[petsc-users] MatMatMul inefficient

[Pierre Jolivet]

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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