[petsc-users] MatMatSolve for MUMPS disappeared?

[Jed Brown]

On Wed, Sep 19, 2012 at 2:52 AM, Alexander Grayver
<agrayver at gfz-potsdam.de>wrote:

> Is MatSolve in any way more efficient than KSPSolve?
> As far as I understand for both cases one ends up with a loop over RHS,
> but KSPSolve allows to use iterative solvers (I don't need them at the
> moment though).
> I would then use latter option and do this loop myself.
>

The loop over KSPSolve configured with -ksp_type preonly -pc_type lu will
do the same thing as MatMatSolve_Basic. If you think you might want to use
iterative methods, this is definitely the way to go. Adding support for
block Krylov methods (that inherently handle multiple right hand sides) is
something we've been discussing for a while and will eventually happen, but
I can't promise a time.


> That is right. The request on that has recently appeared in the mumps
> mailing list and they replied there are no plans to develop it.
> I have usually < 10^3 and this is not a bottleneck, so I'm fine with a
> "inefficient" loop over RHS.
>

How large are the vectors? It's easy for a small number of right hand sides
to overflow memory on rank 0.


>
> But, for those who are chasing performance and have many sprase RHS (e.g.,
> this is the case when one needs to solve many adjoint problems with delta
> function as a RHS) I can say that using MUMPS' multiple RHS mode is a way
> more efficient than solving them in a loop. This is probably related to
> some internals where MUMPS is able to take advantage of getting many RHS at
> once.
>

There are significant efficiencies to handling many RHS at once, but even
if the right hand sides are sparse, the solutions are dense. The savings
from doing special things for the sparse RHS are thus only incremental.
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