[petsc-users] SNES convergence issue

Jed Brown jed at 59A2.org
Mon May 16 08:04:14 CDT 2011

On Mon, May 16, 2011 at 14:54, Tian(ICT) <rongtian at ncic.ac.cn> wrote:

>  12 KSP preconditioned resid norm 5.794625326821e+000 true resid norm
> 5.490686672936e+006 ||Ae||/||Ax|| 7.765003561961e+004
>  13 KSP preconditioned resid norm 7.239823936251e-001 true resid norm
> 4.262009749173e+005 ||Ae||/||Ax|| 6.027391992073e+003
>  14 KSP preconditioned resid norm 6.718858761053e-001 true resid norm
> 7.089916382344e+005 ||Ae||/||Ax|| 1.002665590704e+004
>  15 KSP preconditioned resid norm 6.284315560280e-001 true resid norm
> 4.441924320726e+005 ||Ae||/||Ax|| 6.281829619309e+003
>  16 KSP preconditioned resid norm 6.861179511523e-011 true resid norm
> 1.032548577293e+006 ||Ae||/||Ax|| 1.460244202260e+004

Notice how the true residual is blowing up while the GMRES estimate
converges. (The language here is not correct, "preconditioned resid norm" in
this case is actually the unpreconditioned norm as estimated by FGMRES.)
This indicates that the preconditioner is singular which could be due to
lack of pivoting. If you run with -pc_type none, it should converge
correctly. Do that and see if SNES is converging properly. Also try -pc_type
lu -pc_factor_mat_solver_package superlu (or mumps or umfpack) if you have
any of these packages installed.

What equations are you solving and with what discretization?
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