Hi Lawrence,

if I set up the ksp as below, I get rid of the segfault. However, I don’t get any output from the KSP monitors attached to the subKSPs, only from the KSP monitor of the main KSP.

        up_solver = LinearVariationalSolver(up_problem, solver_parameters=sparams)
        ksp = up_solver.snes.getKSP()
        ksp.setUp()
        ksp.setMonitor(self._ksp_monitor)
        ksp_hdiv = ksp.getPC().getFieldSplitSubKSP()
        ksp_hdiv[0].setMonitor(KSPMonitor('fieldsplit_0',verbose=2))
        ksp_hdiv[1].setMonitor(KSPMonitor('fieldsplit_1',verbose=2))
        with self._ksp_monitor:
            up_solver.solve()

Thanks,

Eike

--

Dr Eike Hermann Mueller
Lecturer in Scientific Computing

Department of Mathematical Sciences
University of Bath
Bath BA2 7AY, United Kingdom

+44 1225 38 5557
e.mueller@bath.ac.uk
http://people.bath.ac.uk/em459/

On 18 Mar 2015, at 15:09, Lawrence Mitchell <lawrence.mitchell@imperial.ac.uk> wrote:


On 18 Mar 2015, at 08:11, Eike Mueller <e.mueller@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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