[petsc-users] MatMatMul inefficient

[Matthew Knepley]

On Mon, Feb 13, 2023 at 9:21 AM Guido Margherita via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> Hi all,
>
> I realised that performing a matrix-matrix multiplication using the
> function MatMatMult it is not at all computationally efficient with respect
> to performing N times a matrix-vector multiplication with MatMul, being N
> the number of columns of the second matrix in the product.
> When I multiply I matrix A  46816 x 46816 to a matrix Q  46816 x 6, the
> MatMatMul function is indeed 6 times more expensive than 6 times a call to
> MatMul, when performed sequentially (0.04056  s vs 0.0062 s ). When the
> same code is run in parallel the gap grows even more, being10 times more
> expensive.
>   Is there an explanation for it?
>

So we can reproduce this, what kind of matrix is A? I am assuming that Q is
dense.

  Thanks,

     Matt


>
> t1 = MPI_Wtime()
> call MatMatMult(A,Q,MAT_INITIAL_MATRIX, PETSC_DEFAULT_REAL, AQ, ierr )
> t2 = MPI_Wtime()
> t_MatMatMul = t2-t1
>
> t_MatMul=0.0
> do j = 0, m-1
>         call MatGetColumnVector(Q, q_vec, j,ierr)
>
>         t1 = MPI_Wtime()
>         call MatMult(A, q_vec, aq_vec, ierr)
>         t2 = MPI_Wtime()
>
>         t_MatMul = t_MatMul + t2-t1
> end do
>
> Thank you,
> Margherita Guido
>
>

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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