<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Jed brings up good points about false negatives, there are also false positives to worry about. This is a bad idea, at least as a default solver. <div><br></div><div>That said, there is work on "adaptive" solvers like bootstrap AMG and using machine learning to formalize what it sounds like Barry is driving at. Perhaps make a '-ksp_type black_box' solver type where you put any heuristics methods like this. (this would require the KSP method to change the PC method which is ugly...)</div><div><br><div>Mark</div><div><br><div><div>On Aug 5, 2010, at 5:36 AM, Matthew Knepley wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">On Thu, Aug 5, 2010 at 2:10 AM, Jed Brown <span dir="ltr"><<a href="mailto:jed@59a2.org">jed@59a2.org</a>></span> wrote:<br><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"> <div class="im">On Thu, 5 Aug 2010 00:21:13 -0500, Matthew Knepley <<a href="mailto:knepley@gmail.com">knepley@gmail.com</a>> wrote:<br> > If someone tells us, "I have a Stokes problem", we could search for 0<br> > diagonals and create the partition for FS.<br> <br> </div>I suppose this would work for some common cases, but there are lot of<br> discretizations and pressure-dependent constitutive relations/boundary<br> conditions that have nonzeros in the pressure block. I think some slip<br> conditions can also produce zero or negative values in the momentum<br> block.<br></blockquote><div><br></div><div>Do not disagree.</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"> I'm not convinced that it's so much to ask people to provide an index<br> set, considering that FieldSplit is a somewhat advanced thing anyway<br> (based on sheer number of choices available, and (typical) sensitivity<br> to those choices).</blockquote><div><br></div><div>I guess the point here is somewhat like the point of DAs. It is a very limited</div><div>thing, but something many people do. So we provide a way to most easily</div><div> do the very limited thing.</div><div><br></div><div>Not sure if this is useful enough, but it might be.</div><div><br></div><div> Matt</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"> <font color="#888888"><br> Jed<br> </font></blockquote></div><br><br clear="all"><br>-- <br>What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.<br>-- Norbert Wiener<br> </blockquote></div><br></div></div></body></html>