial_c);
cudaThreadSynchronize();
cudaEventRecord(stop, 0);
cudaEventSynchronize(stop);
cudaEventElapsedTime(&elapsedTime, start, stop);
// finish up on the CPU side
c = 0;
for (int i = 0; i < blocksPerGrid; i ) {
c = partial_c[i];
}
cudaFreeHost(a);
cudaFreeHost(b);
cudaFreeHost(partial_c);
// free events
cudaEventDestroy(start);
cudaEventDestroy(stop);
printf("计算结果: %fn", c);
return elapsedTime;
}
float cuda_host_alloc_test(int size) {
cudaEvent_t start, stop;
float *aa, *bb, c, *partial_cc;
float *dev_aa, *dev_bb, *dev_partial_cc;
float elapsedTime;
cudaEventCreate(&start);
cudaEventCreate(&stop);
// allocate memory on the CPU side
cudaHostAlloc((void**) &aa, size * sizeof(*aa), cudaHostAllocDefault);
cudaHostAlloc((void**) &bb, size * sizeof(*bb), cudaHostAllocDefault);
cudaHostAlloc((void**) &partial_cc, size * sizeof(*partial_cc), cudaHostAllocDefault);
// allocate the memory on the GPU
cudaMalloc((void**) &dev_aa, size * sizeof(float));
cudaMalloc((void**) &dev_bb, size * sizeof(float));
cudaMalloc((void**) &dev_partial_cc, blocksPerGrid * sizeof(float));
// fill in the host memory with data
for (int i = 0; i < size; i ) {
aa[i] = i;
bb[i] = i * 2;
}
cudaEventRecord(start, 0);
// copy the arrays 'a' and 'b' to the GPU
cudaMemcpy(dev_aa, aa, size * sizeof(float), cudaMemcpyHostToDevice);
cudaMemcpy(dev_bb, bb, size * sizeof(float), cudaMemcpyHostToDevice);
dot<<<blocksPerGrid, threadsPerBlock>>>(size, dev_aa, dev_bb, dev_partial_cc);
// copy the array 'c' back from the GPU to the CPU
/* cudaMemcpy(partial_cc, dev_partial_cc, blocksPerGrid * sizeof(float),
cudaMemcpyDeviceToHost);*/
cudaEventRecord(stop, 0);
cudaEventSynchronize(stop);
cudaEventElapsedTime(&elapsedTime, start, stop);
// finish up on the CPU side
c = 0;
for (int i = 0; i < blocksPerGrid; i ) {
c = partial_cc[i];
}
cudaFree(dev_aa);
cudaFree(dev_bb);
cudaFree(dev_partial_cc);
// free memory on the CPU side
cudaFreeHost(aa);
cudaFreeHost(bb);
cudaFreeHost(partial_cc);
// free events
cudaEventDestroy(start);
cudaEventDestroy(stop);
printf("计算结果: %fn", c);
return elapsedTime;
}
int main(void) {
cudaDeviceProp prop;
int whichDevice;
cudaGetDevice(&whichDevice);
cudaGetDeviceProperties(&prop, whichDevice);
if (prop.canMapHostMemory != 1) {
printf("Device can not map memory.n");
return 0;
}
float elapsedTime;
cudaSetDeviceFlags (cudaDeviceMapHost);
// try it with malloc
elapsedTime = malloc_test(N);
printf("普通主机内存: %3.1f msn", elapsedTime);
// now try it with cudaHostAlloc
elapsedTime = cuda_pinned_alloc_test(N);
printf("零拷贝内存: %3.1f msn", elapsedTime);
// now try it with cudaHostAlloc
elapsedTime = cuda_host_alloc_test(N);
printf("页锁定内存: %3.1f msn", elapsedTime);
}
