Multiply matrix by scalar

Question

I'm a newbie with cuda and cublas.

I want to multiply each element in a matrix (I used cublasSetMatrix) with a scalar value.

Can I use cublasscal() for that? the documentation says it's for a vector.

Thanks.

Answer 1

Yes, you can use it for a matrix scaling operation as well, assuming your matrix is stored contiguously. That means you did an ordinary cudaMalloc with a flat pointer to store the matrix. In that case even though it's a "matrix" it's stored contiguously in memory, and so the storage looks the same as a vector. If you have a MxN matrix, then pass MxN as the number of elements in the vector.

For example, something like (omitting error checking for clarity/brevity):

float *mymatrix, *d_mymatrix;
int size = M*N*sizeof(float);
mymatrix = (float *)malloc(size);
cudaMalloc((void **)&d_mymatrix, size);
... (cublas/handle setup)
cublasSetVector(M*N, sizeof(float), mymatrix, 1, d_mymatrix, 1);
float alpha = 5.0;
cublasSscal(handle, M*N, &alpha, d_mymatrix, 1);