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<p>Hi Matt,</p>
<p>I plotted the memory scalings using different threshold values.
The two scalings are slightly translated (from -22 to -88 mB) but
this gain is neglectable. The 3.6-scaling keeps being robust while
the 3.10-scaling deteriorates.</p>
<p>Do you have any other suggestion?</p>
<p>Thanks<br>
</p>
Myriam <br>
<br>
<div class="moz-cite-prefix">Le 03/02/19 à 02:27, Matthew Knepley a
écrit :<br>
</div>
<blockquote type="cite"
cite="mid:CAMYG4GkPNv4k501_+CpB2yXtxV812PgJLxPLKkvM3zx=3_mLjg@mail.gmail.com">
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<div dir="ltr">On Fri, Mar 1, 2019 at 10:53 AM Myriam
Peyrounette via petsc-users <<a
href="mailto:petsc-users@mcs.anl.gov"
moz-do-not-send="true">petsc-users@mcs.anl.gov</a>>
wrote:<br>
</div>
<div class="gmail_quote">
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">Hi,<br>
<br>
I used to run my code with PETSc 3.6. Since I upgraded the
PETSc version<br>
to 3.10, this code has a bad memory scaling.<br>
<br>
To report this issue, I took the PETSc script ex42.c and
slightly<br>
modified it so that the KSP and PC configurations are the
same as in my<br>
code. In particular, I use a "personnalised" multi-grid
method. The<br>
modifications are indicated by the keyword "TopBridge" in
the attached<br>
scripts.<br>
<br>
To plot the memory (weak) scaling, I ran four calculations
for each<br>
script with increasing problem sizes and computations
cores:<br>
<br>
1. 100,000 elts on 4 cores<br>
2. 1 million elts on 40 cores<br>
3. 10 millions elts on 400 cores<br>
4. 100 millions elts on 4,000 cores<br>
<br>
The resulting graph is also attached. The scaling using
PETSc 3.10<br>
clearly deteriorates for large cases, while the one using
PETSc 3.6 is<br>
robust.<br>
<br>
After a few tests, I found that the scaling is mostly
sensitive to the<br>
use of the AMG method for the coarse grid (line 1780 in<br>
main_ex42_petsc36.cc). In particular, the performance
strongly<br>
deteriorates when commenting lines 1777 to 1790 (in
main_ex42_petsc36.cc).<br>
<br>
Do you have any idea of what changed between version 3.6
and version<br>
3.10 that may imply such degradation?<br>
</blockquote>
<div><br>
</div>
<div>I believe the default values for PCGAMG changed between
versions. It sounds like the coarsening rate</div>
<div>is not great enough, so that these grids are too large.
This can be set using:</div>
<div><br>
</div>
<div> <a
href="https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCGAMGSetThreshold.html"
moz-do-not-send="true">https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCGAMGSetThreshold.html</a></div>
<div><br>
</div>
<div>There is some explanation of this effect on that page.
Let us know if setting this does not correct the
situation.</div>
<div><br>
</div>
<div> Thanks,</div>
<div><br>
</div>
<div> Matt</div>
<div> </div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">
Let me know if you need further information.<br>
<br>
Best,<br>
<br>
Myriam Peyrounette<br>
<br>
<br>
-- <br>
Myriam Peyrounette<br>
CNRS/IDRIS - HLST<br>
--<br>
<br>
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-- <br>
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<div>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</div>
<div><br>
</div>
<div><a
href="http://www.cse.buffalo.edu/%7Eknepley/"
target="_blank" moz-do-not-send="true">https://www.cse.buffalo.edu/~knepley/</a><br>
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<br>
<pre class="moz-signature" cols="72">--
Myriam Peyrounette
CNRS/IDRIS - HLST
--
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