<html>
<head>
<meta content="text/html; charset=ISO-8859-1"
http-equiv="Content-Type">
</head>
<body bgcolor="#FFFFFF" text="#000000">
Jed,<br>
<br>
thanks for your reply.<br>
By using the options you suggested, namely <i>-mg_levels_ksp_type
richardson -mg_levels_pc_type sor</i>, I was able to<br>
solve without bumping into the DIVERGED_INDEFINITE_PC message.
Nevertheless, the number of iterations increases drastically as the
simulation progresses. <br>
The Poisson's equation I am solving arises from a variable-density
projection method for incompressible multi-phase flows. <br>
At each time step the system matrix coefficients change as a
consequence of the change in location of the heavier phase; the rhs
changes<br>
in time because of the change in the velocity field. Usually the
black-box multigrid or the deflated conjugate gradient method are
used to solve efficiently this type of problem: it is my
understanding - please correct me if I am wrong - that AMG is a
generalization of the former.<br>
The only source term acting is gravity; the hydrostatic pressure is
removed from the governing equation in order to accommodate periodic
boundary conditions: this is more a hack than a clean solution.
Could it be the reason behind the poor performances/
DIVERGED_INDEFINITE_PC problem I am experiencing?<br>
Thanks,<br>
<br>
Michele <br>
<br>
<div class="moz-cite-prefix"><br>
<br>
On 05/17/2014 12:26 AM, Jed Brown wrote:<br>
</div>
<blockquote cite="mid:87y4y0uar8.fsf@jedbrown.org" type="cite">
<pre wrap="">Michele Rosso <a class="moz-txt-link-rfc2396E" href="mailto:mrosso@uci.edu"><mrosso@uci.edu></a> writes:
</pre>
<blockquote type="cite">
<pre wrap="">Hi,
I am solving an inhomogeneous Laplacian in 3D (basically a slightly
modified version of example ex34).
The laplacian is discretized by using a cell-center finite difference
7-point stencil with periodic BCs.
I am solving a time-dependent problem so the solution of the laplacian
is repeated at each time step with a different matrix (always SPD
though) and rhs. Also, the laplacian features large magnitude variations
in the coefficients. I solve by means of CG + GAMG as preconditioner.
Everything works fine for a while until I receive a
DIVERGED_INDEFINITE_PC message.
</pre>
</blockquote>
<pre wrap="">
What is changing as you time step? Is there a nonlinearity that
activates suddenly? Especially a bifurcation or perhaps a source term
that is incompatible with the boundary conditions? You could try
-mg_levels_ksp_type richardson -mg_levels_pc_type sor. Can you
reproduce with a small problem?
The configuration looks okay to me.
</pre>
<blockquote type="cite">
<pre wrap="">Before checking my model is incorrect I would like to rule out the
possibility of improper use of the linear solver. I attached the full
output of a serial run with -log-summary -ksp_view
-ksp_converged_reason ksp_monitor_true_residual. I would appreciate if
you could help me in locating the issue.
</pre>
</blockquote>
</blockquote>
<br>
</body>
</html>