[petsc-dev] strangness in Chebyshev estimate of eigenvalues

[Matthew Knepley]

On Fri, Aug 28, 2015 at 6:02 AM, Mark Adams <mfadams at lbl.gov> wrote:

> Is there a good way to get a deterministic rand wrt number of processors?
> Seeding rand for each entry, with its global ID would work I assume.
>
> If drand48 is just one line then why not just copy it tweak it as desired?
>

I did. Its in next.

  Matt


> Cheby is otherwise deterministic, with a deterministic PC, and we have
> equation numbers to make this work. Eigen estimates are a real pain and the
> source of a lot of time consuming AMG errors.  It would be nice to have it
> not depend in any way on processor count.
>
> On Thu, Aug 27, 2015 at 10:12 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>>
>> > On Aug 27, 2015, at 5:35 PM, Jed Brown <jed at jedbrown.org> wrote:
>> >
>> > Barry Smith <bsmith at mcs.anl.gov> writes:
>> >>   This is the default behavior if one just creates a PetscRandom and
>> uses it, so don't be going and setting seeds inside GAMG or Chebyshev.
>> >
>>
>>    Yikes, you are right all three of our current random number generators
>> have global state! What a mess. We should remove all them completely and we
>> can complement Matt's by adding in a more modern version of Sprng that does
>> not use global state. (Stupid Sprng fucks changed to C++)
>>
>>    Barry
>>
>> > You're right (my mistake), but the consequences are dangerous if the
>> > user is holding a PetscRandom across calls to GAMG (or anything that
>> > creates new PetscRandom instances).  The reason is that merely creating
>> > a PetscRandom reseeds the global PRNG.  (What moron thought a global
>> > seed was a good idea?)  Seems to me we should avoid drand48 always so we
>> > don't disrupt the user's source of randomness (leading to really
>> > confusing incorrect results).  We could use the non-disastrous
>> > drand48_r, but it's a GNU extension so we'd need an alternative anyway.
>> >
>> > If Matt changes "rander48" to the rand48_r (or erand48_r) interface,
>> > then we'd be okay.  This would also remove the internal variables that
>> > Matt forget to make static.
>>
>>
>


-- 
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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20150828/434ba309/attachment.html>