<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Thu, Jul 16, 2015 at 8:18 PM, Michele Rosso <span dir="ltr"><<a href="mailto:mrosso@uci.edu" target="_blank">mrosso@uci.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><u></u>
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Barry,<br>
<br>
thank you very much for the detailed answer. I tried what you suggested and it works.<br>
So far I tried on a small system but the final goal is to use it for very large runs. How does PCGAMG compares to PCMG as far as performances and scalability are concerned?<br>
Also, could you help me to tune the GAMG part ( my current setup is in the attached ksp_view.txt file )? <br></div></blockquote><div><br></div><div>I am going to add this to the document today but you can run with -info. This is very noisy so you might want to do the next step at run time. Then grep on GAMG. This will be about 20 lines. Send that to us and we can go from there.</div><div><br></div><div>Mark</div><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div>
<br>
I also tried to use superlu_dist for the LU decomposition on mg_coarse_mg_sub_<br>
-mg_coarse_mg_coarse_sub_pc_type lu<br>
-mg_coarse_mg_coarse_sub_pc_factor_mat_solver_package superlu_dist<br>
<br>
but I got an error:<br>
<br>
****** Error in MC64A/AD. INFO(1) = -2 <br>
****** Error in MC64A/AD. INFO(1) = -2<br>
****** Error in MC64A/AD. INFO(1) = -2<br>
****** Error in MC64A/AD. INFO(1) = -2<br>
****** Error in MC64A/AD. INFO(1) = -2<br>
****** Error in MC64A/AD. INFO(1) = -2<br>
****** Error in MC64A/AD. INFO(1) = -2<br>
symbfact() error returns 0<br>
symbfact() error returns 0<br>
symbfact() error returns 0<br>
symbfact() error returns 0<br>
symbfact() error returns 0<br>
symbfact() error returns 0<br>
symbfact() error returns 0<br>
<br>
<br>
Thank you,<br>
Michele<br>
<br>
<br>
On Thu, 2015-07-16 at 18:07 -0500, Barry Smith wrote:
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<pre><span class="">
<font color="#737373">> On Jul 16, 2015, at 5:42 PM, Michele Rosso <<a href="mailto:mrosso@uci.edu" target="_blank">mrosso@uci.edu</a>> wrote:</font>
<font color="#737373">> </font>
<font color="#737373">> Barry,</font>
<font color="#737373">> </font>
<font color="#737373">> thanks for your reply. So if I want it fixed, I will have to use the master branch, correct?</font>
Yes, or edit mg.c and remove the offending lines of code (easy enough).
<font color="#737373">> </font>
<font color="#737373">> On a side note, what I am trying to achieve is to be able to use how many levels of MG I want, despite the limitation imposed by the local number of grid nodes.</font>
I assume you are talking about with DMDA? There is no generic limitation for PETSc's multigrid, it is only with the way the DMDA code figures out the interpolation that causes a restriction.
<font color="#737373">> So far I am using a borrowed code that implements a PC that creates a sub communicator and perform MG on it.</font>
<font color="#737373">> While reading the documentation I found out that PCMGSetLevels takes in an optional array of communicators. How does this work?</font>
It doesn't work. It was an idea that never got pursued.
<font color="#737373">> Can I can simply define my matrix and rhs on the fine grid as I would do normally ( I do not use kspsetoperators and kspsetrhs ) and KSP would take care of it by using the correct communicator for each level?</font>
No.
You can use the PCMG geometric multigrid with DMDA for as many levels as it works and then use PCGAMG as the coarse grid solver. PCGAMG automatically uses fewer processes for the coarse level matrices and vectors. You could do this all from the command line without writing code.
For example if your code uses a DMDA and calls KSPSetDM() use for example -da_refine 3 -pc_type mg -pc_mg_galerkin -mg_coarse_pc_type gamg -ksp_view
Barry
<font color="#737373">> </font>
<font color="#737373">> Thanks,</font>
<font color="#737373">> Michele</font>
<font color="#737373">> </font>
<font color="#737373">> </font>
<font color="#737373">> </font>
<font color="#737373">> </font>
<font color="#737373">> On Thu, 2015-07-16 at 17:30 -0500, Barry Smith wrote:</font>
<font color="#737373">>> Michel,</font>
<font color="#737373">>> </font>
<font color="#737373">>> This is a very annoying feature that has been fixed in master </font>
</span><font color="#737373">>> <a href="http://www.mcs.anl.gov/petsc/developers/index.html" target="_blank">http://www.mcs.anl.gov/petsc/developers/index.html</a></font><span class="">
<font color="#737373">>> I would like to have changed it in maint but Jed would have a shit-fit :-) since it changes behavior.</font>
<font color="#737373">>> </font>
<font color="#737373">>> Barry</font>
<font color="#737373">>> </font>
<font color="#737373">>> </font>
</span><span class=""><font color="#737373">>> > On Jul 16, 2015, at 4:53 PM, Michele Rosso <<a href="mailto:mrosso@uci.edu" target="_blank">mrosso@uci.edu</a>> wrote:</font>
<font color="#737373">>> > </font>
<font color="#737373">>> > Hi,</font>
<font color="#737373">>> > </font>
<font color="#737373">>> > I am performing a series of solves inside a loop. The matrix for each solve changes but not enough to justify a rebuilt of the PC at each solve.</font>
<font color="#737373">>> > Therefore I am using KSPSetReusePreconditioner to avoid rebuilding unless necessary. The solver is CG + MG with a custom PC at the coarse level.</font>
<font color="#737373">>> > If KSP is not updated each time, everything works as it is supposed to. </font>
<font color="#737373">>> > When instead I allow the default PETSc behavior, i.e. updating PC every time the matrix changes, the coarse level KSP , initially set to PREONLY, is changed into GMRES </font>
<font color="#737373">>> > after the first solve. I am not sure where the problem lies (my PC or PETSc), so I would like to have your opinion on this.</font>
<font color="#737373">>> > I attached the ksp_view for the 2 successive solve and the options stack.</font>
<font color="#737373">>> > </font>
<font color="#737373">>> > Thanks for your help,</font>
<font color="#737373">>> > Michel</font>
<font color="#737373">>> > </font>
<font color="#737373">>> > </font>
<font color="#737373">>> > </font>
<font color="#737373">>> > <ksp_view.txt><petsc_options.txt></font>
<font color="#737373">>> </font>
<font color="#737373">>> </font>
<font color="#737373">>> </font>
<font color="#737373">> </font>
</span></pre>
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