[petsc-users] GAMG PC dependence on right-hand side

[Jed Brown]

John Mousel <john.mousel at gmail.com> writes:

> Jed,
>
> I alternate between solving Helmholtz and Poisson equations in an outer
> loop. The matrix is fixed for both operators during the process, and only
> the right-hand side is updated for each system.

Do you have one KSP or two?  If one, I assume you're trying to preserve
memory, but there is likely more memory in the preconditioner than the
Krylov space and if you want to reuse the preconditioner, you have to
set it up anew each time.

> I just tried -random_seed 1, and it seems to have made all the difference
> in the world!

That's disconcerting because we would like the algorithm to be
insensitive to such things.  How big was the performance difference?
Can you give us some information to reproduce?  (Maybe a smallish
example matrix that demonstrates this problem.)
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