PCFieldSplit and Schur

Matthew Knepley knepley at gmail.com
Tue Nov 24 11:10:51 CST 2009

Can you use the debugger to get a stack trace?


On Tue, Nov 24, 2009 at 10:46 AM, francois pacull <fpacull at fluorem.com>wrote:

> Dear PETSc team,
> I am having a little bit of trouble with the Schur option of PCFieldSplit:
> I would like to apply some special treatments respectively to the five first
> and the two last equations over seven for each grid node of a CFD linear
> system...
> In the present case, the KSP is "gmres" (or "fgmres" if the PC is
> changing), the PC is "asm" (the subdomain IS is defined with
> PCASMSetLocalSubdomains in order to always include all the fields associated
> to the grid points of the overlap), the SUBKSP is "preonly" and the SUBPC is
> "fieldsplit".
> So far the FieldSplit options PC_COMPOSITE_ADDITIVE and
> PC_COMPOSITE_MULTIPLICATIVE work well with fieldsplit_0 and fieldsplit_1 set
> to KSPPREONLY and PCILU in the following way:
> PCFieldSplitGetSubKSP(SubPC,number_of_split,&FieldSplitKSP);
> ... [for the fields 0,1,2,3,4 (fieldsplit_0)]
> KSPSetType(FieldSplitKSP[0],KSPPREONLY);
> KSPGetPC(FieldSplitKSP[0],&FieldSplitPC0);
> PCSetType(FieldSplitPC0,PCILU);
> ... [for the fields 5 and 6 (fieldsplit_1)]
> KSPSetType(FieldSplitKSP[1],KSPPREONLY);
> KSPGetPC(FieldSplitKSP[1],&FieldSplitPC1);
> PCSetType(FieldSplitPC1,PCILU);
> ...
> They also work well when I set the FieldSplitKSP[0] and/or FieldSplitKSP[1]
> to KSPGMRES. However, I always get a segmentation violation when I try to
> use the FieldSplit option PC_COMPOSITE_SCHUR... I guess that I am missing
> something: is there more options to set in this Schur case?
> So far I did not turn on the Schur complement preconditioner option:
> PCFieldSplitSchurPrecondition(subpc,PETSC_FALSE);
> Is there a book or article describing the methods implemented in this
> FieldSplit option?
> The version is petsc-3.0.0-p7...
> Thank you for your help,
> Regards,
> francois pacull.

What most experimenters take for granted before they begin their experiments
is infinitely more interesting than any results to which their experiments
-- Norbert Wiener
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