[petsc-users] Question about SuperLU
Barry Smith
bsmith at petsc.dev
Fri Jun 10 08:32:01 CDT 2022
It is difficult to tell exactly how the preconditioner is being formed with the information below it looks like in the
first case: the original B diagonal block and V diagonal block of the matrix are being factored separately with SuperLU_DIST
second case: the B block is factored with SuperLU_DIST and an explicit approximation to a Schur complement of the V block (Schur complement on eliminating the B block) is formed using "Preconditioner for the Schur complement formed from Sp, an assembled approximation to S, which uses A00's %sdiagonal's inverse" (this is the printout from a KSPView() for this part of the preconditioner).
My guess is you have a "Stokes"-like problem where the V block is identically 0 so, of course, the SuperLU_DIST will fail on it. But the approximation of the Schur complement onto that block is not singular so SuperLU_DIST has no trouble. If I am wrong and the V block is not identically 0 then it may be singular (or possibly, but less likely just badly order) so that SuperLU_DIST encounters a zero pivot.
You can run with -ksp_view_pre to have the KSP print the KSP solver algorithm details BEFORE the linear solve (hence they would get printed despite your failed solve). That would be useful to see exactly what your preconditioner is.
You can use -ksp_view_pmat (with appropriate prefix) to display the matrix that is going to be factored. Thus you can quickly verify what V is.
If you run with -ksp_error_if_not_converged then the solver will stop exactly when the zero pivot is encountered; this would include some information from SuperLU_DIST which might include the row number etc.
Notes on PETSc improvements needed.
1) The man page for KSPCheckSolve() is terribly misleading
2) It would be nice to have a view that displayed the nested fieldsplit preconditioners more clearly
> On Jun 9, 2022, at 5:19 PM, Jorti, Zakariae via petsc-users <petsc-users at mcs.anl.gov> wrote:
>
> Hi,
>
> I am solving non-linear problem that has 5 unknowns {ni, T, E, B, V}, and for the preconditioning part, I am using a FieldSplit preconditioner. At the last fieldsplit/level, we are left with a {B,V} block that tried to precondition in 2 different ways:
> a) SuperLU:
> -fieldsplit_TEBV_fieldsplit_EBV_fieldsplit_BV_ksp_type preonly -fieldsplit_TEBV_fieldsplit_EBV_fieldsplit_BV_pc_type lu -fieldsplit_TEBV_fieldsplit_EBV_fieldsplit_BV_pc_factor_mat_solver_type superlu_dist
> b) a Schur-based fieldsplit preconditioner that uses SuperLU for both V and B blocks:
> -fieldsplit_TEBV_fieldsplit_EBV_fieldsplit_BV_ksp_type gmres -fieldsplit_TEBV_fieldsplit_EBV_fieldsplit_BV_pc_type fieldsplit -fieldsplit_TEBV_fieldsplit_EBV_fieldsplit_BV_pc_fieldsplit_type schur -fieldsplit_TEBV_fieldsplit_EBV_fieldsplit_BV_pc_fieldsplit_schur_precondition selfp -fieldsplit_TEBV_fieldsplit_EBV_fieldsplit_BV_fieldsplit_B_ksp_type preonly -fieldsplit_TEBV_fieldsplit_EBV_fieldsplit_BV_fieldsplit_B_pc_type lu -fieldsplit_TEBV_fieldsplit_EBV_fieldsplit_BV_fieldsplit_B_pc_factor_mat_solver_type superlu_dist -fieldsplit_TEBV_fieldsplit_EBV_fieldsplit_BV_fieldsplit_V_ksp_type preonly -fieldsplit_TEBV_fieldsplit_EBV_fieldsplit_BV_fieldsplit_V_pc_type lu -fieldsplit_TEBV_fieldsplit_EBV_fieldsplit_BV_fieldsplit_V_pc_factor_mat_solver_type superlu_dist
>
> Option a) yields the following error:
> " Linear fieldsplit_ni_ solve converged due to CONVERGED_ATOL iterations 0
> Linear fieldsplit_TEBV_fieldsplit_tau_ solve converged due to CONVERGED_RTOL iterations 1
> Linear fieldsplit_TEBV_fieldsplit_EBV_fieldsplit_EP_ solve converged due to CONVERGED_RTOL iterations 5
> Linear fieldsplit_TEBV_fieldsplit_EBV_fieldsplit_BV_ solve did not converge due to DIVERGED_PC_FAILED iterations 0
> PC failed due to FACTOR_NUMERIC_ZEROPIVOT "
> whereas options b) seems to be working well.
> Is it possible that the SuperLU on the {V,B} block uses a reordering that introduces a zero pivot or could there be another explanation for this error?
>
> Many thanks.
> Best,
>
> Zakariae
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