[petsc-users] ASM vs GASM

[Boyce Griffith]

> On Apr 30, 2019, at 9:06 AM, Mark Adams <mfadams at lbl.gov> wrote:
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> 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.

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 <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 <https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCASMSetOverlap.html>). However, changing the GASM overlap does not make any difference in the convergence history.

-- Boyce
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