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Barry,<BR>
<BR>
thank you. I think in few days I will run a larger test and I will send the log to have better insight.<BR>
Thank you for now.<BR>
<BR>
Michele<BR>
<BR>
<BR>
On Thu, 2015-07-16 at 19:48 -0500, Barry Smith wrote:
<BLOCKQUOTE TYPE=CITE>
<PRE>
<FONT COLOR="#737373">> On Jul 16, 2015, at 7:18 PM, Michele Rosso <<A HREF="mailto:mrosso@uci.edu">mrosso@uci.edu</A>> wrote:</FONT>
<FONT COLOR="#737373">> </FONT>
<FONT COLOR="#737373">> Barry,</FONT>
<FONT COLOR="#737373">> </FONT>
<FONT COLOR="#737373">> thank you very much for the detailed answer. I tried what you suggested and it works.</FONT>
<FONT COLOR="#737373">> 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?</FONT>
Algebraic multigrid has much larger setup times than geometric multigrid, but since it is is running on a much smaller problem with geometric multigrid on top of it that is much less of an issue.
<FONT COLOR="#737373">> Also, could you help me to tune the GAMG part ( my current setup is in the attached ksp_view.txt file )? </FONT>
The defaults are probably fine.
<FONT COLOR="#737373">> </FONT>
<FONT COLOR="#737373">> I also tried to use superlu_dist for the LU decomposition on mg_coarse_mg_sub_</FONT>
<FONT COLOR="#737373">> -mg_coarse_mg_coarse_sub_pc_type lu</FONT>
<FONT COLOR="#737373">> -mg_coarse_mg_coarse_sub_pc_factor_mat_solver_package superlu_dist</FONT>
Yeah, gamg tries to put all the rows of the matrix on one process and leave the other processes empty, I suspect superlu_dist doesn't like empty matrices. Just use -mg_coarse_mg_coarse_sub_pc_type lu it is PETSc's native sequential sparse solver and will run faster than superlu_dist anyways except for really large coarse problems which you don't want anyway.
How many iterations is the multigrid using?
Instead of using -mg_coarse_pc_type some of the PETSc guys are fans of -mg_coarse_pc_type chebyshev which may decrease the iterations slightly.
You can run with -log_summary and send the output, this shows where the time is being spent.
Barry
<FONT COLOR="#737373">> </FONT>
<FONT COLOR="#737373">> but I got an error:</FONT>
<FONT COLOR="#737373">> </FONT>
<FONT COLOR="#737373">> ****** Error in MC64A/AD. INFO(1) = -2 </FONT>
<FONT COLOR="#737373">> ****** Error in MC64A/AD. INFO(1) = -2</FONT>
<FONT COLOR="#737373">> ****** Error in MC64A/AD. INFO(1) = -2</FONT>
<FONT COLOR="#737373">> ****** Error in MC64A/AD. INFO(1) = -2</FONT>
<FONT COLOR="#737373">> ****** Error in MC64A/AD. INFO(1) = -2</FONT>
<FONT COLOR="#737373">> ****** Error in MC64A/AD. INFO(1) = -2</FONT>
<FONT COLOR="#737373">> ****** Error in MC64A/AD. INFO(1) = -2</FONT>
<FONT COLOR="#737373">> symbfact() error returns 0</FONT>
<FONT COLOR="#737373">> symbfact() error returns 0</FONT>
<FONT COLOR="#737373">> symbfact() error returns 0</FONT>
<FONT COLOR="#737373">> symbfact() error returns 0</FONT>
<FONT COLOR="#737373">> symbfact() error returns 0</FONT>
<FONT COLOR="#737373">> symbfact() error returns 0</FONT>
<FONT COLOR="#737373">> symbfact() error returns 0</FONT>
<FONT COLOR="#737373">> </FONT>
<FONT COLOR="#737373">> </FONT>
<FONT COLOR="#737373">> Thank you,</FONT>
<FONT COLOR="#737373">> Michele</FONT>
<FONT COLOR="#737373">> </FONT>
<FONT COLOR="#737373">> </FONT>
<FONT COLOR="#737373">> On Thu, 2015-07-16 at 18:07 -0500, Barry Smith wrote:</FONT>
<FONT COLOR="#737373">>> > On Jul 16, 2015, at 5:42 PM, Michele Rosso <<A HREF="mailto:mrosso@uci.edu">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>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> Yes, or edit mg.c and remove the offending lines of code (easy enough). </FONT>
<FONT COLOR="#737373">>> </FONT>
<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>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> 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>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> </FONT>
<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>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> It doesn't work. It was an idea that never got pursued.</FONT>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> </FONT>
<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>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> No.</FONT>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> 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. </FONT>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> 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 </FONT>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> Barry</FONT>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> </FONT>
<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>
<FONT COLOR="#737373">>> >> <A HREF="http://www.mcs.anl.gov/petsc/developers/index.html">http://www.mcs.anl.gov/petsc/developers/index.html</A></FONT>
<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>
<FONT COLOR="#737373">>> >> > On Jul 16, 2015, at 4:53 PM, Michele Rosso <<A HREF="mailto:mrosso@uci.edu">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>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">>> </FONT>
<FONT COLOR="#737373">> </FONT>
<FONT COLOR="#737373">> <ksp_view.txt></FONT>
</PRE>
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