Bob and Dave,
I have had some discussions on some other stuff (electrostatics) with
Michael Mascagni at Florida State University, who has co-developed SPRNG
(
http://www.cs.fsu.edu/~mascagni/).
He is a nice guy and maybe Bob can chat with him a bit on parallel
random #s.
a.
Robert Duke wrote:
> I think one problem that comes in is when you randomly seed multiple
> processors, you actually don't know when two sequences on two different
> processes will overlap, but at some point they will. If there is a lot
> of state to the generator, presumably any two random seed starting
> points won't overlap "soon", but apparently this can be a big enough
> problem that folks that do MC can spot interprocessor correlations. So
> we really have to be sure of what we are doing. It is interesting to
> me, and I'll pursue it as I get time, as long as Tom doesn't shoot me.
> Then we will be out of the space where we are worrying about provability.
> Regards - Bob
> ----- Original Message ----- From: "David A. Case" <case.scripps.edu>
> To: <amber-developers.scripps.edu>
> Sent: Wednesday, May 07, 2008 9:22 PM
> Subject: Re: amber-developers: Verlet update time and ntt=3 parallel
> scaling
>
>
>> On Wed, May 07, 2008, Ross Walker wrote:
>>
>>> As far as I can tell if our random number generator is any good -
>>> which I
>>> don't know if we have properly checked or not - two sets of random
>>> numbers
>>> from different seeds should not have any correlation. Thus it should be
>>> equally correct (statistically) to do a Langevin run with each processor
>>> having its own random number stream - with simply different seeds for
>>> each
>>> mpi thread. This should be equivalent to having a single random number
>>> stream shared between all processors where each processor makes sure it
>>> doesn't use the same portion of the stream as other processors.
>>
>> I agree with this, but (as Bob points out) it's not clear how you
>> prove it.
>>
>> With the current method, one *assumes* that the single stream of
>> numbers (that
>> you would get with a serial code) is correct, then arranges to get the
>> same
>> results in parallel.
>>
>> The only artifacts I know of have to do with reusing a particular part
>> of the
>> big stream of numbers. Since the period is very long, presumably
>> Ross' scheme
>> would have low probability of having this happen, but without a detailed
>> understanding of the scheme works, you might get fooled. But I think
>> it would
>> be worth the risk.
>>
>>
>> ...dac
>>
>>
>
--
Dr. Adrian E. Roitberg
Associate Professor
Quantum Theory Project and Department of Chemistry
University of Florida PHONE 352 392-6972
P.O. Box 118435 FAX 352 392-8722
Gainesville, FL 32611-8435 Email adrian.qtp.ufl.edu
============================================================================
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Received on Sun May 11 2008 - 06:07:19 PDT
