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<span style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">Hi Matt,</span>
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Thanks for the quick response!</div>
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> I cannot understand precisely what is happening here. You specify a restart</div>
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> size when you setup the KSP. It allocates that many vecs (roughly). Why are</div>
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> there reallocations? Do you increase the restart size during the iteration?</div>
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I don't believe there are any reallocations (I didn't write this solver, but I</div>
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don't see any calls which set the restart size or any other relevant parameter [1]):</div>
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as far as I can tell, the solver just allocates a lot of vectors. I'm working</div>
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off of traces computed by heaptrack, which is my only insight into how this</div>
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works. The allocations come from KSPCreateVecs(), which is called by</div>
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1. KSPFGMRESGetNewVectors() (for about 1.7 GB [2] of memory)</div>
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2. KSPSetUp_GMRES() (for about 300 MB of memory)</div>
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3. KSPSetUp_FGMRES() (for about 264 MB of memory)</div>
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4. KSPSetWorkVecs() (for about 236 MB of memory)</div>
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Is there some relevant set of monitoring flags I can set which will show me how</div>
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many vectors I allocate or use? That would also help.</div>
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Best,</div>
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David</div>
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[1] This is IBAMR's PETScKrylovLinearSolver.</div>
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[2] This is half the total memory we use for side-centered data vectors.</div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Matthew Knepley <knepley@gmail.com><br>
<b>Sent:</b> Tuesday, February 10, 2026 6:28 PM<br>
<b>To:</b> Wells, David <drwells@email.unc.edu><br>
<b>Cc:</b> petsc-users@mcs.anl.gov <petsc-users@mcs.anl.gov><br>
<b>Subject:</b> Re: [petsc-users] Limiting the number of vectors allocated at a time by fgmres etc.</font>
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<div dir="ltr">On Tue, Feb 10, 2026 at 5:32 PM Wells, David via petsc-users <<a href="mailto:petsc-users@mcs.anl.gov">petsc-users@mcs.anl.gov</a>> wrote:</div>
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Hello,</div>
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I've been profiling the memory usage of my solver and it looks like a huge number (roughly half) of allocations are from KSPFGMRESGetNewVectors(). I read through the source code and it looks like these vectors are allocated ten at a time (FGMRES_DELTA_DIRECTIONS)
in a couple of places inside that KSP. Is there a way to change this value?</div>
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<div>We could add an option to change this delta. Actually theory suggests that a constant is not optimal, but rather we should double the number each time. I would also be willing to code that.</div>
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If not - how hard would it be to add an API to set a different initial value for that? These vectors take up a lot of memory and I would rather just one at a time.</div>
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<div>I cannot understand precisely what is happening here. You specify a restart size when you setup the KSP. It allocates that many vecs (roughly). Why are there reallocations? Do you increase the restart size during the iteration?</div>
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<div> Thanks,</div>
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<div> Matt</div>
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Best,</div>
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David Wells</div>
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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>
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<div><a href="https://urldefense.us/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!G_uCfscf7eWS!aBTZ4oaySeEsyg3U98-DZfMmoj0Wg8RUNEzlPdQoEnZ6pfCuP3cZSF7ib3bqpT7GBGVct81F2vjmak_Zva9875mpIc0$" target="_blank">https://www.cse.buffalo.edu/~knepley/</a><br>
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