[petsc-users] MatView to file

[Pierre Jolivet]

> On 12 Aug 2022, at 9:47 PM, Alfredo Jaramillo <ajaramillopalma at gmail.com> wrote:
> 
> Hello Mark,
> But why should this depend on the number of processes?

Because with non-binary formats, the matrix is centralized on the first process, which can become very costly.

Thanks,
Pierre

> thanks
> Alfredo
> 
> On Fri, Aug 12, 2022 at 1:42 PM Mark Adams <mfadams at lbl.gov <mailto:mfadams at lbl.gov>> wrote:
> With 4 million elements you are nowhere near the 32 but integer limit of 2B or 32Gb of memory.
> 
> See the https://petsc.org/main/docs/manualpages/Mat/MatView <https://petsc.org/main/docs/manualpages/Mat/MatView>
> You should go to binary format when doing large matrices.
> 
> Mark
> 
> On Fri, Aug 12, 2022 at 1:00 PM Alfredo Jaramillo <ajaramillopalma at gmail.com <mailto:ajaramillopalma at gmail.com>> wrote:
> Hello Mark,
> Thank you, I added the lines that you sent.
> This only happens when running the code with more than 1 process. With only 1 MPI process the matrix is printed out.
> With 2 processes or more I observed the program begins to allocate RAM until it exceeds the computer capacity (32GB) so I wasn't able to get the stack trace.
> 
> However, I was able to reproduce the problem by compiling src/ksp/ksp/tutorials/ex54.c.html <https://petsc.org/release/src/ksp/ksp/tutorials/ex54.c.html> (modifying line 144) and running it with
> 
> mpirun -np 2 ex54 -ne 1000
> 
> This gives a sparse matrix of order ~1 million. When running ex54 with only one MPI process I don't observe this excessive allocation and the matrix is printed out.
> 
> Thanks,
> Alfredo
> 
> 
> 
> On Fri, Aug 12, 2022 at 10:02 AM Mark Adams <mfadams at lbl.gov <mailto:mfadams at lbl.gov>> wrote:
> You also want:
> 
>     PetscCall(PetscViewerPopFormat(viewer));
>     PetscCall(PetscViewerDestroy(&viewer));
> 
> This should not be a problem.
> If this is a segv and you configure it with   '--with-debugging=1', you should get a stack trace, which would help immensely.
> Or run in a debugger to get a stack trace.
> 
> Thanks,
> Mark
> 
> 
> On Fri, Aug 12, 2022 at 11:26 AM Alfredo Jaramillo <ajaramillopalma at gmail.com <mailto:ajaramillopalma at gmail.com>> wrote:
> Dear developers,
> 
> I'm writing a sparse matrix into a file by doing 
> 
>     if (dump_mat) {
>         PetscViewer viewer;
>         PetscViewerASCIIOpen(PETSC_COMM_WORLD,"mat-par-aux.m",&viewer);
>         PetscViewerPushFormat(viewer, PETSC_VIEWER_ASCII_MATLAB);
>         MatView(A,viewer);
>     }
> 
> This works perfectly for small cases.
> The program crashes for a case where the matrix A is of order 1 million but with only 4 million non-zero elements.
> 
> Maybe at some point petsc is full-sizing A?
> 
> Thank you,
> Alfredo

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