[petsc-users] Code performance for solving multiple RHS

[Harshad Ranadive]

Hi All,

I have currently added the PETSc library with our CFD solver.

In this I need to use KSPSolve(...) multiple time for the same matrix A. I
have read that PETSc does not support passing multiple RHS vectors in the
form of a matrix and the only solution to this is calling KSPSolve multiple
times as in example given here:
http://www.mcs.anl.gov/petsc/petsc-current/src/ksp/ksp/examples/tutorials/ex16.c.html

I have followed this technique, but I find that the performance of the code
is very slow now. I basically have a mesh size of 8-10 Million and I need
to solve the matrix A very large number of times. I have checked that the
statement KSPSolve(..) is taking close to 90% of my computation time.

I am setting up the matrix A, KSPCreate, KSPSetup etc just once at the
start. Only the following statements are executed in a repeated loop

*Loop begin: (say million times !!)*

*   loop over vector length*
*       VecSetValues( ....)*
*   end*

*   VecAssemblyBegin( ... )*
*   VecAssemblyEnd (...)*

*    KSPSolve (...)*

*    VecGetValues*

*Loop end.*

Is there an efficient way of doing this rather than using KSPSolve multiple
times?

Please note my matrix A never changes during the time steps or across the
mesh ... So essentially if I can get the inverse once would it be good
enough?  It has been recommended in the FAQ that matrix inverse should be
avoided but would it be okay to use in my case?

Also could someone please provide an example of how to use MatLUFactor
and MatCholeskyFactor() to find the matrix inverse... the arguments below
were not clear to me.
*IS row *
*IS col*
*const MatFactorInfo *info*

Apologies for a long email and thanks to anyone for help.

Regards
Harshad
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