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<p>Matt, Lawrence</p>
<p><br>
The same problem happens when using gmres with rtol 1e-6 in the<br>
schur complement (attachment "left_schur"). I'm not sure what<br>
this tells us. If I understand Lawrence correctly, the null space<br>
may be attached to the wrong matrix (A11 instead of Sp)?<br>
</p>
<p><br>
Chris<br>
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<font style="font-family:Verdana;font-size:8pt;color:#7D7D7D;font-weight:bold;">dr. ir.</font><font style="font-family:Verdana;font-size:8pt;color:#7D7D7D;font-weight:bold;">
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<div id="divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri, sans-serif" color="#000000"><b>From:</b> Matthew Knepley <knepley@gmail.com><br>
<b>Sent:</b> Wednesday, March 22, 2017 5:39 PM<br>
<b>To:</b> Klaij, Christiaan<br>
<b>Cc:</b> petsc-users@mcs.anl.gov<br>
<b>Subject:</b> Re: [petsc-users] left and right preconditioning with a constant null space</font>
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<div class="gmail_quote">On Wed, Mar 22, 2017 at 4:04 PM, Klaij, Christiaan <span dir="ltr">
<<a href="mailto:C.Klaij@marin.nl" target="_blank">C.Klaij@marin.nl</a>></span> wrote:<br>
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<p>Thanks Matt, I will try your suggestion and let you know. In the<br>
meantime this is what I did to set the constant null space:<br>
<br>
call MatNullSpaceCreate(MPI_COMM_<wbr>WORLD,PETSC_TRUE,0,PETSC_NULL_<wbr>OBJECT,nullsp,ierr); CHKERRQ(ierr)<br>
call MatSetNullSpace(aa_sub(4),<wbr>nullsp,ierr); CHKERRQ(ierr)<br>
call MatNullSpaceDestroy(nullsp,<wbr>ierr); CHKERRQ(ierr)<br>
<br>
where aa_sub(4) corresponds to A11. This is called before<br>
begin/end mat assembly.</p>
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</blockquote>
<div>Hmm, I wonder if the problem is that A11 has the nullspace, but the PC pmat is actually B^T diag(A)^{-1} B. I have</div>
<div>to look where the solver is actually taking the nullspace from. We need to improve the -ksp_view output to make this</div>
<div>stuff much easier to see.</div>
<div><br>
</div>
<div> Matt</div>
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<p>Chris</p>
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<font style="font-family:Verdana; font-size:8pt; color:#7d7d7d; font-weight:bold">dr. ir.</font><font style="font-family:Verdana; font-size:8pt; color:#7d7d7d; font-weight:bold">
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<div id="m_8210822296635488072divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri, sans-serif" color="#000000"><b>From:</b> Matthew Knepley <<a href="mailto:knepley@gmail.com" target="_blank">knepley@gmail.com</a>><br>
<b>Sent:</b> Wednesday, March 22, 2017 4:47 PM<br>
<b>To:</b> Klaij, Christiaan<br>
<b>Cc:</b> <a href="mailto:petsc-users@mcs.anl.gov" target="_blank">petsc-users@mcs.anl.gov</a><br>
<b>Subject:</b> Re: [petsc-users] left and right preconditioning with a constant null space</font>
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<div class="gmail_extra">
<div class="gmail_quote">On Wed, Mar 22, 2017 at 2:58 PM, Klaij, Christiaan <span dir="ltr">
<<a href="mailto:C.Klaij@marin.nl" target="_blank">C.Klaij@marin.nl</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex; border-left:1px #ccc solid; padding-left:1ex">
I'm solving the Navier-Stokes equations using PCFieldSplit type<br>
Schur and Selfp. This particular case has only Neumann conditions<br>
for the pressure field.<br>
<br>
With left preconditioning and no nullspace, I see that the KSP<br>
solver for S does not converge (attachment "left_nonullsp") in<br>
either norm.<br>
<br>
When I attach the constant null space to A11, it gets passed on<br>
to S and the KSP solver for S does converge in the preconditioned<br>
norm only (attachment "left").<br>
<br>
However, right preconditioning uses the unpreconditioned norm and<br>
therefore doesn't converge (attachment "right"), regardless of<br>
whether the nullspace is attached or not. Should I conclude that<br>
right preconditioning cannot be used in combination with a null<br>
space?<br>
</blockquote>
<div><br>
</div>
<div>No, neither of your solves is working. The left preconditioned version is just hiding that fact.</div>
<div><br>
</div>
<div>You should start by checking that the exact solve works. Namely full Schur factorization</div>
<div>with exact solves for A and S. Since you do not have a matrix for S (unless you tell it do</div>
<div>use "full"), just use a very low (1e-10) tolerance. My guess is that something is off with</div>
<div>your null space specification.</div>
<div><br>
</div>
<div> Thanks,</div>
<div><br>
</div>
<div> Matt</div>
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Chris<br>
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