[petsc-users] minres and cholesky or icc preconditioner

[Umut Tabak]

Dear all,

I am trying to solve some linear systems with minres where the operator 
matrices can become indefinite depending on \lambda and moreover both 
the operator matrices and the right hand side is changing in a loop.

A = K - \lambda M;

I used the basic example, ex1.c, and tried supply some command line 
arguments to see what is going on for the linear solves with minres.

Use of -pc_type cholesky or icc  does not return an error however I 
could not understand this because the operator matrix is not 
PD(symmetric however) so I did not get how cholesky or icc might be used 
as a preconditioner in this case. However, the results are fine, but the 
iteration numbers are a bit high maybe I should play with the tolerance, 
because I am not looking for the very close approximate solution, a 
coarse representation of the solution might also help me. This is 
because I would like to use these solution vectors in some other context 
mixed with some other vectors.

And the other question is that backslash of MATLAB solves these systems 
faster than umfpack or mumps interface of PETSc, is this due to the fact 
that my PETSc configuration is the default debug one(I read somewhere in 
the documentation some time ago, mentioning that there can be important 
performance differences, but could not locate that right now.)?

I would appreciate clarification on these points.

Best,
Umut

-- 
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