One benefit of the proposal method used in nessai is it allows for simple parallelisation of the likelihood evaluation since new live points are drawn in batches and then stored. The likelihood can therefore be precomputed and stored later use.
Likelihood parallelisation can be enabled in nessai by setting the keyword argument n_pool when calling FlowSampler. This determines the size of the multiprocessing pool to use for evaluating the likelihood.
Note
If running nessai via a job scheduler such as HTCondor, remember to set the number of requested CPUs accordingly.
Alternatively, nessai can use a user-defined pool. This is specified by setting the pool argument in NestedSampler or FlowSampler. Some variables must be initialised when creating the pool, this is done using :py:func:`~nessai.utils.multiprocessing.initialise_pool_variables`:
from multiprocessing import Pool
from nessai.utils.multiprocessing import initialise_pool_variables
model = GaussianModel()
pool = Pool(
processes=2,
initializer=initialise_pool_variables,
initargs=(model,),
)pool can then passed to the pool keyword argument when setting up the sampler.
ray includes a distributed multiprocessing pool that can also be used with nessai. Simply import ray.util.multiprocessing.Pool instead of the standard pool and initialise using the method described above.
When a pool object is passed to nessai it tries to determine how many processes the pool contains and (if the likelihood is vectorised) uses this information to determine the chunk size when evaluating the likelihood. If it can not determine this, then likelihood vectorisation will be disabled. This can be avoided by specifying n_pool and max_threads when initialising the sampler.
.. literalinclude:: ../examples/parallelisation_example.py
:language: python