[
https://issues.apache.org/jira/browse/RNG-50?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16562006#comment-16562006
]
Alex D Herbert edited comment on RNG-50 at 7/31/18 11:40 AM:
-------------------------------------------------------------
I've made a simple Maven project that tests the use of the {{log(n!}} function:
[Poisson Sampler Test
Code|https://github.com/aherbert/poisson-sampler-test-code]
Since the function outside the algorithm loop can be pre-computed I've ignored
this from the statistics. The table below shows the number of calls made to
{{log(n!)}} and the summary statistics on the distribution of {{n}}. For
reference the expected standard deviation of all samples from the Poisson
distribution is shown.
||Mean||Samples||log(n!) calls||Calls/sample||Min n||Max n||Av n||SD n||Poisson
SD||
|40|2000000|15336|0.007668|11|77|42.676382368283775|14.47432293750233|6.324555320336759|
|45|2000000|13343|0.0066715|13|85|47.662144944914935|15.252118330974465|6.708203932499369|
|50|2000000|11930|0.005965|17|92|52.95188600167645|15.989613273366508|7.0710678118654755|
|55|2000000|10748|0.005374|18|100|57.59666914774842|16.894476407792787|7.416198487095663|
|60|2000000|9723|0.0048615|24|108|62.777332099146356|17.525350974045864|7.745966692414834|
|65|2000000|9005|0.0045025|31|110|67.82809550249861|18.2532893886245|8.06225774829855|
|70|2000000|8166|0.004083|34|118|72.43350477590008|19.056570931403215|8.366600265340756|
|75|2000000|7694|0.003847|38|129|77.51286716922277|19.75367031023264|8.660254037844387|
|80|2000000|6989|0.0034945|38|130|82.77063957647732|20.279865491852842|8.94427190999916|
|85|2000000|6651|0.0033255|47|134|87.96977898060442|20.8263169458762|9.219544457292887|
|90|2000000|6314|0.003157|47|144|92.9708584098828|21.38232095182317|9.486832980505138|
|95|2000000|5948|0.002974|49|145|97.51008742434432|22.124775693124754|9.746794344808963|
|100|2000000|5574|0.002787|56|151|102.7929673484033|22.699920614277534|10.0|
|200|2000000|2654|0.001327|139|267|203.6525998492841|31.75185127203293|14.142135623730951|
|400|2000000|1316|6.58E-4|315|504|403.35562310030394|44.98486072660934|20.0|
|800|2000000|657|3.285E-4|691|947|803.1278538812785|63.848062553855826|28.284271247461902|
|1600|2000000|336|1.68E-4|1442|1776|1604.6607142857142|86.91136884271837|40.0|
|3200|2000000|155|7.75E-5|2977|3429|3210.2903225806454|124.33892557133235|56.568542494923804|
|6400|2000000|74|3.7E-5|6088|6699|6378.472972972973|174.87476716507024|80.0|
|12800|2000000|58|2.9E-5|12335|13135|12803.775862068966|256.02912684516065|113.13708498984761|
|25600|2000000|19|9.5E-6|25102|26111|25542.052631578947|358.73101555410426|160.0|
|51200|2000000|10|5.0E-6|50301|51893|51056.6|605.0541206281124|226.27416997969522|
|102400|2000000|3|1.5E-6|101510|103388|102729.66666666667|1057.375209343386|320.0|
Interestingly the section of code that uses {{log(n!)}} is called about 0.7
down to 0.3 % of the time. So the value of a cache of size 80 is debatable.
Reading the algorithm it is clear that the {{log(n!)}} is called with a value
that is approximately the same as the sample that will be returned (it is just
adjusted by a Poisson sample using a mean in the range of 0-1). Thus
{{log(n!)}} is called with a value from the Poisson distribution. With high
mean (>40) this is approximately a Gaussian and so the value of n will will be
approximately in the range {{mean +/- 3*sqrt(mean)}}, with {{sqrt(mean)}} the
expected standard deviation of the Poisson distribution. In the results the
mean is slightly higher than the Poisson mean but the standard deviation is
more than double. This is due the sub-sample of the Poisson that is used when
calling {{log(n!)}}, i.e. this executes only part of the time for all samples.
If a cache is to be used then it could be made smarter and only the range of n
expected given the mean should be cached. This will have some value when the
user wants to compute thousands of samples, but the exact crossover point when
a cache is beneficial would require use-case testing.
In light of these results perhaps the cache should be removed or made optional.
Given that a lot of the initial computation at the start of the main algorithm
loop is the same I have created a new {{NoCachePoissonSampler}} that
precomputes this. It actually delegates the work of sampling to an internal
class that is specialised for small mean or large mean. This is less readable
than the original but runs about 2x faster for big mean and is comparable for
small mean to the existing version. It has little penalty when run inside a
loop for a single use.
{noformat}
Mean 10 Single construction (33017430) vs Loop construction
(43150013) (1.306886.2x faster)
Mean 10 Single construction (33017430) vs Loop construction with no cache
(38633093) (1.170082.2x faster)
Mean 10 Single construction (33017430) vs Single construction with no cache
(29106732) (0.881557.2x faster)
Mean 15 Single construction (44054388) vs Loop construction
(49272725) (1.118452.2x faster)
Mean 15 Single construction (44054388) vs Loop construction with no cache
(47975491) (1.089006.2x faster)
Mean 15 Single construction (44054388) vs Single construction with no cache
(40819826) (0.926578.2x faster)
Mean 20 Single construction (49668097) vs Loop construction
(56182968) (1.131168.2x faster)
Mean 20 Single construction (49668097) vs Loop construction with no cache
(52115198) (1.049269.2x faster)
Mean 20 Single construction (49668097) vs Single construction with no cache
(46318785) (0.932566.2x faster)
Mean 25 Single construction (59471099) vs Loop construction
(64330381) (1.081708.2x faster)
Mean 25 Single construction (59471099) vs Loop construction with no cache
(60546587) (1.018084.2x faster)
Mean 25 Single construction (59471099) vs Single construction with no cache
(54591773) (0.917955.2x faster)
Mean 30 Single construction (68146512) vs Loop construction
(74374546) (1.091392.2x faster)
Mean 30 Single construction (68146512) vs Loop construction with no cache
(70451976) (1.033831.2x faster)
Mean 30 Single construction (68146512) vs Single construction with no cache
(63940806) (0.938284.2x faster)
Mean 35 Single construction (79878953) vs Loop construction
(84966196) (1.063687.2x faster)
Mean 35 Single construction (79878953) vs Loop construction with no cache
(87798333) (1.099142.2x faster)
Mean 35 Single construction (79878953) vs Single construction with no cache
(89916831) (1.125664.2x faster)
Mean 40 Single construction (44851418) vs Loop construction
(150102896) (3.346670.2x faster)
Mean 40 Single construction (44851418) vs Loop construction with no cache
(64463988) (1.437279.2x faster)
Mean 40 Single construction (44851418) vs Single construction with no cache
(20547465) (0.458123.2x faster)
Mean 45 Single construction (44565349) vs Loop construction
(142062133) (3.187726.2x faster)
Mean 45 Single construction (44565349) vs Loop construction with no cache
(56858335) (1.275842.2x faster)
Mean 45 Single construction (44565349) vs Single construction with no cache
(20147275) (0.452084.2x faster)
Mean 50 Single construction (43771462) vs Loop construction
(146827129) (3.354403.2x faster)
Mean 50 Single construction (43771462) vs Loop construction with no cache
(53001835) (1.210877.2x faster)
Mean 50 Single construction (43771462) vs Single construction with no cache
(19761750) (0.451476.2x faster)
Mean 55 Single construction (43445931) vs Loop construction
(163112456) (3.754378.2x faster)
Mean 55 Single construction (43445931) vs Loop construction with no cache
(52989752) (1.219671.2x faster)
Mean 55 Single construction (43445931) vs Single construction with no cache
(19555998) (0.450123.2x faster)
Mean 60 Single construction (43694078) vs Loop construction
(166955616) (3.821012.2x faster)
Mean 60 Single construction (43694078) vs Loop construction with no cache
(53799074) (1.231267.2x faster)
Mean 60 Single construction (43694078) vs Single construction with no cache
(19645395) (0.449612.2x faster)
Mean 65 Single construction (43766713) vs Loop construction
(179079936) (4.091693.2x faster)
Mean 65 Single construction (43766713) vs Loop construction with no cache
(55747299) (1.273737.2x faster)
Mean 65 Single construction (43766713) vs Single construction with no cache
(20493758) (0.468250.2x faster)
Mean 70 Single construction (43975538) vs Loop construction
(189005650) (4.297972.2x faster)
Mean 70 Single construction (43975538) vs Loop construction with no cache
(53124058) (1.208037.2x faster)
Mean 70 Single construction (43975538) vs Single construction with no cache
(19395643) (0.441055.2x faster)
Mean 75 Single construction (45913218) vs Loop construction
(205133974) (4.467863.2x faster)
Mean 75 Single construction (45913218) vs Loop construction with no cache
(55565851) (1.210236.2x faster)
Mean 75 Single construction (45913218) vs Single construction with no cache
(19380347) (0.422108.2x faster)
Mean 80 Single construction (43494472) vs Loop construction
(204741747) (4.707305.2x faster)
Mean 80 Single construction (43494472) vs Loop construction with no cache
(53357258) (1.226760.2x faster)
Mean 80 Single construction (43494472) vs Single construction with no cache
(19207700) (0.441612.2x faster)
{noformat}
was (Author: alexherbert):
I've made a simple Maven project that tests the use of the {{log(n!}} function:
[Poisson Sampler Test
Code|https://github.com/aherbert/poisson-sampler-test-code]
Since the function outside the algorithm loop can be pre-computed I've ignored
this from the statistics.
{noformat}
Mean 40 Samples=2000000 log(n!)= 15336 ( 0.00767 / sample) min= 11
max= 77 mean= 42.676 SD= 14.474 Poisson SD= 6.325
Mean 45 Samples=2000000 log(n!)= 13343 ( 0.00667 / sample) min= 13
max= 85 mean= 47.662 SD= 15.252 Poisson SD= 6.708
Mean 50 Samples=2000000 log(n!)= 11930 ( 0.00597 / sample) min= 17
max= 92 mean= 52.952 SD= 15.990 Poisson SD= 7.071
Mean 55 Samples=2000000 log(n!)= 10748 ( 0.00537 / sample) min= 18
max= 100 mean= 57.597 SD= 16.894 Poisson SD= 7.416
Mean 60 Samples=2000000 log(n!)= 9723 ( 0.00486 / sample) min= 24
max= 108 mean= 62.777 SD= 17.525 Poisson SD= 7.746
Mean 65 Samples=2000000 log(n!)= 9005 ( 0.00450 / sample) min= 31
max= 110 mean= 67.828 SD= 18.253 Poisson SD= 8.062
Mean 70 Samples=2000000 log(n!)= 8166 ( 0.00408 / sample) min= 34
max= 118 mean= 72.434 SD= 19.057 Poisson SD= 8.367
Mean 75 Samples=2000000 log(n!)= 7694 ( 0.00385 / sample) min= 38
max= 129 mean= 77.513 SD= 19.754 Poisson SD= 8.660
Mean 80 Samples=2000000 log(n!)= 6989 ( 0.00349 / sample) min= 38
max= 130 mean= 82.771 SD= 20.280 Poisson SD= 8.944
Mean 85 Samples=2000000 log(n!)= 6651 ( 0.00333 / sample) min= 47
max= 134 mean= 87.970 SD= 20.826 Poisson SD= 9.220
Mean 90 Samples=2000000 log(n!)= 6314 ( 0.00316 / sample) min= 47
max= 144 mean= 92.971 SD= 21.382 Poisson SD= 9.487
Mean 95 Samples=2000000 log(n!)= 5948 ( 0.00297 / sample) min= 49
max= 145 mean= 97.510 SD= 22.125 Poisson SD= 9.747
Mean 100 Samples=2000000 log(n!)= 5574 ( 0.00279 / sample) min= 56
max= 151 mean= 102.793 SD= 22.700 Poisson SD= 10.000
Mean 200 Samples=2000000 log(n!)= 2654 ( 0.00133 / sample) min= 139
max= 267 mean= 203.653 SD= 31.752 Poisson SD= 14.142
Mean 400 Samples=2000000 log(n!)= 1316 (0.000658 / sample) min= 315
max= 504 mean= 403.356 SD= 44.985 Poisson SD= 20.000
Mean 800 Samples=2000000 log(n!)= 657 (0.000329 / sample) min= 691
max= 947 mean= 803.128 SD= 63.848 Poisson SD= 28.284
Mean 1600 Samples=2000000 log(n!)= 336 (0.000168 / sample) min= 1442
max= 1776 mean= 1604.661 SD= 86.911 Poisson SD= 40.000
Mean 3200 Samples=2000000 log(n!)= 155 (7.75e-05 / sample) min= 2977
max= 3429 mean= 3210.290 SD=124.339 Poisson SD= 56.569
Mean 6400 Samples=2000000 log(n!)= 74 (3.70e-05 / sample) min= 6088
max= 6699 mean= 6378.473 SD=174.875 Poisson SD= 80.000
Mean 12800 Samples=2000000 log(n!)= 58 (2.90e-05 / sample) min= 12335
max= 13135 mean= 12803.776 SD=256.029 Poisson SD=113.137
Mean 25600 Samples=2000000 log(n!)= 19 (9.50e-06 / sample) min= 25102
max= 26111 mean= 25542.053 SD=358.731 Poisson SD=160.000
Mean 51200 Samples=2000000 log(n!)= 10 (5.00e-06 / sample) min= 50301
max= 51893 mean= 51056.600 SD=605.054 Poisson SD=226.274
Mean 102400 Samples=2000000 log(n!)= 3 (1.50e-06 / sample) min=101510
max=103388 mean=102729.667 SD=1057.375 Poisson SD=320.000
{noformat}
Interestingly the section of code that uses {{log(n!)}} is called about 0.7
down to 0.3 % of the time. So the value of a cache of size 80 is debatable.
Reading the algorithm it is clear that the {{log(n!)}} is called with a value
that is approximately the same as the sample that will be returned (it is just
adjusted by a Poisson sample using a mean in the range of 0-1). Thus
{{log(n!)}} is called with a value from the Poisson distribution. With high
mean (>40) this is approximately a Gaussian and so the value of n will will be
approximately in the range {{mean +/- 3*sqrt(mean)}}, with {{sqrt(mean)}} the
expected standard deviation of the Poisson distribution. In the results the
mean is slightly higher than the Poisson mean but the standard deviation is
more than double. This is due the sub-sample of the Poisson that is used when
calling {{log(n!)}}, i.e. this executes only part of the time for all samples.
If a cache is to be used then it could be made smarter and only the range of n
expected given the mean should be cached. This will have some value when the
user wants to compute thousands of samples, but the exact crossover point when
a cache is beneficial would require use-case testing.
In light of these results perhaps the cache should be removed or made optional.
Given that a lot of the initial computation at the start of the main algorithm
loop is the same I have created a new {{NoCachePoissonSampler}} that
precomputes this. It actually delegates the work of sampling to an internal
class that is specialised for small mean or large mean. This is less readable
than the original but runs about 2x faster for big mean and is comparable for
small mean to the existing version. It has little penalty when run inside a
loop for a single use.
{noformat}
Mean 10 Single construction (33017430) vs Loop construction
(43150013) (1.306886.2x faster)
Mean 10 Single construction (33017430) vs Loop construction with no cache
(38633093) (1.170082.2x faster)
Mean 10 Single construction (33017430) vs Single construction with no cache
(29106732) (0.881557.2x faster)
Mean 15 Single construction (44054388) vs Loop construction
(49272725) (1.118452.2x faster)
Mean 15 Single construction (44054388) vs Loop construction with no cache
(47975491) (1.089006.2x faster)
Mean 15 Single construction (44054388) vs Single construction with no cache
(40819826) (0.926578.2x faster)
Mean 20 Single construction (49668097) vs Loop construction
(56182968) (1.131168.2x faster)
Mean 20 Single construction (49668097) vs Loop construction with no cache
(52115198) (1.049269.2x faster)
Mean 20 Single construction (49668097) vs Single construction with no cache
(46318785) (0.932566.2x faster)
Mean 25 Single construction (59471099) vs Loop construction
(64330381) (1.081708.2x faster)
Mean 25 Single construction (59471099) vs Loop construction with no cache
(60546587) (1.018084.2x faster)
Mean 25 Single construction (59471099) vs Single construction with no cache
(54591773) (0.917955.2x faster)
Mean 30 Single construction (68146512) vs Loop construction
(74374546) (1.091392.2x faster)
Mean 30 Single construction (68146512) vs Loop construction with no cache
(70451976) (1.033831.2x faster)
Mean 30 Single construction (68146512) vs Single construction with no cache
(63940806) (0.938284.2x faster)
Mean 35 Single construction (79878953) vs Loop construction
(84966196) (1.063687.2x faster)
Mean 35 Single construction (79878953) vs Loop construction with no cache
(87798333) (1.099142.2x faster)
Mean 35 Single construction (79878953) vs Single construction with no cache
(89916831) (1.125664.2x faster)
Mean 40 Single construction (44851418) vs Loop construction
(150102896) (3.346670.2x faster)
Mean 40 Single construction (44851418) vs Loop construction with no cache
(64463988) (1.437279.2x faster)
Mean 40 Single construction (44851418) vs Single construction with no cache
(20547465) (0.458123.2x faster)
Mean 45 Single construction (44565349) vs Loop construction
(142062133) (3.187726.2x faster)
Mean 45 Single construction (44565349) vs Loop construction with no cache
(56858335) (1.275842.2x faster)
Mean 45 Single construction (44565349) vs Single construction with no cache
(20147275) (0.452084.2x faster)
Mean 50 Single construction (43771462) vs Loop construction
(146827129) (3.354403.2x faster)
Mean 50 Single construction (43771462) vs Loop construction with no cache
(53001835) (1.210877.2x faster)
Mean 50 Single construction (43771462) vs Single construction with no cache
(19761750) (0.451476.2x faster)
Mean 55 Single construction (43445931) vs Loop construction
(163112456) (3.754378.2x faster)
Mean 55 Single construction (43445931) vs Loop construction with no cache
(52989752) (1.219671.2x faster)
Mean 55 Single construction (43445931) vs Single construction with no cache
(19555998) (0.450123.2x faster)
Mean 60 Single construction (43694078) vs Loop construction
(166955616) (3.821012.2x faster)
Mean 60 Single construction (43694078) vs Loop construction with no cache
(53799074) (1.231267.2x faster)
Mean 60 Single construction (43694078) vs Single construction with no cache
(19645395) (0.449612.2x faster)
Mean 65 Single construction (43766713) vs Loop construction
(179079936) (4.091693.2x faster)
Mean 65 Single construction (43766713) vs Loop construction with no cache
(55747299) (1.273737.2x faster)
Mean 65 Single construction (43766713) vs Single construction with no cache
(20493758) (0.468250.2x faster)
Mean 70 Single construction (43975538) vs Loop construction
(189005650) (4.297972.2x faster)
Mean 70 Single construction (43975538) vs Loop construction with no cache
(53124058) (1.208037.2x faster)
Mean 70 Single construction (43975538) vs Single construction with no cache
(19395643) (0.441055.2x faster)
Mean 75 Single construction (45913218) vs Loop construction
(205133974) (4.467863.2x faster)
Mean 75 Single construction (45913218) vs Loop construction with no cache
(55565851) (1.210236.2x faster)
Mean 75 Single construction (45913218) vs Single construction with no cache
(19380347) (0.422108.2x faster)
Mean 80 Single construction (43494472) vs Loop construction
(204741747) (4.707305.2x faster)
Mean 80 Single construction (43494472) vs Loop construction with no cache
(53357258) (1.226760.2x faster)
Mean 80 Single construction (43494472) vs Single construction with no cache
(19207700) (0.441612.2x faster)
{noformat}
> 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
> Priority: Minor
> 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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