[petsc-users] MatMatMul inefficient

[Guido Margherita]

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/