[petsc-users] ASM vs GASM

Fande Kong fdkong.jd at gmail.com
Tue Apr 30 10:23:12 CDT 2019

On Tue, Apr 30, 2019 at 7:40 AM Boyce Griffith via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> On Apr 30, 2019, at 9:06 AM, Mark Adams <mfadams at lbl.gov> wrote:
>> Allowing GASM to construct the "outer" subdomains from the
>> non-overlapping "inner" subdomains, and using "exact" subdomain solvers
>> (subdomain KSPs are using FGMRES+ILU with an rtol of 1e-12), I get
>> convergence in ~2 iterations in the preconditioned residual norm to an rtol
>> of 1e-12. If I look at the true residual norm, however, it stagnates around
>> 1e-4.
> That PC is singular.
> Yes. I am confused about why GASM is giving a singular PC but ASM is not
> with the same subdomains.

We could not tell much without the info of detailed solver setup. The
overlapping function was implemented by me and Dmitry a couple of years
ago, and it is trick when a subdomain is shared by multiple cores.  Do you
mind to start from our example? Or setup an example for us to demonstrate
your issues?

At least please use "-ksp_view" (linear solver) or "-snes_view" (nonlinear
solver) to print more information that will help us a bit.

> One difference between them is that the default GASM overlap is 0 (
> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCGASMSetOverlap.html),
> but the default ASM overlap is 1 (
> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCASMSetOverlap.html
> ).

For your particular application, not sure you need any overlap since there
are two different PDEs on two different subdomains. It may be fine to run
the PC without the overlapping domain.   It is a definitely interesting
application for GASM.


> However, changing the GASM overlap does not make any difference in the
> convergence history.
> -- Boyce
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