Parallel uniform sampling

[segasai]

This is a preliminary version of patch that enables parallel uniform sampling. The idea is to get away from this propose/evolve for uniform distributions where evolve doesn't really do anything other than evaluate logl on a single pt, but instead make propose a no-op for uniform, and evolve() actually do the sampling inside the boundary till a satisfactory pt is found.

While implementing this I realized that after some changes all the different samplers in nestedsamplers.py may not need to be there as they do something very similar (but that's tbd).

ATM some tests fail, but I think that's fixable.

Also some class members had to be renamed. I.e. .bound attribute is really misleading in the classes as it was storing the list of all the historic bounds, so I renamed that to bound_list.

[coveralls]

Pull Request Test Coverage Report for Build 7952108689

Details

• -12 of 225 (94.67%) changed or added relevant lines in 6 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 91.863%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
py/dynesty/sampling.py	28	30	93.33%
py/dynesty/bounding.py	30	34	88.24%
py/dynesty/sampler.py	136	142	95.77%
<!--	Total:	213	225	94.67%	-->

Files with Coverage Reduction	New Missed Lines	%
py/dynesty/bounding.py	1	92.24%
<!--	Total:	1		-->

Totals
Change from base Build 7180723466:	0.2%
Covered Lines:	4053
Relevant Lines:	4412

---

💛 - Coveralls

[joshspeagle]

!!!! This is an excellent proposed change! Very interested to see how the other internal refactoring might also work.

[segasai]

Thanks @joshspeagle Just out of curiosity i've actually refactored nestedsamplers. It was surprisingly easy to get rid of those classes there which removed few hundred lines (of codes + comments). I am not 100%, but 95% certain that's a good change just because it's easier to understand it now (and there was so little different between those samplers)

[segasai]

The demonstration of this patch. The following uniform sampling of Eggbox with single bound takes 36 minutes in the current dynesty : 9528it [36:29, 4.35it/s, +1000 | bound: 8600 | nc: 1 | ncall: 12936194 | eff(%): 0.081 | loglstar: -inf < 243.000 < inf | logz: 235.824 +/- 0.079 | dlogz: 0.000 > 0.100] With the patch it takes 3 minutes.

9490it [03:16, 48.25it/s, +1000 | bound: 999 | nc: 1 | ncall: 12943702 | eff(%): 0.081 | loglstar: -inf < 243.000 < inf | logz: 235.863 +/- 0.078 | dlogz: 0.000 > 0.100]

And if I bump the number of threads to 36 , it takes 50 seconds. I.e. basically it scales properly, while previously uniform sampler basically didn't really scale with multiple threads.

import numpy as np
import dynesty
import multiprocessing as mp

nlive = 1000

def loglike_egg(x):
    logl = ((2 + np.cos(x[0] / 2) * np.cos(x[1] / 2))**5)
    return logl

def prior_transform_egg(x):
    return x * 10 * np.pi

LOGZ_TRUTH = 235.856

def test_bounds():
    bound, sample = 'single', 'unif'
    # stress test various boundaries
    ndim = 2
    rstate = np.random.default_rng(444)
    with mp.Pool(4) as poo:
        sampler = dynesty.NestedSampler(loglike_egg,
                                        prior_transform_egg,
                                        ndim,
                                        nlive=nlive,
                                        bound=bound,
                                        sample=sample,
                                        rstate=rstate,
                                        pool=poo,
                                        queue_size=4)
        sampler.run_nested(dlogz=0.1, print_progress=True)
        assert (abs(LOGZ_TRUTH - sampler.results.logz[-1])
                < 5. * sampler.results.logzerr[-1])

if __name__ == '__main__':
    test_bounds()

[segasai]

It would be great to have some review of this patch @joshspeagle if you have time, before the patch gets bigger.

My current thinking is the patch is mostly positive with a few negatives listed below:

• Previously I had a test of custom sampler like that _SAMPLERS["custom"] = MultiEllipsoidSampler. I had to disable this, but I don't actually know if anyone in the wild is using this functionality.
• The new logic in propose_live has too many ifs() for my liking because it has to deal with combinations of different bounds/samplers. I think this needs to be cleaned up.
• kwargs is used to communicate the bound and a few other variables to sample_bound_unif. Maybe that needs to be changed and we need to expand SampleArgument https://github.com/joshspeagle/dynesty/blob/3f100253dd53fd302ee84f7f25f010b3bf70ad29/py/dynesty/sampling.py#L24 rather than abuse kwargs
• Renaming of a few attributes (there were .method/.sampling attributes, now there is only .sampling). The .bound is renamed to .bound_list.
• Some interface change to bounding classes, so they can be used interchangeably. Maybe having an BoundingInterface class is a good idea...
• I don't have an actual example, but I could imagine the case where the new implementation is slower. Maybe the case of very high acceptance rate, very fast logl function, and very large bound, in this case the overhead from pickling the bound and sending it to the pool worker may be significant. Probably this is not a big worry, since if acceptance fraction is high, things are easy no matter what.

Among things I'd like to do, but I don't want to do in this patch: I think with this patch it's becoming easier to do #391. We already now can pack update_slice or update_rwalk in SliceSampler RWalkSampler objects and make sure that things like history https://github.com/joshspeagle/dynesty/blob/3f100253dd53fd302ee84f7f25f010b3bf70ad29/py/dynesty/nestedsamplers.py#L145 is stored inside those objects. But I'd prefer to do that separately, as this requires more thought.

[ColmTalbot]

In Bilby, we rely on the custom sampler (see here), although I'd love to see this supported in dynesty. We mostly use this to allow additional data to be passed through the kwargs, specifically current live points.

[segasai]

Thanks for the link @ColmTalbot. I think those interfaces will certainly break with this patch, which is I think unfortunate, but I think if we combine this change with the new sampler interface ( #391), hopefully it'll be worthwhile. I did particularly want to implement the ensemble (a-la emcee) proposal in any new sampler interface.

What I need to implement the new way of sampling is to formalize the exact interface for those 'inner samplers' (rwalk/slice/uniform)

It is clear each sampler needs the sample() interface as defined in sampling.py, They also need update() interface as currently written in update_slice() etc. But I think my understanding is some samplers also need extra arguments passed to evolve_point, so therefore each sampler also needs some sort of generate_args(Sampler). In the simplest form it would just set the kwargs to whatever you want.