[petsc-dev] Infinite loop in A*B

[Pierre Jolivet]

> On 1 Mar 2021, at 6:29 AM, Zhang, Hong <hzhang at mcs.anl.gov> wrote:
> 
> Pierre,
> This is a bug in MatMatMultSymbolic_MPIAIJ_MPIDense() during optimization of block column size of B. Run your code with
> '-matmatmult_Bbn 1', the infinite loop should not occur.

Thanks Hong, I can confirm this option makes the more complex use case run smoothly as well. 

> I'll try to figure out a fix tomorrow.

Great.

Thanks,
Pierre

> Hong
> From: Zhang, Hong <hzhang at mcs.anl.gov>
> Sent: Sunday, February 28, 2021 11:05 PM
> To: Pierre Jolivet <pierre at joliv.et>; For users of the development version of PETSc <petsc-dev at mcs.anl.gov>; Zhang, Hong <hzhang at mcs.anl.gov>
> Subject: Re: [petsc-dev] Infinite loop in A*B
>  
> The infinite loop in MatMatMultNumeric_MPIAIJ_MPIDense()
> for (i=0; i<BN; i+=n) {
> }
> is cause by n=contents->workB->cmap->n=0 (line 590 in mpimatmatmult.c)
> Hong
> From: petsc-dev <petsc-dev-bounces at mcs.anl.gov> on behalf of Zhang, Hong via petsc-dev <petsc-dev at mcs.anl.gov>
> Sent: Sunday, February 28, 2021 10:33 PM
> To: Pierre Jolivet <pierre at joliv.et>; For users of the development version of PETSc <petsc-dev at mcs.anl.gov>
> Subject: Re: [petsc-dev] Infinite loop in A*B
>  
> I can reproduce the hang with 
> mpiexec -n 2 ./matmatmult
> 
> It seems in an infinite loop of calling MatDensePlaceArray() from 
> 
> #0  MatDensePlaceArray (mat=0xda5c50, array=0xd15e60)
>     at /home/hongsu/soft/petsc/src/mat/impls/dense/mpi/mpidense.c:2047
> #1  0x00007fa0d13bf4f7 in MatDenseGetSubMatrix_SeqDense (A=0xcfb2b0, cbegin=0,
>     cend=0, v=0xd90370)
>     at /home/hongsu/soft/petsc/src/mat/impls/dense/seq/dense.c:2997
> #2  0x00007fa0d13c574e in MatDenseGetSubMatrix (A=0xcfb2b0, cbegin=0, cend=0,
>     v=0xd90370) at /home/hongsu/soft/petsc/src/mat/impls/dense/seq/dense.c:3371
> #3  0x00007fa0d13db5ce in MatDenseGetSubMatrix_MPIDense (A=0xca5250, cbegin=0,
>     cend=0, v=0x7ffe87d41de0)
>     at /home/hongsu/soft/petsc/src/mat/impls/dense/mpi/mpidense.c:1835
> #4  0x00007fa0d13c574e in MatDenseGetSubMatrix (A=0xca5250, cbegin=0, cend=0,
>     v=0x7ffe87d41de0)
>     at /home/hongsu/soft/petsc/src/mat/impls/dense/seq/dense.c:3371
> #5  0x00007fa0d179c2fa in MatMatMultNumeric_MPIAIJ_MPIDense (A=0xc55490,
>     B=0xca5250, C=0xd282b0)
>     at /home/hongsu/soft/petsc/src/mat/impls/aij/mpi/mpimatmatmult.c:593
> #6  0x00007fa0d1181331 in MatProductNumeric_AB (mat=0xd282b0)
>     at /home/hongsu/soft/petsc/src/mat/interface/matproduct.c:567
> #7  0x00007fa0d1182c14 in MatProductNumeric (mat=0xd282b0)
>     at /home/hongsu/soft/petsc/src/mat/interface/matproduct.c:679
> #8  0x00007fa0d115ef69 in MatProduct_Private (A=0xc55490, B=0xca5250,
>     scall=MAT_INITIAL_MATRIX, fill=-2, ptype=MATPRODUCT_AB, C=0x7ffe87d42018)
>     at /home/hongsu/soft/petsc/src/mat/interface/matrix.c:9405
> ---Type <return> to continue, or q <return> to quit---
> #9  0x00007fa0d115f274 in MatMatMult (A=0xc55490, B=0xca5250, scall=MAT_INITIAL_MATRIX, fill=-2,
>     C=0x7ffe87d42018) at /home/hongsu/soft/petsc/src/mat/interface/matrix.c:9445
> #10 0x000000000040130a in main (argc=2, argv=0x7ffe87d42108) at ex1.c:20
> 
> I'll try to figure out what is going on. If anyone has a clue, please help. The above stack comes from 'release' branch.
> Hong
> From: petsc-dev <petsc-dev-bounces at mcs.anl.gov> on behalf of Pierre Jolivet <pierre at joliv.et>
> Sent: Sunday, February 28, 2021 4:17 PM
> To: For users of the development version of PETSc <petsc-dev at mcs.anl.gov>
> Subject: [petsc-dev] Infinite loop in A*B
>  
> Hello,
> The following MWE loops indefinitely for MPI_Comm_size in {2; 3}.
> Nothing fancy, just MatAIJ and MatDense.
> The problem is either in MatMPIDenseScatter() or MatMatMultSymbolic_MPIAIJ_MPIDense(), I believe, so if someone familiar with those routines can figure out a hot fix, I’m all ears.
> I could of course switch to a MatMult(), but the same infinite loop happens in another more complex code with
> A = rows=8, cols=35212
> B = rows=35212, cols=9
> So I’ll need a fix eventually.
> 
> Thanks,
> Pierre

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