[petsc-dev] use of random numbers in GAMG

[Andrs, David]

On Sat, Aug 8, 2015 at 2:29 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
>   Mark,
>
>     So GAMG uses random numbers in the implementation, this means
> identical runs on machines with different random number generates can
> produce noticeable different convergence histories. This means that the "no
> change" tests in the nightly tests indicate change (i.e. a problem) when
> this perhaps not a problem. So it is impossible to trust the results of the
> tests.
>
>    Is there anything we can do to alleviate this problem? Are the random
> number usage fundamental to the algorithm? Should we distribute with PETSc
> a "random number generator" that will generate the same results on all
> systems (if that is even possible?)?
>

​We had a similar problem in MOOSE where tests using RNG were returning
slightly different results. We ended up using mtwist
<http://www.cs.hmc.edu/~geoff/mtwist.html>, however, the author now
recommends to use PCG <http://www.pcg-random.org>.

​Hope that helps,
--
David​



>
>   Thanks
>
>   Barry
>
>
> Note that on the same machine if you run the same program twice with our
> default system provided "random number generator" you will get the same
> result, this is an issue with running on different kinds of machines and
> getting different results.
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20150808/d3509be3/attachment.html>