<div>Hi Barry,</div>
<div>Thank you very much for the explanation. It is good to know there are more options to start with.</div>
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<div>Yan</div>
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<div class="gmail_quote">On Tue, Sep 15, 2009 at 8:32 PM, Barry Smith <span dir="ltr"><<a href="mailto:bsmith@mcs.anl.gov">bsmith@mcs.anl.gov</a>></span> wrote:<br>
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<div class="im"><br>On Sep 15, 2009, at 6:04 PM, Ryan Yan wrote:<br><br>
<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">Thanks Jed.<br><br>And in the term ||Ae||/||Ax||, PETSc impilictly assume that we use zero as initial guess and here the x really means exact solution, right?<br>
</blockquote><br></div> If you call KSPSetInitialGuessNonzero() then PETSc will use the initial value in x as the initial guess. The default convergence tests<br>use the ratio of || r_current||/ || b ||. Thus if you start with a great initial guess it may not iterate much at all. You can use also -ksp_converged_use_initial_residual_norm<br>
or KSPDefaultConvergedSetUIRNorm() to have it use the ratio of || r_current||/|| r_initial|| for the relative convergence test.<br><br> You can also provide your own convergence test with KSPSetConvergenceTest().<br><font color="#888888"><br>
Barry</font>
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<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid"><br>Yan<br><br><br><br><br>On Tue, Sep 15, 2009 at 6:41 PM, Jed Brown <<a href="mailto:jed@59a2.org" target="_blank">jed@59a2.org</a>> wrote:<br>
Ryan Yan wrote:<br>> But can you say a little more about what happened in the first scenario, I<br>> did not quite fellow you. Let's say, If I set -ksp_rtol 1e-2, what does this<br>> mean in the left PC case. More specifically, which term will be considered<br>
> as a stop creteria for the KSP solve.<br><br>Look at the first column (preconditioned residual norm). Notice that<br>this is decreasing by 1e-2 and 1e-3 respectively in your examples.<br><br>Jed<br><br><br></blockquote>
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