[petsc-users] Multiple solves with PCMG fail

[Barry Smith]

  Can you send us the code: to petsc-maint at mcs.anl.gov or petsc-users at mcs.anl.gov ? Or something that reproduces the problem?

  Barry

> On Oct 20, 2014, at 3:31 AM, Filippo Leonardi <filippo.leonardi at sam.math.ethz.ch> wrote:
> 
> Hi,
> 
> I have a very specific problem that I cannot figure out with PCMG and multiple 
> solves. I got a linear system that I solve many times, with same matrix but 
> different RHS. I can successfully solve the system with standard techniques, 
> such as default solver or LU or PCGAMG. Even MG works if I destroy the ksp 
> each time or I recompute the matrices at each time. But when I try and go to 
> MG and not recomputing the matrices each time the solver fails. Any idea?
> 
> Here some detail: the setup:
> ierr = KSPSetDMActive(ksp, PETSC_FALSE); CHKERRQ(ierr);
> ierr = KSPSetDM(ksp,  da); CHKERRQ(ierr);
> ierr = KSPSetComputeOperators(ksp, ComputeMatrix, ctx); CHKERRQ(ierr);
> ierr = KSPSetComputeRHS(ksp, ComputeRHS, ctx); CHKERRQ(ierr);
> ierr = KSPSetFromOptions(ksp); CHKERRQ(ierr);
> 
> Then I solve as usual, for a large number of time steps:
> ierr = KSPSolve(ksp, NULL, NULL); CHKERRQ(ierr);
> ierr = KSPGetSolution(ksp, &phi);
> 
> The solver converges and does that in a reasonable number of iterations:
> Linear solve converged due to CONVERGED_RTOL iterations 7
> And ksp_view and ksp_monitor do not show any weird stuff.
> 
> - Weirdly enough using any solver (for instance cg+bjacobi or gamg) everything 
> works (So the matrix and RHS are working fine).
> - But the problem persists with Galerkin matrices (-pc_mg_galerkin) (So is not 
> a ComputeMatrix problem).
> - If I do:
> ierr = KSPSetComputeOperators(ksp, ComputeMatrix, this); CHKERRQ(ierr);
> between each solve or destroy the ksp entirely each time the solution is also 
> perfect (So is not a boundary scaling or other stuff problem).
> - If I run MG with only 2 levels (so just coarse) I also get a fine result.
> 
> Setup RHS is called each time as expected, setup matrix is called just once, 
> also as expected.
> 
> The only thing I can think is that MG does not update some value that actually 
> needs to be recomputed.
> 
> Any idea?
> 
> The solution is not that different from:
> http://www.mcs.anl.gov/petsc/petsc-current/src/ksp/ksp/examples/tutorials/ex29.c.html
> 
> Best,
> Filippo