CUDA Performance - Always return different values - cuda

Question

This my code:
using namespace std;
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
const int N = 8000;
void fillArray(int *data, int count) {
for (int i = 0; i < count; i++)
data[i] = rand() % 100;
}
__global__ void add(int* a, int *b, int *c) {
int tid = threadIdx.x + blockIdx.x * blockDim.x;
if (tid < N) {
c[tid] = a[tid] + b[tid];
}
}
__global__ void subtract(int* a, int *b, int *c) {
int tid = threadIdx.x + blockIdx.x * blockDim.x;
if (tid < N) {
c[tid] = a[tid] - b[tid];
}
}
__global__ void multiply(int* a, int *b, int *c) {
int tid = threadIdx.x + blockIdx.x * blockDim.x;
if (tid < N) {
c[tid] = a[tid] * b[tid];
}
}
__global__ void divide(int* a, int *b, int *c) {
int tid = threadIdx.x + blockIdx.x * blockDim.x;
if (tid < N) {
c[tid] = a[tid] / b[tid];
}
}
__global__ void modu(int* a, int *b, int *c) {
int tid = threadIdx.x + blockIdx.x * blockDim.x;
if (tid < N) {
c[tid] = a[tid] % b[tid];
}
}
__global__ void neg(int *data, int *c) {
int tid = threadIdx.x + blockIdx.x * blockDim.x;
if (tid < N) {
c[tid] = -data[tid];
}
}
float duration(int *devA, int *devB, int *devC, int blocksPerGrid, int threadsPerBlock) {
cudaEvent_t start, stop;
float elapsedTime;
cudaEventCreate(&start);
cudaEventCreate(&stop);
cudaEventRecord(start, 0);
int hArrayC[N];
add<<<blocksPerGrid, threadsPerBlock>>>(devA, devB,devC);
cudaMemcpy(hArrayC,devC,N*sizeof(int),cudaMemcpyDeviceToHost);
subtract<<<blocksPerGrid, threadsPerBlock>>>(devA, devB,devC);
cudaMemcpy(hArrayC,devC,N*sizeof(int),cudaMemcpyDeviceToHost);
multiply<<<blocksPerGrid, threadsPerBlock>>>(devA, devB,devC);
cudaMemcpy(hArrayC,devC,N*sizeof(int),cudaMemcpyDeviceToHost);
divide<<<blocksPerGrid, threadsPerBlock>>>(devA, devB,devC);
cudaMemcpy(hArrayC,devC,N*sizeof(int),cudaMemcpyDeviceToHost);
modu<<<blocksPerGrid, threadsPerBlock>>>(devA, devB,devC);
cudaMemcpy(hArrayC,devC,N*sizeof(int),cudaMemcpyDeviceToHost);
neg<<<blocksPerGrid, threadsPerBlock>>>(devA,devC);
cudaMemcpy(hArrayC,devC,N*sizeof(int),cudaMemcpyDeviceToHost);
neg<<<blocksPerGrid, threadsPerBlock>>>(devB,devC);
cudaMemcpy(hArrayC,devC,N*sizeof(int),cudaMemcpyDeviceToHost);
cudaEventRecord(stop, 0);
cudaEventSynchronize(stop);
cudaEventElapsedTime(&elapsedTime, start, stop);
cudaEventDestroy(start);
cudaEventDestroy(stop);
return elapsedTime;
}
int main(void) {
int *a, *b;
a = new int[N];
b = new int [N];
float dur = 0;
int *devA, *devB,*devC;
cudaMalloc((void**) &devA, N * sizeof(int));
cudaMalloc((void**) &devB, N * sizeof(int));
cudaMalloc((void**) &devC, N * sizeof(int));
fillArray(a, N);
fillArray(b, N);
cudaMemcpy(devA, a, N * sizeof(int), cudaMemcpyHostToDevice);
cudaMemcpy(devB, b, N * sizeof(int), cudaMemcpyHostToDevice);
dur = duration(devA, devB, devC,N, 1);
cout << "Global memory version:\n";
cout << "Process completed in " << dur;
cout << " for a data set of " << N << " integers.";
cudaFree(devA);
cudaFree(devB);
delete [] a;
delete [] b;
return 0;
}
What i want to know the total miliseconds in duration function. But miliseconds always return in different values. Sometimes it is 10 ms sometimes it is 0.78652 sometimes it is 30 miliseconds.Why? What is wrong with my code?

Answer 1

This may be caused by the loading/unloading of the NVIDIA drivers. Think of it as an initialization step for the GPU.
You can either set your GPU to persistence mode:
nvidia-smi -pm 1
Or you could run a dummy kernel before timing your GPU code to trigger the loading of the drivers:
__global__ void dummy()
{
// This kernel does nothing, this is just a "warm-up"
}
// Before your cudaEventRecord etc.
dummy<<<blocksPerGrid, threadsPerBlock>>>();
Or maybe just use cudaThreadSynchronize() before timing your kernels.