On Mon, May 7, 2012 at 12:15 PM, <coco at dmi.unict.it> wrote: > I am trying to use a matrix-free method to solve a linear system with > Petsc. In particular, I noticed that if I solve the same linear system with > the same KSP solver, the same preconditioner, and the same options, I > obtain different convergence results if I solve the system with the full- > or free-matrix method. Is that plausible? > > Here is a piece of code: > > //Method #1: > ierr=MatCreate(PETSC_COMM_**WORLD,&M); CHKERRQ(ierr); > ierr=MatSetType(M, MATMPIAIJ); CHKERRQ(ierr); > ierr=MatSetSizes(M,PETSC_**DECIDE,PETSC_DECIDE,N,M); CHKERRQ(ierr); > [...filling the matrix...] > ierr = KSPCreate(PETSC_COMM_WORLD,&**ksp);CHKERRQ(ierr); > ierr = KSPSetOperators(ksp,M,M,**DIFFERENT_NONZERO_PATTERN);** > CHKERRQ(ierr); > ierr = KSPSetTolerances(ksp,1.e-12,**PETSC_DEFAULT,PETSC_DEFAULT,** > PETSC_DEFAULT);CHKERRQ(ierr); > ierr = KSPSolve(ksp,RHS,U);CHKERRQ(**ierr); > > //Method #2: > extern PetscErrorCode UserMult(Mat,Vec,Vec); > ierr=MatCreateShell(PETSC_**COMM_WORLD,N,M,n,M,ctx,&M1); CHKERRQ(ierr); > ierr=MatShellSetOperation(M,**MATOP_MULT,(void(*)(void)) UserMult); > CHKERRQ(ierr); > ierr=MatAssemblyBegin(M,MAT_**FINAL_ASSEMBLY); CHKERRQ(ierr); > ierr=MatAssemblyEnd(M,MAT_**FINAL_ASSEMBLY); CHKERRQ(ierr); > ierr = KSPCreate(PETSC_COMM_WORLD,&**ksp1);CHKERRQ(ierr); > ierr = KSPSetOperators(ksp1,M1,M1,**DIFFERENT_NONZERO_PATTERN);** > CHKERRQ(ierr); > ierr = KSPSetTolerances(ksp1,1.e-12,**PETSC_DEFAULT,PETSC_DEFAULT,** > PETSC_DEFAULT);CHKERRQ(ierr); > ierr = KSPSolve(ksp1,RHS,U1);CHKERRQ(**ierr); > > In particular, the method #1 converges in 15 iterations, while the method > #2 in more than 1000. >
What preconditioner? How did you determine that the matrix-free method applies the same linear operator? -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120507/9f922359/attachment.htm>
