[FLASH-USERS] Random Number Generators in FLASH

[Carlo Graziani]

Hi Jason.

That's right, a per-block initialization is what I was thinking.  This
might come up in particle initialization, for example.

Carlo

On 02/24/2016 03:13 PM, Jason Galyardt wrote:
> Hi Carlo,
>
> Thanks for the response. I agree with your comments on MPI implementation and general reproducibility of scientific results. One practical question, though: In order to decouple the random sequence from the MPI configuration, you would need to set the random seed separately for each block, with the seed for each RNG calculated as you suggest from some static, block-specific quantity. However, in the FLASH architecture, you only have one RNG per MPI processor. For my case, this is not a problem, since I'm only using the RNG at initialization; I can reset the seed for each block and not worry about affecting the others on a given MPI processor. Is this in line with your thinking?
>
> Thanks,
> Jason
>
>
> On Wed, Feb 24, 2016 at 3:27 PM, Carlo Graziani <carlo at oddjob.uchicago.edu <mailto:carlo at oddjob.uchicago.edu>> wrote:
>
>     On 02/24/2016 01:56 PM, Jason Galyardt wrote:
>
>         Dear FLASH Users,
>
>         In FLASH 4.3, there is a random number generator (RNG) unit, located at source/flashUtilities/rng. There are two implementations: the default is located at source/flashUtilities/rng, and the other is located at source/flashUtilities/rng/mt_rng. The default implementation seems to use the Fortran 90 intrinsic RNG facilities (random_seed() and random_number()), while the mt_rng implementation uses the 32-bit Mersenne Twister.
>
>         Has anyone used the Mersenne Twister implementation before? Is there a recommended prescription for setting the seed on each processor of an MPI job? The latter is complicated by the fact that mt_rng seems to want an integer array of length 624 instead of a single integer seed which can be constructed from the processor ID and a global, user-provided seed.
>
>         Thanks,
>         Jason
>
>         ----
>         Jason Galyardt
>         Department of Physics and Astronomy
>         University of Georgia
>
>
>     Hi Jason.
>
>     As I recall, this functionality was introduced in order to obtain
>     cross-platform reproducibility of results.  The Fortan 90 intrinsics
>     random_seed() and random_number() produce platform-dependent results, so in
>     cases where it is important that pseudorandomness should be repeatable across
>     platforms, it is desirable to have a portable high-quality RNG, such as
>     the Mersenne twister.
>
>     I don't know whether a recommended seeding prescription was ever articulated.
>     It should be obvious, though, that if RNG initialization is bound to MPI rank
>     rather than to a stable in-application computational structure (e.g. hashed
>     coordinates of local block center, or Morton ordering of a block) the advantage
>     of reproducibility is promptly lost, since one has now tied RNG initialization
>     to the system-level details of MPI process allocation, so that the same
>     simulation performed with different number of MPI processes necessarily
>     produces different results.
>
>     Bit-for-bit simulation reproducibility, even despite RNG calls, is important
>     for regression testing.  It is also (I would argue) very valuable in general
>     scientific simulation, particularly in cases of flows where small differences
>     can be amplified by irreducible instabilities.  However, it is not obtainable
>     without some effort, so the effort involved had better pass some cost/benefit
>     analysis for you. If you feel it is more trouble than it is worth, the standard
>     F90 intrinsics should probably produces adequate randomness.
>
>     Carlo
>
>     --
>     Carlo Graziani (773) 702-7973 <tel:%28773%29%20702-7973> (Voice)
>     University of Chicago Flash Center (773) 702-6645 <tel:%28773%29%20702-6645> (FAX)
>     5747 South Ellis Avenue     -------------------------------------
>     Jones 314                  | The less a statesman amounts to, the
>     Chicago, IL 60637          | more he loves the flag.
>     carlo at oddjob.uchicago.edu <mailto:carlo at oddjob.uchicago.edu>  |               --- Kin Hubbard
>
>
>


-- 
Carlo Graziani                                 (773) 702-7973 (Voice)
University of Chicago Flash Center             (773) 702-6645 (FAX)
5747 South Ellis Avenue     -------------------------------------
Jones 314                  | The less a statesman amounts to, the
Chicago, IL 60637          | more he loves the flag.
carlo at oddjob.uchicago.edu  | 		   --- Kin Hubbard