Jed Brown wrote: > On Wed, Apr 6, 2011 at 11:36, Thomas Witkowski > <thomas.witkowski at tu-dresden.de > <mailto:thomas.witkowski at tu-dresden.de>> wrote: > > This brings me back to the starting point of my current problem: > how to set splits for MatNest correctly? PCFieldSplitSetFields > seems not to work in my case. > > > PCFieldSplitSetFields() is a special case that only works when fields > are interlaced. I would like to get rid of it, but there are still > some cases where it can provide a faster implementation in the special > case so we'll wait until the faster implementation can be accessed > through the generic interface. > > > When I want to make use of PCFieldSplitSetIS for splitting > MatNest, which indices should I provide? The MatNest I've created > consists of the to main diagonal blocks with indices 0 to n and 0 > to m. > > > This does not make sense because these index sets overlap. Maybe you > want to use (0:m) and (m:m+n), then these are the index sets you pass > to PCFieldSplitSetIS(). With two processes, you would usually have > > is0 = [(0:m0), (m0+n0:m0+n0+m1)] > is1 = [(m0:m0+n0), (m0+n0+m1:m0+n0+m1+n1)] > > This is the decomposition that is set up automatically by > MatCreateNest when you pass PETSC_NULL for the index sets. > > You should be able to retrieve the index sets that MatNest created > using MatNestGetSubMats(), but this is not implemented (Dave and I > discussed it and agree, but a patch has not been pushed yet). Doing it this way seems to work correctly, at least for the initialization. When solving the system, I get a segdev. Here is the stack (I make use of the current petsc-dev):
#0 0x00000000008ce6b3 in PetscMemcpy (a=0x188bba0, b=0x2, n=128) at /fastfs/witkowsk/software/petsc-dev/include/petscsys.h:1777 #1 0x00000000008d0935 in VecScatterBegin_1 (ctx=0x18a4460, xin=0x17b1d70, yin=0x1889840, addv=INSERT_VALUES, mode=SCATTER_FORWARD) at /fastfs/witkowsk/software/petsc-dev/src/vec/vec/utils/vpscat.h:11 4 #2 0x00000000008c6953 in VecScatterBegin (inctx=0x18a4460, x=0x17b1d70, y=0x1889840, addv=INSERT_VALUES, mode=SCATTER_FORWARD) at /fastfs/witkowsk/software/petsc-dev/src/vec/vec/utils/vscat.c:1575 #3 0x00000000007160b6 in PCApply_FieldSplit_Schur (pc=0x182f4e0, x=0x17b1d70, y=0x17c2640) at /fastfs/witkowsk/software/petsc-dev/src/ksp/pc/impls/fieldsplit/fieldsplit.c:565 #4 0x0000000000c7015c in PCApply (pc=0x182f4e0, x=0x17b1d70, y=0x17c2640) at /fastfs/witkowsk/software/petsc-dev/src/ksp/pc/interface/precon.c:384 #5 0x00000000007ff6e3 in KSPSolve_PREONLY (ksp=0x17fe730) at /fastfs/witkowsk/software/petsc-dev/src/ksp/ksp/impls/preonly/preonly.c:26 #6 0x000000000077b246 in KSPSolve (ksp=0x17fe730, b=0x17b1d70, x=0x17c2640) at /fastfs/witkowsk/software/petsc-dev/src/ksp/ksp/interface/itfunc.c:426 .... The problem is that the data field (vec->data) of the nested vectors is empty. I created the nested vectors exactly in the same way as in /src/ksp/ksp/examples/tests/ex22.c. When I would make a VecGetArray(myvec, &vecptr) on a nested vector myvec, than vecptr is not a vailde pointer. This is the reason for the segdev. Is it a bug in PETSc or some problem of my usage of PETSc's interface? Thomas
