[Trilinos-Users] aztecoo+sundance problem

[Mike Heroux]

Gideon,
 
This is often a symptom of a poorly conditioned preconditioner.  In other
words, the LU factors of ILU are poorly conditioned, which can be true even
if your original matrix is well-conditioned.  You can check if this is the
case by calling the Condest() method on your AztecOO object.  If the value
comes back as 1E+14 or more, then this is likely the issue.  It is possible
to reduce the condition number and still have an effective preconditioner by
using the absolute and relative threshold values to perturb the diagonal of
the preconditioner.  This is described in the AztecOO User Guide, starting
on page 27:
 
http://software.sandia.gov/trilinos/packages/aztecoo/AztecOOUserGuide.pdf
 
If this is not the problem, let us know.  Also, there may be other simpler
ways to reduce the condition number of the preconditioner, but we need to
know more about your problem and the particular ILU parameters you are
using.
 
Mike
 


  _____  

From: trilinos-users-bounces at software.sandia.gov
[mailto:trilinos-users-bounces at software.sandia.gov] On Behalf Of Gideon
Simpson
Sent: Wednesday, January 17, 2007 7:52 PM
To: trilinos-users at software.sandia.gov
Subject: [Trilinos-Users] aztecoo+sundance problem


I've been getting the following error when solving some problems that come
out of Sundance, using GMRES+DomainDecomposition+ILU 

***************************************************************

Warning: a breakdown in this method
has occurred and solution has not converged.

Solver: gmres
number of iterations: 1

Recursive residual = nan

Calculated Norms Requested Norm
-------------------------------------------- --------------

||r||_2 / ||r0||_2: 1.000000e+00 1.000000e-12

***************************************************************



Solution time: 9.740000 (sec.)
total iterations: 1
Epetra ERROR -2, ../../../../packages/aztecoo/src/AztecOO.cpp, line 824
Solve failed! state = Crashed
message=Aztec solver failed: numerical breakdown
iters taken = 1
final residual = nan
matrix written to badMatrix.dat
RHS vector written to badVector.dat

The matrix and RHS data files look fine, and when I solve the problem using
an Amesos direct solver, the answer is consistent. What is the nature of
this error and how might I correct it?


-Gideon Simpson
Department of Applied Physics and Applied Mathematics
Columbia University



-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://software.sandia.gov/mailman/private/trilinos-users/attachments/20070117/4f1512a7/attachment.html