[Numpy-discussion] skip samples in random number generator

Brad Buran bburan at alum.mit.edu
Thu Oct 2 18:13:58 EDT 2014


Thanks for the great input.  The idea of implementing jump-ahead in
numpy.random would be a very nice feature, but I don't currently have
the time to work on implementing such a feature.  For now, it seems
the simplest approach is to cache the RandomState and reuse that
later.

Brad

On Thu, Oct 2, 2014 at 2:24 PM, Robert Kern <robert.kern at gmail.com> wrote:
> On Thu, Oct 2, 2014 at 9:52 PM, Sturla Molden <sturla.molden at gmail.com> wrote:
>> Robert Kern <robert.kern at gmail.com> wrote:
>>
>>> No one needs small jumps of arbitrary size. The real use case for
>>> jumping is to make N parallel streams that won't overlap. You pick a
>>> number, let's call it `jump_steps`, much larger than any single run of
>>> your system could possibly consume (i.e. the number of core PRNG
>>> variates pulled is << `jump_steps`). Then you can initializing N
>>> parallel streams by initializing RandomState once with a seed, storing
>>> that RandomState, then jumping ahead by `jump_steps`, storing *that*
>>> RandomState, by `2*jump_steps`, etc. to get N RandomState streams that
>>> will not overlap. Give those to your separate processes and let them
>>> run.
>>>
>>> So the alternative may actually be to just generate and distribute
>>> *one* set of these jump coefficients for a really big jump size but
>>> still leaves you enough space for a really large number of streams
>>> (fortunately, 2**19937-1 is a really big number).
>>
>> DCMT might be preferred in this case. It works the same, except you have N
>> "random state" streams with characteristic polynomials that are distinct
>> and relatively prime to each other. Thus each of the N processes will get
>> an independent stream of random numbers, without any chance of overlap.
>>
>> http://www.math.sci.hiroshima-u.ac.jp/∼m-mat/MT/DC/dc.html
>
> Yes, but that would require rewriting much of numpy.random to allow
> replacing the core generator. This would work out-of-box because it's
> just manipulating the state of the current core generator.
>
> --
> Robert Kern
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