BLAS and CUBLAS

Question

I'm wondering about NVIDIA's cuBLAS Library. Does anybody have experience with it? For example if I write a C program using BLAS will I be able to replace the calls to BLAS with calls to cuBLAS? Or even better implement a mechanism which let's the user choose at runtime?

What about if I use the BLAS Library provided by Boost with C++?

Answer 1

The answer by janneb is incorrect, cuBLAS is not a drop-in replacement for a CPU BLAS. It assumes data is already on the device, and the function signatures have an extra parameter to keep track of a cuBLAS context.

However, coming in CUDA 6.0 is a new library called NVBLAS which provides exactly this "drop-in" functionality. It intercepts Level3 BLAS calls (GEMM, TRSV, etc) and automatically sends them to the GPU, effectively tiling the PCIE transfer with on-GPU computation.

There is some information here: https://developer.nvidia.com/cublasxt, and CUDA 6.0 is available to CUDA registered developers today.

Full docs will be online once CUDA 6.0 is released to the general public.

Answer 2

CUBLAS does not wrap around BLAS. CUBLAS also accesses matrices in a column-major ordering, such as some Fortran codes and BLAS.

I am more used to writing code in C, even for CUDA. A code written with CBLAS (which is a C wrap of BLAS) can easily be change into a CUDA code. Be aware that Fortran codes that use BLAS are quite different from C/C++ codes that use CBLAS. Fortran and BLAS normally store matrices or double arrays in column-major ordering, but C/C++ normally handle Row-major ordering. I normally handle this problem writing saving the matrices in a 1D arrays, and use #define to write a macro toa access the element i,j of a matrix as:

/* define macro to access Aij in the row-wise array A[M*N] */
#define indrow(ii,jj,N) (ii-1)*N+jj-1 /* does not depend on rows M  */
/* define macro to access Aij in the col-wise array A[M*N] */
#define indcol(ii,jj,M) (jj-1)*M+ii-1 

CBLAS library has a well organize parameters and conventions (const enum variables) to give to each function the ordering of the matrix. Beware that also the storage of matrices vary, a row-wise banded matrix is not stored the same as a column-wise band matrix.

I don't think there are mechanics to allow the user to choose between using BLAS or CUBLAS, without writing the code twice. CUBLAS also has on most function calls a "handle" variable that does not appear on BLAS. I though of #define to change the name at each function call, but this might not work.