<html><head><meta http-equiv="Content-Type" content="text/html; charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Apr 30, 2019, at 9:06 AM, Mark Adams <<a href="mailto:mfadams@lbl.gov" class="">mfadams@lbl.gov</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><br class=""><br class="">
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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.<br class=""><br class=""></blockquote><div class=""><br class=""></div><div class="">That PC is singular. </div></div></div>
</div></blockquote></div><div class=""><br class=""></div>Yes. I am confused about why GASM is giving a singular PC but ASM is not with the same subdomains.<div class=""><br class=""></div><div class="">One difference between them is that the default GASM overlap is 0 (<a href="https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCGASMSetOverlap.html" class="">https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCGASMSetOverlap.html</a>), but the default ASM overlap is 1 (<a href="https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCASMSetOverlap.html" class="">https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCASMSetOverlap.html</a>). However, changing the GASM overlap does not make any difference in the convergence history.</div><div class=""><div class=""><br class=""></div><div class="">-- Boyce</div></div></body></html>