[petsc-users] Different solution while running in parallel

[Zhang, Hong]

Karhik,
Can you find out the condition number of your matrix?
Hong

________________________________
From: petsc-users <petsc-users-bounces at mcs.anl.gov> on behalf of Karthikeyan Chockalingam - STFC UKRI via petsc-users <petsc-users at mcs.anl.gov>
Sent: Wednesday, November 16, 2022 6:04 PM
To: petsc-users at mcs.anl.gov <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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