[petsc-users] behavior of running same codes on different clusters many times

[Matthew Knepley]

On Wed, Aug 27, 2014 at 3:54 PM, Xiangdong <epscodes at gmail.com> wrote:

> Hello everyone,
>
> When I ran the same petsc codes on different clusters many times, one
> cluster always produces the same results, while the other one varies in
> terms of number of iterations for SNES and KSP convergence. If the problem
> is ill-conditioned, the round-off errors may cause different number of
> iterations to converge. However, what puzzles me is that the number of
> iterations on one machine is always the same while the other varies. I use
> same codes with same version of PETSc. The differences I can think about
> are MPI implementations (mvapich2 vs openmpi), compiler (icc vs gcc) and
> the network between the nodes. Could these factors cause these different
> behaviors?
>

Some machines (like BlueGene) have very consistent behavior, including the
ordering of communication operations, whereas
other machines do not. For ill-conditioned problems, this can make a real
difference. However, what this is telling you is that
your answer is not really any more meaningful than that spread.

   Matt


> Thank you.
>
> Best,
> Xiangdong
>



-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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