[firedrake] More detailled breakdown of PETSc timings / higher order geometric MG

[Lawrence Mitchell]

On 18 Mar 2015, at 08:11, Eike Mueller <e.mueller at bath.ac.uk> wrote:

> Dear all,
> 
> to get a more detailled breakdown of the PETSc fieldsplit preconditioner I now tried
> 
>        ksp = up_solver.snes.getKSP()
>        ksp.setMonitor(self._ksp_monitor)
>        ksp_hdiv = ksp.getPC().getFieldSplitSubKSP()
>        ksp_hdiv.setMonitor(self._ksp_monitor)
> 
> to attach my own KSP monitor to the solver for the HDiv system. I can then use that to work out the time per iteration and number of iterations of the velocity mass matrix solve. I suspect that for some reason the same PC (preonly+bjacobi+ILU) is less efficient for my standalone velocity mass matrix solve, possibly because the ilu does not work due to the wrong dof-ordering (I observe that preonly+bjacobi+ILU is not faster than cg+jacobi for my inversion, but in the fieldsplit case there is a significant difference).
> 
> However, the third line of the code above crashes with a nasty segfault in PETSc:
> 
>  File "/Users/eikemueller/PostDocBath/EllipticSolvers/Firedrake_workspace/firedrake-helmholtzsolver/source/gravitywaves.py", line 475, in solve
>    pc_hdiv = ksp.getPC().getFieldSplitSubKSP()
>  File "PC.pyx", line 384, in petsc4py.PETSc.PC.getFieldSplitSubKSP (src/petsc4py.PETSc.c:136328)
> petsc4py.PETSc.Error: error code 85
> [0] PCFieldSplitGetSubKSP() line 1662 in /Users/eikemueller/PostDocBath/EllipticSolvers/petsc/src/ksp/pc/impls/fieldsplit/fieldsplit.c
> [0] PCFieldSplitGetSubKSP_FieldSplit_Schur() line 1259 in /Users/eikemueller/PostDocBath/EllipticSolvers/petsc/src/ksp/pc/impls/fieldsplit/fieldsplit.c
> [0] MatSchurComplementGetKSP() line 317 in /Users/eikemueller/PostDocBath/EllipticSolvers/petsc/src/ksp/ksp/utils/schurm.c
> [0] Null argument, when expecting valid pointer
> [0] Null Object: Parameter # 1
> 
You probably needed to call

up_solver.snes.setUp() (and maybe up_solver.snes.setFromOptions(), once you've set the petsc options appropriately) before you can pull the schur complement KSPs out.

Lawrence

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