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<div class="moz-cite-prefix">On 10/22/2013 09:16 PM, Matthew Knepley
wrote:<br>
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cite="mid:CAMYG4Gk+BN13p3MD_-OBrawkhbdP0Tz8cGDPONneddef_H=iwg@mail.gmail.com"
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<div class="gmail_extra">There are no blackbox preconditioners.
You must find one that matches your system. The right thing to
do</div>
<div class="gmail_extra">is to lookup the literature on your
problem and see what other people have done. It should be easy
to</div>
<div class="gmail_extra">replicate in PETSc.</div>
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+ The first thing to investigate is the nature of the matrices, is
that symmetric or not since you use GMRES I assume it is
non-symmetric.<br>
+ Second, do you have any ideas on the distribution of the
eigenvalues(or values of the smallest and largest singular values)<br>
+ If the problem is ill-conditioned(of course this is from personal
experience), leave iterative methods as soon as possible and stick
to direct solver unless you can find a well performing
preconditioner(which is again from experience, very difficult to
find).<br>
+ Iterative methods really require one to know what he/she is doing
and there is a vast literature on that which I find difficult to
follow from time to time as an engineer.<br>
+ However for well conditioned problems, their performance is
amazing, although almost all the practical engineering problems are
ill-conditioned. Moreover the picture gets even worse if two or
three physics are coupled(the issue of scaling between different
field variables).<br>
<br>
These are the points from my experience. <br>
Good luck.<br>
Best,<br>
Umut<br>
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