<div dir="ltr">From what you're describing, it sounds like the solver you're using is GMRES (if you are using the default), preconditioned with fieldsplit with nested CG-Jacobi solves. That is, your preconditioner involves inner CG solves on each field, so is a much "heavier". This seems consistent with your observation of fewer (outer) Krylov iterations but much more work being done per iteration.<div><br></div><div>This should all be visible with -ksp_view. <div><br></div><div>Do you see what you expect if, instead of CG/Jacobi on each block, you use Preonly/Jacobi on each block?</div></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Nov 7, 2017 at 2:43 PM, Bernardo Rocha <span dir="ltr"><<a href="mailto:bernardomartinsrocha@gmail.com" target="_blank">bernardomartinsrocha@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">Hello everyone,</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small"><br></div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">I have a general question about the performance of the PCFieldSplit</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">that I'm not sure if I understood properly.</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small"><br></div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">Consider a simple Poisson problem discretized by FEM into a system Ax=b </div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">which is then solved by CG and Jacobi.</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small"><br></div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">Then, I create a "vectorial Poisson" problem by simply adding another block</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">of this problem to create a block-like version of it. </div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">Something like</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">[ [A, 0]</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">  [0, A]] </div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">then I create a PCFieldSplit with CG and Jacobi for each block.</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small"><br></div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">Either with additive or multiplicative fieldsplit, the PC is much better and solves it </div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">with fewer iterations than the scalar case. However, the execution time taken by</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">the PCFieldSplit is much bigger than the simple Jacobi for the scalar case.</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small"><br></div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">(From -log_view I see all the time difference in PCApply)</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small"><br></div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">Why is this happening?</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small"><br></div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">Best regards,</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small">Bernardo</div><div class="gmail_default" style="font-family:arial,helvetica,sans-serif;font-size:small"><br></div></div>
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