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Dear Mark, Dear Matthew, <br>
<br>
in order to discard load imbalance as the cause of the reported weak
scaling issue in the GAMG preconditioner <br>
set-up stage (as you said, we were feeding GAMG with a suboptimal
mesh distribution, having empty processors, <br>
among others), we simplified the weak scaling test by considering
the standard body-fitted trilinear (Q1) FE discretization <br>
of the 3D Poisson problem on a unit cube discretized with a <b><i>uniform,
structured hexahedral mesh,</i></b> <i><b>partitioned</b></i><i><b><br>
</b></i><i><b>optimally (by hand) among processors</b></i>, with a
fixed load/core of 30**3 hexahedra/core. Thus, all processors have
the same load<br>
(up-to strong Dirichlet boundary conditions on the subdomains
touching the global boundary), and the edge-cut is minimum.<br>
<br>
We used the following GAMG preconditioner options:<br>
<br>
-pc_type gamg<br>
-pc_gamg_type agg<br>
-pc_gamg_est_ksp_type cg<br>
-mg_levels_esteig_ksp_type cg<br>
-mg_coarse_sub_pc_type cholesky<br>
-mg_coarse_sub_pc_factor_mat_ordering_type nd<br>
-pc_gamg_process_eq_limit 50<br>
-pc_gamg_square_graph 10<br>
-pc_gamg_agg_nsmooths 1<br>
<br>
The results that we obtained for 48 (4x4x3 subdomains), 10,368
(24x24x18 subdomains), and 16,464 <br>
(28x28x21 subdomains) CPU cores are as follows:<br>
<br>
**preconditioner set up**<br>
[0.9844961860, <b>7.017674042</b>, <b>12.10154881</b>]<br>
<br>
**PCG stage**<br>
[0.5849160422, 1.515251888, 1.859617710]<br>
<br>
**number of PCG iterations**<br>
[9,14,15]<br>
<br>
As you can observe, <b>there is still a significant time increase
when scaling the problem from 48 to 10K/16K MPI tasks</b><b><br>
</b><b>for the preconditioner setup stage. </b>This time increase
is not as significant for the PCG stage.<b> </b>Please find
attached the combined <br>
output of -ksp_view and -log_view for these three points of the weak
scaling curve.<br>
<br>
Given these results, I am starting to suspect that something within
the underlying software + hardware stack might be<br>
responsible for this. I am using OpenMPI 1.10.7 + Intel compilers
version 18.0. The underlying supercomputer is MN-IV at <br>
BSC
(<a class="moz-txt-link-freetext" href="https://www.bsc.es/marenostrum/marenostrum/technical-information">https://www.bsc.es/marenostrum/marenostrum/technical-information</a>).
Have you ever conducted a weak scaling test<br>
of GAMG with OpenMPI on a similar computer architecture? Can you
share your experience with us? <br>
(versions tested, outcome, etc.)<br>
<br>
We also tried an alternative MPI library, Intel(R) MPI Library for
Linux* OS, Version 2018 Update 4 Build 20180823 (id: 18555),<br>
<b>without success. </b>For this MPI library, the preconditioner
set-up stage crashes (find attached stack frames, and internal MPI
library<br>
errors) for the largest two core counts (it did not crash for the 48
CPU cores case), while it did not crash with OpenMPI 1.10.7.<br>
Have you ever experienced errors like the ones<br>
attached? Is there anyway to set up PETSc such that the subroutine
that crashes is replaced by an alternative implementation of<br>
the same concept? (this would be just a workaround). It might be a
BUG in the Intel MPI library, although I cannot confirm it. We also
got <br>
these errors with the unfitted FEM+space-filling curves version of
our code.<br>
<br>
Thanks a lot for your help and valuable feedback!<br>
Best regards,<br>
Alberto.<br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
<div class="moz-cite-prefix">On 08/11/18 17:29, Mark Adams wrote:<br>
</div>
<blockquote
cite="mid:CADOhEh57G=gxTBb506iNiRC1_zJSayfs6ryA4DqorpJVw=hAcw@mail.gmail.com"
type="cite">
<div dir="ltr">
<div class="gmail_quote">
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF"><br>
I did not configured PETSc with ParMetis support. Should
I? <br>
<br>
I figured it out when I tried to use
"-pc_gamg_repartition". PETSc complained that it was not
compiled with ParMetis support.</div>
</blockquote>
<div><br>
</div>
<div>You need ParMetis, or some parallel mesh partitioner,
configured to use repartitioning. I would guess that
"-pc_gamg_repartition" would not help and might hurt,
because it just does the coarse grids, not the fine grid.
But it is worth a try. Just configure with
--download-parmetis</div>
<div><br>
</div>
<div>The problem is that you are using space filling curves on
the background grid and are getting empty processors.
Right? The mesh setup phase is not super optimized, but
your times</div>
<div><br>
</div>
<div>And you said in your attachment that you added the near
null space, but just the constant vector. I trust you mean
the three translational rigid body modes. That is the
default and so you should not see any difference. If you
added one vector of all 1s then that would be bad. You also
want the rotational rigid body modes. Now, you are
converging pretty well and if your solution does not have
much rotation in it the the rotational modes are not needed,
but they are required for optimality in general.</div>
<div> <br>
</div>
</div>
</div>
</blockquote>
<br>
<pre class="moz-signature" cols="72">--
Alberto F. Martín-Huertas
Senior Researcher, PhD. Computational Science
Centre Internacional de Mètodes Numèrics a l'Enginyeria (CIMNE)
Parc Mediterrani de la Tecnologia, UPC
Esteve Terradas 5, Building C3, Office 215,
08860 Castelldefels (Barcelona, Spain)
Tel.: (+34) 9341 34223
<a class="moz-txt-link-abbreviated" href="mailto:e-mail:amartin@cimne.upc.edu">e-mail:amartin@cimne.upc.edu</a>
FEMPAR project co-founder
web: <a class="moz-txt-link-freetext" href="http://www.fempar.org">http://www.fempar.org</a>
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</pre>
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