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|
"""Analysis backends --- :mod:`MDAnalysis.analysis.backends`
============================================================
.. versionadded:: 2.8.0
The :mod:`backends` module provides :class:`BackendBase` base class to
implement custom execution backends for
:meth:`MDAnalysis.analysis.base.AnalysisBase.run` and its
subclasses.
.. SeeAlso:: :ref:`parallel-analysis`
.. _backends:
Backends
--------
Three built-in backend classes are provided:
* *serial*: :class:`BackendSerial`, that is equivalent to using no
parallelization and is the default
* *multiprocessing*: :class:`BackendMultiprocessing` that supports
parallelization via standard Python :mod:`multiprocessing` module
and uses default :mod:`pickle` serialization
* *dask*: :class:`BackendDask`, that uses the same process-based
parallelization as :class:`BackendMultiprocessing`, but different
serialization algorithm via `dask <https://dask.org/>`_ (see `dask
serialization algorithms
<https://distributed.dask.org/en/latest/serialization.html>`_ for details)
Classes
-------
"""
import warnings
from typing import Callable
from MDAnalysis.lib.util import is_installed
class BackendBase:
"""Base class for backend implementation.
Initializes an instance and performs checks for its validity, such as
``n_workers`` and possibly other ones.
Parameters
----------
n_workers : int
number of workers (usually, processes) over which the work is split
Examples
--------
.. code-block:: python
from MDAnalysis.analysis.backends import BackendBase
class ThreadsBackend(BackendBase):
def apply(self, func, computations):
from multiprocessing.dummy import Pool
with Pool(processes=self.n_workers) as pool:
results = pool.map(func, computations)
return results
import MDAnalysis as mda
from MDAnalysis.tests.datafiles import PSF, DCD
from MDAnalysis.analysis.rms import RMSD
u = mda.Universe(PSF, DCD)
ref = mda.Universe(PSF, DCD)
R = RMSD(u, ref)
n_workers = 2
backend = ThreadsBackend(n_workers=n_workers)
R.run(backend=backend, unsupported_backend=True)
.. warning::
Using `ThreadsBackend` above will lead to erroneous results, since it
is an educational example. Do not use it for real analysis.
.. versionadded:: 2.8.0
"""
def __init__(self, n_workers: int):
self.n_workers = n_workers
self._validate()
def _get_checks(self):
"""Get dictionary with ``condition: error_message`` pairs that ensure the
validity of the backend instance
Returns
-------
dict
dictionary with ``condition: error_message`` pairs that will get
checked during ``_validate()`` run
"""
return {
isinstance(self.n_workers, int)
and self.n_workers
> 0: f"n_workers should be positive integer, got {self.n_workers=}",
}
def _get_warnings(self):
"""Get dictionary with ``condition: warning_message`` pairs that ensure
the good usage of the backend instance
Returns
-------
dict
dictionary with ``condition: warning_message`` pairs that will get
checked during ``_validate()`` run
"""
return dict()
def _validate(self):
"""Check correctness (e.g. ``dask`` is installed if using ``backend='dask'``)
and good usage (e.g. ``n_workers=1`` if backend is serial) of the backend
Raises
------
ValueError
if one of the conditions in :meth:`_get_checks` is ``True``
"""
for check, msg in self._get_checks().items():
if not check:
raise ValueError(msg)
for check, msg in self._get_warnings().items():
if not check:
warnings.warn(msg)
def apply(self, func: Callable, computations: list) -> list:
"""map function `func` to all tasks in the `computations` list
Main method that will get called when using an instance of
``BackendBase``. It is equivalent to running ``[func(item) for item in
computations]`` while using the parallel backend capabilities.
Parameters
----------
func : Callable
function to be called on each of the tasks in computations list
computations : list
computation tasks to apply function to
Returns
-------
list
list of results of the function
"""
raise NotImplementedError
class BackendSerial(BackendBase):
"""A built-in backend that does serial execution of the function, without any
parallelization.
Parameters
----------
n_workers : int
Is ignored in this class, and if ``n_workers`` > 1, a warning will be
given.
.. versionadded:: 2.8.0
"""
def _get_warnings(self):
"""Get dictionary with ``condition: warning_message`` pairs that ensure
the good usage of the backend instance. Here, it checks if the number
of workers is not 1, otherwise gives warning.
Returns
-------
dict
dictionary with ``condition: warning_message`` pairs that will get
checked during ``_validate()`` run
"""
return {
self.n_workers
== 1: "n_workers is ignored when executing with backend='serial'"
}
def apply(self, func: Callable, computations: list) -> list:
"""
Serially applies `func` to each task object in ``computations``.
Parameters
----------
func : Callable
function to be called on each of the tasks in computations list
computations : list
computation tasks to apply function to
Returns
-------
list
list of results of the function
"""
return [func(task) for task in computations]
class BackendMultiprocessing(BackendBase):
"""A built-in backend that executes a given function using the
:meth:`multiprocessing.Pool.map <multiprocessing.pool.Pool.map>` method.
Parameters
----------
n_workers : int
number of processes in :class:`multiprocessing.Pool
<multiprocessing.pool.Pool>` to distribute the workload
between. Must be a positive integer.
Examples
--------
.. code-block:: python
from MDAnalysis.analysis.backends import BackendMultiprocessing
import multiprocessing as mp
backend_obj = BackendMultiprocessing(n_workers=mp.cpu_count())
.. versionadded:: 2.8.0
"""
def apply(self, func: Callable, computations: list) -> list:
"""Applies `func` to each object in ``computations`` using `multiprocessing`'s `Pool.map`.
Parameters
----------
func : Callable
function to be called on each of the tasks in computations list
computations : list
computation tasks to apply function to
Returns
-------
list
list of results of the function
"""
from multiprocessing import Pool
with Pool(processes=self.n_workers) as pool:
results = pool.map(func, computations)
return results
class BackendDask(BackendBase):
"""A built-in backend that executes a given function with *dask*.
Execution is performed with the :func:`dask.compute` function of
:class:`dask.delayed.Delayed` object (created with
:func:`dask.delayed.delayed`) with ``scheduler='processes'`` and
``chunksize=1`` (this ensures uniform distribution of tasks among
processes). Requires the `dask package <https://docs.dask.org/en/stable/>`_
to be `installed <https://docs.dask.org/en/stable/install.html>`_.
Parameters
----------
n_workers : int
number of processes in to distribute the workload
between. Must be a positive integer. Workers are actually
:class:`multiprocessing.pool.Pool` processes, but they use a different and
more flexible `serialization protocol
<https://docs.dask.org/en/stable/phases-of-computation.html#graph-serialization>`_.
Examples
--------
.. code-block:: python
from MDAnalysis.analysis.backends import BackendDask
import multiprocessing as mp
backend_obj = BackendDask(n_workers=mp.cpu_count())
.. versionadded:: 2.8.0
"""
def apply(self, func: Callable, computations: list) -> list:
"""Applies `func` to each object in ``computations``.
Parameters
----------
func : Callable
function to be called on each of the tasks in computations list
computations : list
computation tasks to apply function to
Returns
-------
list
list of results of the function
"""
from dask.delayed import delayed
import dask
computations = [delayed(func)(task) for task in computations]
results = dask.compute(
computations,
scheduler="processes",
chunksize=1,
num_workers=self.n_workers,
)[0]
return results
def _get_checks(self):
"""Get dictionary with ``condition: error_message`` pairs that ensure the
validity of the backend instance. Here checks if ``dask`` module is
installed in the environment.
Returns
-------
dict
dictionary with ``condition: error_message`` pairs that will get
checked during ``_validate()`` run
"""
base_checks = super()._get_checks()
checks = {
is_installed("dask"): (
"module 'dask' is missing. Please install 'dask': "
"https://docs.dask.org/en/stable/install.html"
)
}
return base_checks | checks
|
