Alex D Herbert created RNG-50:
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Summary: PoissonSampler single use speed improvements
Key: RNG-50
URL: https://issues.apache.org/jira/browse/RNG-50
Project: Commons RNG
Issue Type: Improvement
Affects Versions: 1.0
Reporter: Alex D Herbert
Attachments: PoissonSamplerTest.java
The Sampler architecture of {{org.apache.commons.rng.sampling.distribution}} is
nicely written for fast sampling of small dataset sizes. The constructors for
the samplers do not check the input parameters are valid for the respective
distributions (in contrast to the old
{{org.apache.commons.math3.random.distribution}} classes). I assume this is a
design choice for speed. Thus most of the samplers can be used within a loop to
sample just one value with very little overhead.
The {{PoissonSampler}} precomputes log factorial numbers upon construction if
the mean is above 40. This is done using the {{InternalUtils.FactorialLog}}
class. As of version 1.0 this internal class is currently only used in the
{{PoissonSampler}}.
The cache size is limited to 2*PIVOT (where PIVOT=40). But it creates and
precomputes the cache every time a PoissonSampler is constructed if the mean is
above the PIVOT value.
Why not create this once in a static block for the PoissonSampler?
{code:java}
/** {@code log(n!)}. */
private static final FactorialLog factorialLog;
static
{
factorialLog = FactorialLog.create().withCache((int) (2 *
PoissonSampler.PIVOT));
}
{code}
This will make the construction cost of a new {{PoissonSampler}} negligible. If
the table is computed dynamically as a static construction method then the
overhead will be in the first use. Thus the following call will be much faster:
{code:java}
UniformRandomProvider rng = ...;
int value = new PoissonSampler(rng, 50).sample();
{code}
I have tested this modification (see attached file) and the results are:
{noformat}
Mean 40 Single construction ( 7330792) vs Loop construction
(24334724) (3.319522.2x faster)
Mean 40 Single construction ( 7330792) vs Loop construction with static
FactorialLog ( 7990656) (1.090013.2x faster)
Mean 50 Single construction ( 6390303) vs Loop construction
(19389026) (3.034132.2x faster)
Mean 50 Single construction ( 6390303) vs Loop construction with static
FactorialLog ( 6146556) (0.961857.2x faster)
Mean 60 Single construction ( 6041165) vs Loop construction
(21337678) (3.532047.2x faster)
Mean 60 Single construction ( 6041165) vs Loop construction with static
FactorialLog ( 5329129) (0.882136.2x faster)
Mean 70 Single construction ( 6064003) vs Loop construction
(23963516) (3.951765.2x faster)
Mean 70 Single construction ( 6064003) vs Loop construction with static
FactorialLog ( 5306081) (0.875013.2x faster)
Mean 80 Single construction ( 6064772) vs Loop construction
(26381365) (4.349935.2x faster)
Mean 80 Single construction ( 6064772) vs Loop construction with static
FactorialLog ( 6341274) (1.045591.2x faster)
{noformat}
Thus the speed improvements would be approximately 3-4 fold for single use
Poisson sampling.
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