[petsc-users] MatMatMul inefficient

[Guido Margherita]

Hi,

You can find the reproducer at this link https://github.com/margheguido/Miniapp_MatMatMul , including the matrices I used.
I have trouble undrerstanding what is different in my case from the one you referenced me to.

Thank you so much,
Margherita

Il giorno 13 feb 2023, alle ore 3:51 PM, Pierre Jolivet <pierre at joliv.et> ha scritto:

Could you please share a reproducer?
What you are seeing is not typical of the performance of such a kernel, both from a theoretical or a practical (see fig. 2 of https://joliv.et/article.pdf) point of view.

Thanks,
Pierre

On 13 Feb 2023, at 3:38 PM, Guido Margherita via petsc-users <petsc-users at mcs.anl.gov> wrote:

A is a sparse MATSEQAIJ, Q is dense.

Thanks,
Margherita

Il giorno 13 feb 2023, alle ore 3:27 PM, knepley at gmail.com ha scritto:

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/


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