CUDA Device To Device transfer expensive

问题:

I have written some code to try to swap quadrants of a 2D matrix for FFT purposes, that is stored in a flat array.

    int leftover = W-dcW;

    T *temp;
    T *topHalf;
cudaMalloc((void **)&temp, dcW * sizeof(T));

    //swap every row, left and right
    for(int i = 0; i < H; i++)
    {
        cudaMemcpy(temp, &data[i*W], dcW*sizeof(T),cudaMemcpyDeviceToDevice);
        cudaMemcpy(&data[i*W],&data[i*W+dcW], leftover*sizeof(T), cudaMemcpyDeviceToDevice);
        cudaMemcpy(&data[i*W+leftover], temp, dcW*sizeof(T), cudaMemcpyDeviceToDevice); 
    }

cudaMalloc((void **)&topHalf, dcH*W* sizeof(T));
    leftover = H-dcH;
    cudaMemcpy(topHalf, data, dcH*W*sizeof(T), cudaMemcpyDeviceToDevice);
    cudaMemcpy(data, &data[dcH*W], leftover*W*sizeof(T), cudaMemcpyDeviceToDevice);
    cudaMemcpy(&data[leftover*W], topHalf, dcH*W*sizeof(T), cudaMemcpyDeviceToDevice);

Notice that this code takes device pointers, and does DeviceToDevice transfers.

Why does this seem to run so slow? Can this be optimized somehow? I timed this compared to the same operation on host using regular memcpy and it was about 2x slower.

Any ideas?

回答1:

I ended up writing a kernel to do the swaps. This was indeed faster than the Device to Device memcpy operations

回答2:

Perhaps the following solution to perform the 2d fftshift in CUDA would be of interest:

#define IDX2R(i,j,N) (((i)*(N))+(j))

__global__ void fftshift_2D(double2 *data, int N1, int N2)
{
    int i = threadIdx.y + blockDim.y * blockIdx.y;
    int j = threadIdx.x + blockDim.x * blockIdx.x;

    if (i < N1 && j < N2) {
        double a = pow(-1.0, (i+j)&1);

        data[IDX2R(i,j,N2)].x *= a;
        data[IDX2R(i,j,N2)].y *= a;
    }
}

It consists in multiplying the matrix to be transformed by a chessboard of 1s and -1s which is equivalent to the multiplication by exp(-j*(n+m)*pi) and thus to shifts in both directions in the conjugate domain.

You have to call this kernel before and after the application of the CUFFT.

One pro is that memory movements/swapping are avoided.

IMPROVEMENT IN SPEED

Following the suggestion received at the NVIDIA Forum, improved speed can be achieved as by changing the instruction

double a = pow(-1.0,(i+j)&1);

to

double a = 1-2*((i+j)&1);

to avoid the use of the slow routine pow.