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Thanks Matt, that was my guess actually.<br>
<br>
On 09.01.2012 17:46, Matthew Knepley wrote:
<blockquote
cite="mid:CAMYG4GkWbMuE6hNWEEQOqtkZzjUdzmg28wzrpm4u7wp1hDTwzg@mail.gmail.com"
type="cite">On Mon, Jan 9, 2012 at 10:37 AM, Alexander Grayver <span
dir="ltr"><<a moz-do-not-send="true"
href="mailto:agrayver@gfz-potsdam.de">agrayver@gfz-potsdam.de</a>></span>
wrote:<br>
<div class="gmail_quote">
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;">
<div bgcolor="#ffffff" text="#000000"> I see this is due to
preconditioner, but could you explain shortly what
particularly causes this problem? It's not obvious to me.<br>
</div>
</blockquote>
<div><br>
</div>
<div>The size of the block being factorized by ILU(0) decreases
as p increases. Thus, it is</div>
<div>a weaker preconditioner. I recommend Yousef Saad's book
which explains all this clearly.</div>
<div><br>
</div>
<div> Matt</div>
<div> </div>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt
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<div bgcolor="#ffffff" text="#000000"> Thanks.<br>
<br>
On 09.01.2012 17:36, Matthew Knepley wrote:
<blockquote type="cite">On Mon, Jan 9, 2012 at 10:31 AM,
Alexander Grayver <span dir="ltr"><<a
moz-do-not-send="true"
href="mailto:agrayver@gfz-potsdam.de" target="_blank">agrayver@gfz-potsdam.de</a>></span>
wrote:<br>
<div class="gmail_quote">
<blockquote class="gmail_quote" style="margin: 0pt 0pt
0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;"> I have tested default
GMRES+ILU(0) solver for problem of size ~200000 and I
observed that both convergence rate and accuracy
degrade when I increase number of processes.<br>
E.g., number of iterations almost doubles when going
from 1 to 4 processors.<br>
<br>
I noticed that ILU(0) is somehow represented by
sequential matrices internally and feel that this
might be a reason?<br>
</blockquote>
<div><br>
</div>
<div>The default is Block Jacobi+ILU(0) and GMRES(30)
which will definitely degrade as the number of</div>
<div>processes is increased. You would need to use an
optimal preconditioner like Multigrid if you want</div>
<div>a constant number of iterates.</div>
<div><br>
</div>
<div> Matt</div>
<div> </div>
<blockquote class="gmail_quote" style="margin: 0pt 0pt
0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204);
padding-left: 1ex;"> Regards,<br>
Alexander<br>
</blockquote>
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<span class="HOEnZb"><font color="#888888">
<div><br>
</div>
-- <br>
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<br>
</font></span></blockquote>
<br>
</div>
</blockquote>
</div>
<br>
<br clear="all">
<div><br>
</div>
-- <br>
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<br>
</blockquote>
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