[petsc-users] Different solution while running in parallel

[Karthikeyan Chockalingam - STFC UKRI]

  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.


This email and any attachments are intended solely for the use of the named recipients. If you are not the intended recipient you must not use, disclose, copy or distribute this email or any of its attachments and should notify the sender immediately and delete this email from your system. UK Research and Innovation (UKRI) has taken every reasonable precaution to minimise risk of this email or any attachments containing viruses or malware but the recipient should carry out its own virus and malware checks before opening the attachments. UKRI does not accept any liability for any losses or damages which the recipient may sustain due to presence of any viruses.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20221117/4715e253/attachment.html>
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: out.txt
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20221117/4715e253/attachment.txt>