<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Just to add to Jed and Matt's comments:<div><br></div><div>1) What are your equations (I really don't care what physics you are modeling, its the equations that we see). Is is a scalar div (alpha(x) grad ) u?</div><div><br></div><div>2) Try using hyper (-pc_type hyper -pc_hypre_type boomeramg). Look at "<span style="font-family: arial; font-size: 14px; line-height: 23px; ">PCApply 448</span>", this should be like 10-20. Configure your systems with --download-hyper=1 if you do not have it.</div><div><br></div><div>3) Why do you say it is ill conditioned? Stretched grids? large jumps in material coefficients? Why do you think it is illoconditioned?</div><div><br></div><div><div><div>On Nov 3, 2012, at 1:52 PM, Matthew Knepley <<a href="mailto:knepley@gmail.com">knepley@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">On Sat, Nov 3, 2012 at 1:38 PM, w_ang_temp <span dir="ltr"><<a href="mailto:w_ang_temp@163.com" target="_blank">w_ang_temp@163.com</a>></span> wrote:<br><div class="gmail_extra"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<div>Hello, Matthew</div>
<div> </div>
<div> I just mean that the problem that I am resolving is a finite element problem. The linear system of it is true elliptic equations.</div>
<div>I heared that AMG was an efficient solver, so I just want to have a try about AMG and find that if it is efficient.</div></div></div></blockquote><div><br></div><div>And I meant it when I said, you MUST look it up .Next time you ask us what AMG can do, please include</div>
<div>a reference for a paper in which they are attacking this problem with it and we can help.</div><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<div> By the way, I want to confirm a conception. In my view, AMG itself can be a solver like gmres. It can also be used as a preconditioner </div>
<div>like jacobi and is used by combining with other solver. Is it right? If it is right, how use AMG solver?</div></div></div></blockquote><div><br></div><div>This is true of almost all KSP and PC objects. These are all jsut approximate solvers.</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"><div style="line-height:1.7;font-size:14px;font-family:arial"><div style="line-height:1.7;font-size:14px;font-family:arial">
<div><span style="line-height:1.7"> </span></div>
<div> Thanks.</div>
<div> </div>
<div> Jim<br><br></div>
<div></div>
<div></div>
<div><br></div>>在 2012-11-04 01:21:59，"Matthew Knepley" <<a href="mailto:knepley@gmail.com" target="_blank">knepley@gmail.com</a>> 写道：<br>
<blockquote style="BORDER-LEFT:#ccc 1px solid;MARGIN:0px 0px 0px 0.8ex;PADDING-LEFT:1ex">><On Sat, Nov 3, 2012 at 1:17 PM, w_ang_temp <span dir="ltr"><<a href="mailto:w_ang_temp@163.com" target="_blank">w_ang_temp@163.com</a>></span> wrote:<br>
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<div><span style="LINE-HEIGHT:1.7">>At 2012-11-04 01:08:26,"Jed Brown" <</span><a style="LINE-HEIGHT:1.7" href="mailto:jedbrown@mcs.anl.gov" target="_blank">jedbrown@mcs.anl.gov</a><span style="LINE-HEIGHT:1.7">> wrote:</span><br>
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<blockquote style="BORDER-LEFT:#ccc 1px solid;MARGIN:0px 0px 0px 0.8ex;PADDING-LEFT:1ex"><p>>1. What kind of equation are you solving? AMG is not working well if it takes that many iterations.<br></p><p>>I just deal with the typical soil-water coupled geotechnical problems. It is a typical finite element equation. The matrix is 30000X30000 and ill-conditioned.</p></blockquote></div></blockquote>
<div><br></div>
<div>>We are now at the root of your problem. Solvers do not work on discretizations, they work on equations. No</div>
<div>>solver is designed for "finite elements", and there is no typical finite element problem.</div>
<div><br></div>
<div>>Multigrid works best on elliptic equations with smooth coefficients. Without that, you have to do special things.</div>
<div><br></div>
<div>>I can tell from the above discussion that you have not spent a lot of time researching successful preconditioning</div>
<div>>strategies for your problem in the literature. This is always the first step to building a high performance solver.</div>
<div><br></div>
<div> > Thanks,</div>
<div><br></div>
<div>> Matt</div>
<div> </div>
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<blockquote style="BORDER-LEFT:#ccc 1px solid;MARGIN:0px 0px 0px 0.8ex;PADDING-LEFT:1ex"><p>>2.</p>
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<div>
<div><font color="#ff0000" size="4" face="courier new, monospace"><b> ##########################################################</b></font></div>
<div><font color="#ff0000" size="4" face="courier new, monospace"><b> # #</b></font></div>
<div><font color="#ff0000" size="4" face="courier new, monospace"><b> # WARNING!!! #</b></font></div>
<div><font color="#ff0000" size="4" face="courier new, monospace"><b> # #</b></font></div>
<div><font color="#ff0000" size="4" face="courier new, monospace"><b> # This code was compiled with a debugging option, #</b></font></div>
<div><font color="#ff0000" size="4" face="courier new, monospace"><b> # To get timing results run ./configure #</b></font></div>
<div><font color="#ff0000" size="4" face="courier new, monospace"><b> # using --with-debugging=no, the performance will #</b></font></div>
<div><font color="#ff0000" size="4" face="courier new, monospace"><b> # be generally two or three times faster. #</b></font></div>
<div><font color="#ff0000" size="4" face="courier new, monospace"><b> # #</b></font></div>
<div><font color="#ff0000" size="4" face="courier new, monospace"><b> ##########################################################</b></font></div></div>
<div class="gmail_extra">>It is true a debugging version. And I used the same version dealing with the same problem, one preconditioner is asm<br>>and the other is amg. The time with amg is about 3 times as with asm. I do not know the reason. And I also do not know the </div>
<div class="gmail_extra">>meaning of '[0]PCSetData_AGG bs=1 MM=7601'.<br><br></div></blockquote></div><br><br><span title="neteasefooter"><span></span></span></blockquote></div><br><br clear="all"><span class="HOEnZb"><font color="#888888">
<div><br></div>-- <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></font></span></div>
</blockquote></div></div><br><br><span title="neteasefooter"><span></span></span></blockquote></div><br><br clear="all"><div><br></div>-- <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>
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