[petsc-users] Fwd: nonzero prescribed boundary condition

[Jed Brown]

On Thu, Sep 8, 2011 at 19:57, amrit poudel <amrit_pou at hotmail.com> wrote:

> After running my simulation multiple times on a multiprocessor computer
> I've just verified that using iterative solver (default gmres) in PETSc  to
> solve a linear system of equations ( Cx=b)  with more than 2 processors
> setting ALWAYS lead to erroneous result. Running identical code with
> identical setting except for the number of processors ( set this to 2)
> ALWAYS gives me correct result .
>

You have to explain what "erroneous result" means here.


>
> I am really not sure what is the point behind including iterative solvers
> if they result into erroneous result on a multiprocessor computer. The
> result I get from multiprocessor computer is a complete garbage, so I am
> really not talking about small percentage of error here.  Also, if somebody
> could enlighten why the iterative solvers are error prone on multiprocessors
> that will be highly appreciated.
>

Well let's not jump to conclusions. Iterative solvers can fail, as can
direct solvers, but it's more common that (a) the problem you are building
is not actually the same or the results are being misinterpreted or (b) the
default methods are not working well and needs some algorithmic adjustments
for your problem.

Please explain what kind of problem you are solving, how you are going about
it, and what symptoms you have observed.
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