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<div class="moz-cite-prefix"> Thanks for replies. Here I attached
the log_summary for the large and small problems. The DoFs for the
large problem is 4 times of that for the small problem. Few
observations are listed here:<br>
1, the total number of iterations does not change much from the
small problem to the large one;<br>
2, the time elapsed for KSPSolve() for the large problem is less
than 4 times of that for the small problem;<br>
3, the time elapsed for PCSet() for the large problem is more than
10 times of that for the small problem;<br>
4, the time elapsed for PCGAMGProl_AGG for the large problem is
more than 20 times of that for the small problem;<br>
In my code, I have solved the Poisson equation for twice with
difference RHS; however, the observation above is almost
consistent for these two times.<br>
Do these observation indicate that I should switch my PC from
GAMG to MG for solving Poisson equation in a single process?<br>
<br>
best,<br>
Alan<br>
<br>
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<blockquote
cite="mid:CAMYG4GnQ1quSBomBtEyWg8cdKt00VUm4_=98qNxY1BJs1LxU5g@mail.gmail.com"
type="cite">
<div dir="ltr">On Tue, Aug 6, 2013 at 2:31 PM, Karl Rupp <span
dir="ltr"><<a moz-do-not-send="true"
href="mailto:rupp@mcs.anl.gov" target="_blank">rupp@mcs.anl.gov</a>></span>
wrote:<br>
<div class="gmail_extra">
<div class="gmail_quote">
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">Hi
Alan,<br>
<br>
please use -log_summary to get profiling information on
the run. What is<br>
the bottleneck? Is it the number of solver iterations
increasing<br>
significantly? If so, consider changing the preconditioner
options (more<br>
levels!). I don't expect a direct solver to be any faster
in the 180k<br>
case for a Poisson problem.<br>
</blockquote>
<div><br>
</div>
<div>Mudpack is geometric multigrid: <a
moz-do-not-send="true"
href="http://www2.cisl.ucar.edu/resources/legacy/mudpack">http://www2.cisl.ucar.edu/resources/legacy/mudpack</a></div>
<div>
This should be faster.</div>
<div><br>
</div>
<div> Matt</div>
<div> </div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">Best
regards,<br>
Karli<br>
<div class="">
<div class="h5"><br>
<br>
On 08/06/2013 02:22 PM, Alan wrote:<br>
> Dear all,<br>
> I hope you're having a nice day.<br>
> I have a quick question on solving Poisson
equation with KSP solvers<br>
> (/src/ksp/ksp/example/tutorial/ex29.c).
Currently, I run this solver with:<br>
> -pc_type gamg -ksp_type cg -pc_gamg_agg_nsmooths
1 -mg_levels_ksp_max_it<br>
> 1 -mg_levels_ksp_type richardson -ksp_rtol 1.0e-7<br>
> It performs very well in parallel computation and
scalability is fine.<br>
> However, if I run it with a single process, the
KSP solver is much<br>
> slower than direct ones, i.e. Mudpack. Briefly,
the speed difference<br>
> between the KSP solver and the direct solver is
negligible on dealing<br>
> with small problems (i.e.36k DoFs ) but becomes
very huge for moderate<br>
> large problems (i.e. 180k DoFs). Although the
direct solver inherently<br>
> has better performance for moderate large
problems in the single<br>
> process, I wonder if any setup or approach can
improve the performance<br>
> of this KSP Poisson solver with the single
process? or even make it<br>
> obtain competitive speed (a little bit slower is
fine) against direct<br>
> solvers.<br>
><br>
> thanks in advance,<br>
> Alan<br>
><br>
<br>
</div>
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<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
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