<div><div dir="auto">I think your run with -pc_type mg is defining a multigrid hierarchy with a only single level. (A single level mg PC would also explain the 100+ iterations required to converge.) The gamg configuration is definitely coarsening your problem and has a deeper hierarchy. A single level hierarchy will require less memory than a multilevel hierarchy. </div></div><div dir="auto"><br></div><div dir="auto">Cheers,</div><div dir="auto">Dave</div><div><br><div class="gmail_quote"><div dir="ltr">On Wed 24. Nov 2021 at 19:03, Matthew Knepley <<a href="mailto:knepley@gmail.com">knepley@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div dir="ltr">On Wed, Nov 24, 2021 at 12:26 PM Karthikeyan Chockalingam - STFC UKRI <<a href="mailto:karthikeyan.chockalingam@stfc.ac.uk" target="_blank">karthikeyan.chockalingam@stfc.ac.uk</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">
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<p class="MsoNormal">Hello,<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">I would like to understand why more memory is consumed by -pc_type gamg compared to -pc_type mg for the same problem size<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">ksp/ksp/tutorial: ./ex45 -da_grid_x 368 -da_grid_x 368 -da_grid_x 368 -ksp_type cg<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">-pc_type mg<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Maximum (over computational time) process memory: total 1.9399e+10 max 9.7000e+09 min 9.6992e+09<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">-pc_type gamg<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Maximum (over computational time) process memory: total 4.9671e+10 max 2.4836e+10 min 2.4835e+10<u></u><u></u></p>
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<u></u><u></u></p>
<p class="MsoNormal">Am I right in understanding that the memory limiting factor is ‘max 2.4836e+10’ as it is the maximum memory used at any given time?</p></div></div></blockquote><div><br></div><div>Yes, I believe so.</div><div><br></div><div>GAMG is using A_C = P^T A P, where P is the prolongation from coarse to fine, in order to compute the coarse operator A_C, rather than</div><div>rediscretization, since it does not have any notion of discretization or coarse meshes. This takes more memory.</div><div><br></div><div> Thanks,</div><div><br></div><div> Matt</div></div></div><div dir="ltr"><div class="gmail_quote"><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div lang="EN-GB"><div><p class="MsoNormal"><u></u><u></u></p>
<p class="MsoNormal">I have attached the -log_view output of both the preconditioners.
<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">Best regards,<u></u><u></u></p>
<p class="MsoNormal">Karthik.<u></u><u></u></p>
<p class="MsoNormal"><u></u> <u></u></p>
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</blockquote></div><br clear="all"><div><br></div>-- <br><div dir="ltr"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><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/~knepley/" target="_blank">https://www.cse.buffalo.edu/~knepley/</a><br></div></div></div></div></div></div></div></div>
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