<html><head><meta http-equiv="Content-Type" content="text/html; charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><div class=""><br class=""></div> Loic,<div class=""><br class=""></div><div class=""> From your code it looks like you are using a DM. Is it a DMDA or a shell DM? If it is a DMDA then the process is intended to be pretty straightforward. PCTELESCOPE should create a new DMDA that has the same properties as the coarse grid DMDA but lives on a smaller number of ranks. From this you can just provide multiple levels of coarsening of the DMDA to produce the smaller multigrid problems. </div><div class=""><br class=""></div><div class=""> Can you let us know more details of what exactly goes wrong with attempts? It is likely straightforward to get things working, perhaps due to our unclear documentation.</div><div class=""><br class=""></div><div class=""> Barry</div><div class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On May 16, 2022, at 3:28 PM, Loic Gouarin <<a href="mailto:loic.gouarin@polytechnique.edu" class="">loic.gouarin@polytechnique.edu</a>> wrote:</div><br class="Apple-interchange-newline"><div class="">
<meta http-equiv="content-type" content="text/html; charset=UTF-8" class="">
<div text="#000000" bgcolor="#FFFFFF" class=""><p class="">Hello,</p><p class="">I could have posted my message on the user list but it seems to
me that it's more in the petsc development part. Don't hesitate to
tell me if I'm wrong.<br class="">
I am developing a code called cafes that studies fluid-particle
interactions in a Stokes fluid. For that, we implemented the whole
solver in matrix free on a cartesian grid. The preconditioner is
naturally a geometrical multigrid for the velocity. The set of
matrix free for each level is set by hand (I worked with Matthew
on this a long time ago now!). <br class="">
I would have liked to implement telescope on this part. I started
it but I didn't get out of it. So I would like to have your help.<br class="">
What I tried to do was to describe my classical multigrid up to a
level N1 and then take the coarse solver and apply the telescope
preconditioner to it up to a level N2. I don't know if this is the
best way but it seemed the most intuitive. The problem is that I
could not get the coarse matrix because the setup is not done yet
(I use a DMKSPSetComputeOperators for the matrices). <br class="">
</p><p class="">The construction of the matrix free for each level is here:
<a class="moz-txt-link-freetext" href="https://github.com/gouarin/cafes/blob/master/cafes/problem/stokes.hpp#L59-L88">https://github.com/gouarin/cafes/blob/master/cafes/problem/stokes.hpp#L59-L88</a></p><p class="">The description of the Stokes preconditioner is described here :
<a class="moz-txt-link-freetext" href="https://github.com/gouarin/cafes/blob/master/cafes/problem/stokes.hpp#L109-L162">https://github.com/gouarin/cafes/blob/master/cafes/problem/stokes.hpp#L109-L162</a></p><p class="">I have tried multiple implementations without success. For me,
the start is to add at line 105 something like</p>
<div style="color: #d4d4d4;background-color: #1e1e1e;font-family: Menlo, Monaco, 'Courier New', monospace;font-weight: normal;font-size: 12px;line-height: 18px;white-space: pre;" class=""><div class=""><span style="color: #57a64a;" class="">KSP coarse;</span></div><div class=""><span style="color: #57a64a;" class="">ierr = PCMGGetCoarseSolve(pc_i, &coarse);CHKERRQ(ierr);</span></div><span style="color: #57a64a;" class="">
</span></div>
<div style="color: #d4d4d4;background-color: #1e1e1e;font-family: Menlo, Monaco, 'Courier New', monospace;font-weight: normal;font-size: 12px;line-height: 18px;white-space: pre;" class=""><span style="color: #57a64a;" class="">PC pc_coarse;</span><div class=""><span style="color: #57a64a;" class="">ierr = KSPGetPC(coarse, &pc_coarse);CHKERRQ(ierr);</span></div><div class=""><span style="color: #57a64a;" class="">ierr = PCSetType(pc_coarse, PCTELESCOPE);CHKERRQ(ierr);</span></div></div>
<div class=""><span style="color: #57a64a;" class=""><br class="">
</span></div>
<div class="">Then, to create the several matrices of the levels of
TELESCOPE, I thought using the same process as before with
DMKSPSetComputeOperators. But it doesn't work...<br class="">
</div>
<div class="">Could you tell me how I can make it work ?</div>
<div class=""><br class="">
</div>
<div class="">Thanks,</div>
<div class="">Loic<br class="">
</div>
<div class=""><br class="">
</div>
<div class=""><span style="color: #57a64a;" class=""></span></div>
</div>
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