4.2.4. Parallel analysis

Added in version 2.8.0: Parallelization of analysis classes was added during Google Summer of Code 2023 by @marinegor and MDAnalysis GSoC mentors.

This section explains the implementation and background for parallelization with the MDAnalysis.analysis.base.AnalysisBase, what users and developers need to know, when you should use parallelization (almost always!), and when you should abstain from doing so (rarely).

4.2.4.1. How to use parallelization

In order to use parallelization in a built-in analysis class SomeAnalysisClass, simply check which backends are available (see Backends for backends that are generally available), and then just enable them by providing backend='multiprocessing' and n_workers=... to SomeClass.run(...) method:

u = mda.Universe(...)
my_run = SomeClass(trajectory)
assert SomeClass.get_supported_backends() == ('serial', 'multiprocessing', 'dask')

my_run.run(backend='multiprocessing', n_workers=12)

For some classes, such as MDAnalysis.analysis.rms.RMSF (in its current implementation), split-apply-combine parallelization isn’t possible, and running them will be impossible with any but the serial backend.

Note

Parallelization is getting added to existing analysis classes. Initially, only MDAnalysis.analysis.rms.RMSD supports parallel analysis, but we aim to increase support in future releases.

4.2.4.2. How does parallelization work

The main idea behind its current implementation is that a trajectory analysis is often trivially parallelizable, meaning you can analyze all frames independently, and then merge them in a single object. This approach is also known as “split-apply-combine”, and isn’t new to MDAnalysis users, since it was first introduced in PMDA [1]. Version 2.8.0 of MDAnalysis brings this approach to the main library.

4.2.4.2.1. split-apply-combine

The following scheme explains the current AnalysisBase.run protocol (user-implemented methods are highlighted in orange):

In short, after checking input parameters and configuring the backend, AnalysisBase splits all the frames into computation groups (equally sized sequential groups of frames to be processed by each worker). All groups then get split between workers of a backend configured early, the main instance gets serialized and distributed between workers, and then _compute() method gets called for all frames of a computation group. Within this method, a user-implemented _single_frame() method gets applied to each frame in a computation group. After that, the main instance gets an object that will combine all the objects from other workers, and all instances get merged with an instance of MDAnalysis.analysis.results.ResultsGroup. Then, a normal user-implemented _compute() method is called.

Parallelization is fully compatible with existing code and does not break any existing code pre-2.8.0. The parallelization protocol mimics the single-process workflow where possible. Thus, user-implemented methods such as _prepare(), _single_frame() and _conclude() won’t need to know they are operating on an instance within the main python process, or on a remote instance, since the executed code is the same in both cases.

4.2.4.2.2. Methods in `AnalysisBase` for parallelization

4.2.4.2.2.1. For developers of new analysis tools

If you want to write your own parallel analysis class, you have to implement _prepare(), _single_frame() and _conclude(). You also have to denote if your analysis can run in parallel by following the steps under Adding parallelization to your own analysis class.

4.2.4.2.2.2. For MDAnalysis developers

From a developer point of view, there are a few methods that are important in order to understand how parallelization is implemented:

The first two methods share the functionality of _setup_frames(). _define_run_frames() is run once during analysis, as it checks that input parameters start, stop, step or frames are consistent with the given trajectory and prepares the slicer object that defines the iteration pattern through the trajectory. _prepare_sliced_trajectory() assigns to the self._sliced_trajectory attribute, computes the number of frames in it, and fills the self.frames and self.times arrays. In case the computation will be later split between other processes, this method will be called again on each of the computation groups.

The method _configure_backend() performs basic health checks for a given analysis class – namely, it compares a given backend (if it’s a str instance, such as 'multiprocessing') with the list of builtin backends (and also the backends implemented for a given analysis subclass), and configures a MDAnalysis.analysis.backends.BackendBase instance accordingly. If the user decides to provide a custom backend (any subclass of MDAnalysis.analysis.backends.BackendBase, or anything with an apply() method), it ensures that the number of workers wasn’t specified twice (during backend initialization and in run() arguments).

After a backend is configured, _setup_computation_groups() splits the frames prepared earlier in self._prepare_sliced_trajectory into a number of groups, by default equal to the number of workers.

In the _compute() method, frames get initialized again with _prepare_sliced_trajectory(), and attributes necessary for a specific analysis get initialized with the _prepare() method. Then the function iterates over self._sliced_trajectory, assigning self._frame_index and self._ts as frame index (within a computation group) and timestamp, and also setting respective self.frames and self.times array values.

After _compute() has finished, the main analysis instance calls the _get_aggregator() method, which merges the self.results attributes from other processes into a single MDAnalysis.analysis.results.Results instance, making it look for the subsequent _conclude() method as if the run was performed in a serial fashion, without parallelization.

4.2.4.3. Helper classes for parallelization

4.2.4.3.1. `ResultsGroup`

MDAnalysis.analysis.results.ResultsGroup extends the functionality of the MDAnalysis.analysis.results.Results class. Since the Results class is basically a dictionary that also keeps track of assigned attributes, it is possible to iterate over all these attributes later. ResultsGroup does exactly that: given a list of the Results objects with the same attributes, it applies a specific aggregation function to every attribute, and stores it as a same attribute of the returned object:

from MDAnalysis.analysis.results import ResultsGroup, Results
group = ResultsGroup(lookup={'mass': ResultsGroup.float_mean})
obj1 = Results(mass=1)
obj2 = Results(mass=3)
assert group.merge([obj1, obj2]) == Results(mass=2.0)

4.2.4.3.2. `BackendBase`

MDAnalysis.analysis.backends.BackendBase holds all backend attributes, and also implements an MDAnalysis.analysis.backends.BackendBase.apply() method, applying a given function to a list of its parameters, but in a parallel fashion. Although in AnalysisBase it is used to apply a _compute function, in principle it can be used to any arbitrary function and arguments, given they’re serializable.

4.2.4.4. When to use parallelization? (Known limitations)

For now, the syntax for running parallel analysis is explicit, meaning by default the serial version will be run, and the parallelization won’t be enabled by default. Although we expect the parallelization to be useful in most cases, there are some known caveats from the inital benchmarks.

4.2.4.4.1. Fast `_single_frame` compared to reading from disk

In all cases, parallelization will not be useful only when frames are being processed faster than being read from the disk, otherwise reading is the bottleneck here. Hence, you’ll benefit from parallelization only if you have relatively much compute per frame, or a fast drive, as illustrated below:

In other words, if you have fast analysis (say, MDAnalysis.analysis.rms.RMSD) and a slow HDD drive, you are likely to not get any benefits from parallelization. Otherwise, you should be fine.

4.2.4.4.2. Serialization issues

For built-in analysis classes, the default serialization with both multiprocessing and dask is known to work. If you’re using some custom analysis class that e.g. stores a non-serializable object in one of its attributes, you might get a serialization error (PicklingError if you’re using a multiprocessing backend). If you want to get around that, we suggest trying backend='dask' (it uses dask serialization engine instead of pickle).

4.2.4.4.3. Out of memory issues

If you have large memory footprint of each worker, you can run into out-of-memory errors (i.e. your server freezes when executing a run). In this case we suggest decreasing the number of workers from all available CPUs (that you can get with multiprocessing.cpu_count()) to a smaller number.

4.2.4.4.4. Progress bar is missing

It is yet not possible to get a progress bar running with any parallel backend. If you want an ETA of your analysis, we suggest running it in serial mode for the first 10-100 frames with verbose=True, and then running it with multiple workers. Processing time scales almost linearly, so you can get your ETA by dividing serial ETA by the number of workers.

4.2.4.5. Adding parallelization to your own analysis class

If you want to add parallelization to your own analysis class, first make sure your algorithm allows you to do that, i.e. you can process each frame independently. Then it’s rather simple – let’s look at the actual code that added parallelization to the MDAnalysis.analysis.rms.RMSD:

from MDAnalysis.analysis.base import AnalysisBase
from MDAnalysis.analysis.results import ResultsGroup

class RMSD(BackendBase):
    @classmethod
    def get_supported_backends(cls):
        return ('serial', 'multiprocessing', 'dask',)

    _analysis_algorithm_is_parallelizable = True

    def _get_aggregator(self):
        return ResultsGroup(lookup={'rmsd': ResultsGroup.ndarray_vstack})

That’s it! The first two methods are boilerplate – get_supported_backends() returns a tuple with built-in backends that will work for your class (if there are no serialization issues, it should be all three), and _is_parallelizable is True (which is set to False in AnalysisBase, hence we have to re-define it), and _get_aggregator() will be used as described earlier. Note that MDAnalysis.analysis.results also provides a few convenient functions (defined as class methods of ResultsGroup) for results aggregation:

So you’ll likely find appropriate functions for basic aggregation there.

4.2.4.6. Writing custom backends

In order to write your custom backend (e.g. using dask.distributed), inherit from the MDAnalysis.analysis.backends.BackendBase and (re)-implement __init__() and apply() methods. Optionally, you can implement methods for validation of correct backend initialization – _get_checks() and _get_warnings(), that would raise an exception or give a warning, respectively, when a new class instance is created:

MDAnalysis.analysis.backends._get_checks()
MDAnalysis.analysis.backends._get_warnings()

from MDAnalysis.analysis.backends import BackendBase
class ThreadsBackend(BackendBase):
    def __init__(self, n_workers: int, starting_message: str = "Useless backend"):
        self.n_workers = n_workers
        self.starting_message = starting_message
        self._validate()

    def _get_warnings(self):
        return {True: 'warning: this backend is useless'}

    def _get_checks(self):
        return {isinstance(self.n_workers, int), 'error: self.n_workers is not an integer'}

    def apply(self, func, computations):
        from multiprocessing.dummy import Pool

        with Pool(processes=self.n_workers) as pool:
            print(self.starting_message)
            results = pool.map(func, computations)
        return results

In order to use a custom backend with another analysis class that does not explicitly support it, you must explicitly state that you’re about to use an unsupported_backend by passing the keyword argument unsupported_backend=True:

from MDAnalysis.analysis.rms import RMSD
R = RMSD(...) # setup the run
n_workers = 2
backend = ThreadsBackend(n_workers=n_workers)
R.run(backend=backend, unsupported_backend=True)

In this way, you will override the check for supported backends.

Warning

When you use unsupported_backend=True you should make sure that you get the same results as when using a supported backend for which the analysis class was tested.

Before reporting a problem with an analysis class, make sure you tested it with a supported backend. When reporting always mention if you used unsupported_backend=True.

References