  We propose different implementations of the sparse matrix--dense vector
multiplication (\spmv{}) for finite fields and rings $\Zb/m\Zb$. We take
advantage of graphic card processors (GPU) and multi-core architectures. Our
aim is to improve the speed of \spmv{} in the \linbox library, and henceforth
the speed of its black box algorithms. Besides, we use this and a new
parallelization of the sigma-basis algorithm in a parallel block Wiedemann rank
implementation over finite fields.
