[petsc-users] Preconditioner question ASM vs SOR

[Erik Andreassen]

I think I understand; does this mean that it is possible to change the sub PC type to SOR? Or is there a specific reason why the sub type is ILU?

The reason why I am asking, is that when I look at the residuals coming out of PCMG, they seem to be largest on local domain boundaries (corresponding to the partitions). Therefore, I’d like to use ASM with overlap instead of SOR as the preconditioner on the coarser levels (to smooth these errors out), but with the ASM/ILU combination this does not work at all (in accordance with your comment on high frequencies).

Thanks again,
Erik

From: Matthew Knepley [mailto:knepley at gmail.com]
Sent: 17. september 2015 14:12
To: Erik Andreassen
Cc: petsc-users at mcs.anl.gov
Subject: Re: [petsc-users] Preconditioner question ASM vs SOR

On Thu, Sep 17, 2015 at 7:07 AM, Erik Andreassen <erand at mek.dtu.dk<mailto:erand at mek.dtu.dk>> wrote:
I have played a bit with preconditioners for the different levels in PCMG, and among others I have tried additive Schwartz (ASM), which does not seem to work very well – especially with no overlap. I’m curious to hear if anyone can explain why ASM with no overlap performs so much worse than SOR?

The point of a smoother is to wipe out the high frequencies in the error. SOR does this (provably for the Laplacian), but there is no
reason ASM/ILU should do this. If this does not happen, then these parts of the solution remain uncorrected when you go to the
coarse grid, and the convergence breaks down.

   Matt

Thanks,
Erik

PS: I had a previous question about setting levels in GAMG, and my first impression is (in line with what Mark Adams wrote) that it is better to leave it to figure it out automatically. Also when using it as the coarse level preconditioner in a PCMG.



--
What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
-- Norbert Wiener
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