[petsc-users] Different solution while running in parallel

Thu Nov 17 10:13:25 CST 2022

Hi Matt,

I tested two sizes manually for the Poisson problem with homogenous Dirichlet boundary conditions (on all nodes on the boundary) and they both produced the right result when run serially using PCLU

  1.  2 elements x 2 elements (total nodes 9 but 1 dof)

A = 10.6667   b = 4    x = 0.375

  1.  3 elements x 3 elements (total nodes 16 but 4 dof)
              A = 10.6667 -1.33333 -1.33333 -1.33333
                     -1.33333 10.6667 -1.33333 -1.33333
                    -1.33333 -1.33333 10.6667 -1.33333
                      -1.33333 -1.33333 -1.33333 10.6667

              b = {4 4 4 4}^T
              x = (0.6 0.6 0.6 0.6)

        Since, it is solvable not sure if the system can be singular?

I have attached the runs for case (2) run on one and two cores. Parallel run produces zero vector for x.

I used MatZeroRowsColumns to set the Dirichlet boundary conditions by zeroing the entries in the matrix corresponding to the boundary nodes.

Best,
Karthik.

From: Matthew Knepley <knepley at gmail.com>
Date: Thursday, 17 November 2022 at 15:16
To: Chockalingam, Karthikeyan (STFC,DL,HC) <karthikeyan.chockalingam at stfc.ac.uk>
Cc: Zhang, Hong <hzhang at mcs.anl.gov>, petsc-users at mcs.anl.gov <petsc-users at mcs.anl.gov>
Subject: Re: [petsc-users] Different solution while running in parallel
Using options instead of code will make your life much easier.

Two thing are wrong here:

  1) Your solver is doing no iterates because the initial residual is very small, 5.493080158227e-15. The LU does not matter.
       In order to check the condition number of your system, run with -pc_type svd -pc_svd_monitor

  2) Your parallel run also does no iterates

  0 KSP none resid norm 6.951601853367e-310 true resid norm 1.058300524426e+01 ||r(i)||/||b|| 8.819171036882e-01

but the true residual is not small. That means that your system is singular, but you have given a consistent RHS.

  Thanks,

     Matt

On Thu, Nov 17, 2022 at 9:37 AM Karthikeyan Chockalingam - STFC UKRI <karthikeyan.chockalingam at stfc.ac.uk<mailto:karthikeyan.chockalingam at stfc.ac.uk>> wrote:
Hi Matt and Hong,

Thank you for your response.
I made the following changes, to get the desired output

    PetscReal norm; /* norm of solution error */

    PetscInt  its;

    KSPConvergedReason reason;

    PetscViewerAndFormat *vf;

    PetscViewerAndFormatCreate(PETSC_VIEWER_STDOUT_WORLD, PETSC_VIEWER_DEFAULT, &vf);

    ierr = KSPView(ksp, PETSC_VIEWER_STDOUT_SELF);CHKERRQ(ierr);

    KSPSolve(ksp, b, x);

    ierr = KSPMonitorTrueResidual(ksp,its,norm,vf);CHKERRQ(ierr);
    ierr = KSPMonitorSingularValue(ksp, its, norm, vf);CHKERRQ(ierr);

I have attached the outputs from both the runs. As before, I am also printing A, b, and x.

I wonder if it is a memory issue related to mpi library employed. I am currently using openmpi – should I instead use mpich?

Kind regards,
Karthik.

From: Matthew Knepley <knepley at gmail.com<mailto:knepley at gmail.com>>
Date: Thursday, 17 November 2022 at 12:19
To: Zhang, Hong <hzhang at mcs.anl.gov<mailto:hzhang at mcs.anl.gov>>
Cc: petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov> <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov>>, Chockalingam, Karthikeyan (STFC,DL,HC) <karthikeyan.chockalingam at stfc.ac.uk<mailto:karthikeyan.chockalingam at stfc.ac.uk>>
Subject: Re: [petsc-users] Different solution while running in parallel
On Wed, Nov 16, 2022 at 9:07 PM Zhang, Hong via petsc-users <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov>> wrote:
Karhik,
Can you find out the condition number of your matrix?

Also, run using

  -ksp_view -ksp_monitor_true_residual -ksp_converged_reason

and send the two outputs.

  Thanks,

      Matt

Hong

________________________________
From: petsc-users <petsc-users-bounces at mcs.anl.gov<mailto:petsc-users-bounces at mcs.anl.gov>> on behalf of Karthikeyan Chockalingam - STFC UKRI via petsc-users <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov>>
Sent: Wednesday, November 16, 2022 6:04 PM
To: petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov> <petsc-users at mcs.anl.gov<mailto:petsc-users at mcs.anl.gov>>
Subject: [petsc-users] Different solution while running in parallel

  Hello,

   I tried to solve a (FE discretized) Poisson equation using PCLU. For some reason I am getting different solutions while running the problem on one and two cores. I have attached the output file (out.txt) from both the runs. I am printing A, b and x from both the runs – while A and b are the same but the solution seems is different.

I am not sure what I doing wrong.

Below is my matrix, vector, and solve setup.

    Mat A;

    Vec b, x;

    ierr = MatCreate(PETSC_COMM_WORLD, &A); CHKERRQ(ierr);

    ierr = MatSetType(A, MATMPIAIJ); CHKERRQ(ierr);

    ierr = MatSetSizes(A, PETSC_DECIDE, PETSC_DECIDE, N, N); CHKERRQ(ierr);

    ierr = MatMPIAIJSetPreallocation(A,d_nz, NULL, o_nz, NULL); CHKERRQ(ierr);

    ierr = MatSetOption(A,MAT_SYMMETRIC,PETSC_TRUE); CHKERRQ(ierr);

    ierr = MatCreateVecs(A, &b, &x); CHKERRQ(ierr);

    KSP           ksp;

    PC            pc;

    KSPCreate(PETSC_COMM_WORLD, &ksp);

    KSPSetOperators(ksp, A, A);

    ierr = KSPSetType(ksp,KSPPREONLY);CHKERRQ(ierr);

    ierr = KSPGetPC(ksp,&pc);CHKERRQ(ierr);

    ierr = PCSetType(pc,PCLU);CHKERRQ(ierr);

    ierr = PCFactorSetMatSolverType(pc,MATSOLVERMUMPS);CHKERRQ(ierr);

    KSPSolve(ksp, b, x);

Thank you for your help.

Karhik.

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