<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Like using the Krylov preconditioner as a Richardson KSP solver with max_its = N? <br class=""><div><blockquote type="cite" class=""><div class="">On Jun 22, 2018, at 1:49 PM, Matthew Knepley <<a href="mailto:knepley@gmail.com" class="">knepley@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><div class="gmail_quote"><div dir="ltr" class="">On Fri, Jun 22, 2018 at 1:47 PM Boyce Griffith <<a href="mailto:griffith@cims.nyu.edu" class="">griffith@cims.nyu.edu</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-style: solid; border-left-color: rgb(204, 204, 204); padding-left: 1ex;"><div dir="auto" class=""><div class=""></div><div class="">Can you set up the preconditioner so that you can just use GMRES?</div></div></blockquote><div class=""><br class=""></div><div class="">So I think what Boyce is saying is, can't you fix the number of iterates in the inner Krylov solvers so that it becomes a linear</div><div class="">operator and you can use GMRES?</div><div class=""><br class=""></div><div class=""> Thanks,</div><div class=""><br class=""></div><div class=""> Matt</div><div class=""> </div><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-style: solid; border-left-color: rgb(204, 204, 204); padding-left: 1ex;"><div dir="auto" class=""><div class="">On Jun 22, 2018, at 4:33 PM, Nishant Nangia <<a href="mailto:nishantnangia329@gmail.com" target="_blank" class="">nishantnangia329@gmail.com</a>> wrote:<br class=""><br class=""></div><blockquote type="cite" class=""><div class=""><div dir="ltr" class="">Hi,<div class=""><br class=""></div><div class="">I am solving a saddle point system using a shell preconditioner (which itself uses Krylov solvers, hence the use of FGMRES). I had added the option to re-scale parts of the saddle point system to minimize loss of floating point precision for cases where there are varying orders of magnitude in the system/unknowns.</div><div class=""><br class=""></div><div class="">I wanted to show that re-scaling can alleviate large differences between the preconditioned and unpreconditioned residual norms. However, I notice that FGMRES only supports right preconditioning, meaning the preconditioned residual is never formed/used (I think).</div><div class=""><br class=""></div><div class="">Is there any way to form the preconditioned norm for FGMRES, or does it just not make sense in the context of right-preconditioned iterative solvers? Is there any way to show that the re-scaling is improving the solver convergence (i.e. showing that it ensures that the true and relative residual are close to each other)?<br clear="all" class=""><div class=""><div class="m_-6010472951141135462gmail_signature" data-smartmail="gmail_signature"><div dir="ltr" class=""><div class=""><div dir="ltr" class=""><br class=""><b class="">Nishant Nangia</b><div class="">Northwestern University<br class=""><div class="">Ph.D. Candidate | Engineering Sciences and Applied Mathematics<br class=""></div></div><div class=""><span style="font-family: arial; font-size: small;" class="">Tech L386</span><br class=""></div></div></div></div></div></div></div></div></div></blockquote></div></blockquote></div><br clear="all" class=""><div class=""><br class=""></div>--<span class="Apple-converted-space"> </span><br class=""><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr" class=""><div class=""><div dir="ltr" class=""><div class="">What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.<br class="">-- Norbert Wiener</div><div class=""><br class=""></div><div class=""><a href="http://www.caam.rice.edu/~mk51/" target="_blank" class="">https://www.cse.buffalo.edu/~knepley/</a></div></div></div></div></div></div></div></blockquote></div><br class=""></body></html>