[petsc-dev] Should pARMS converge in parallel?

[Jed Brown]

Jose, thanks for adding pARMS.

I wonder if there is something wrong or if I'm just not using it correctly.
I'm testing with ksp ex2 and it works fine in serial:

$ mpiexec -n 1 ./ex2 -m 50 -n 50 -ksp_converged_reason -pc_type parms
Linear solve converged due to CONVERGED_RTOL iterations 10
Norm of error 9.34118e-05 iterations 10

but it diverges in parallel:

$ mpiexec -n 2 ./ex2 -m 50 -n 50 -ksp_converged_reason -pc_type parms
Linear solve did not converge due to DIVERGED_DTOL iterations 630
Norm of error 3.12589e+06 iterations 630

I can make it "converge" by turning off restart, but the algebraic residual
is actually large indicating that the preconditioner is actually nonlinear.

$ mpiexec -n 2 ./ex2 -m 50 -n 50 -ksp_converged_reason -pc_type parms
-ksp_gmres_restart 1000
Linear solve converged due to CONVERGED_RTOL iterations 81
Norm of error 30.4982 iterations 81

Indeed, using FGMRES makes it converge

$ mpiexec -n 2 ./ex2 -m 50 -n 50 -ksp_converged_reason -pc_type parms
-ksp_gmres_restart 1000 -ksp_type fgmres
Linear solve converged due to CONVERGED_RTOL iterations 71
Norm of error 0.000150845 iterations 71

but avoiding restarts is really critical

$ mpiexec -n 2 ./ex2 -m 50 -n 50 -ksp_converged_reason -pc_type parms
-ksp_gmres_restart 30 -ksp_type fgmres
Linear solve converged due to CONVERGED_RTOL iterations 771
Norm of error 0.00202382 iterations 771


What do you think about making the preconditioner linear by default? Is
there a way to make it more tolerant of restarts, at least for such a simple
problem (2D Laplace, 5-point stencil, uniform grid)?
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