Ad-hoc Distributed Computation

by Matthew Rocklin

tl;dr: We enable ad-hoc distributed computations with a concurrent.futures interface.


The distributed project prototype provides distributed computing on a cluster in pure Python.

concurrent.futures interface

The distributed.Executor interface mimics the concurrent.futures stdlib package.

from distributed import Executor
executor = Executor('')  # address of center-node in cluster

def inc(x):
    return x + 1

>>> x = executor.submit(inc, 1)
>>> x
<Future: status: waiting, key: inc-8bca9db48807c7d8bf613135d01b875f>

The submit call executes inc(1) on a remote machine. The returned future serves as a proxy to the remote result. We can collect this result back to the local process with the .result() method:

>>> x.result()  # transfers data from remote to local process

Data Locality

However, we don't want to do this; moving data from remote workers to the local process is expensive. We avoid calling .result().

By default the result of the computation (2) stays on the remote computer where the computation occurred. We avoid data transfer by allowing submit calls to directly accept futures as arguments:

>>> y = executor.submit(inc, x)  # no need to call x.result()

The computation for y will happen on the same machine where x lives. This interface deviates from concurrent.futures where we would have to wait on x before submiting y. We no longer have to do the following:

>>> y = executor.submit(inc, x.result())  # old concurrent.futures API

This is useful for two reasons:

  1. Avoids moving data to the local process: We keep data on the distributed network. This avoids unnecessary data transfer.
  2. Fire and forget: We don't have to reason about which jobs finish before which others. The scheduler takes care of this for us.

Ad hoc computations

For ad-hoc computations we often want the fine-grained control that a simple and efficient executor.submit(func, *args) function provides. Ad-hoc computations often have various function calls that depend on each other in strange ways; they don't fit a particular model or framework.

We don't need this fine-grained control for typical embarrassingly parallel work, like parsing JSON and loading it into a database. In these typical cases the bulk operations like map and reduce provided by MapReduce or Spark fit well.

However when computations aren't straightforward and don't fit nicely into a framework then we're stuck performing groupby and join gymnastics over strange key naming schemes to coerce our problem into the MapReduce or Spark programming models. If we're not operating at the petabyte scale then these programming models might be overkill.

The .submit function has an overhead of about a millisecond per call (not counting network latency). This might be crippling at the petabyte scale, but it's quite convenient and freeing at the medium-data scale. It lets us use a cluster of machines without strongly mangling our code or our conceptual model of what's going on.

Example: Binary Tree Reduction

As an example we perform a binary tree reduction on a sequence of random arrays.

This is the kind of algorithm you would find hard-coded into a library like Spark or dask.array/dask.dataframe but that we can accomplish by hand with some for loops while still using parallel distributed computing. The difference here is that we're not limited to the algorithms chosen for us and can screw around more freely.

finish           total             single array output
    ^          /        \
    |        z1          z2        neighbors merge
    |       /  \        /  \
    |     y1    y2    y3    y4     neighbors merge
    ^    / \   / \   / \   / \
start   x1 x2 x3 x4 x5 x6 x7 x8    many array inputs
  • Make sixteen, million element random arrays on the cluster:
import numpy as np
xs =, [1000000] * 16, pure=False)
  • Add neighbors until there is only one left:
while len(xs) > 1:
    xs = [executor.submit(add, xs[i], xs[i + 1])
          for i in range(0, len(xs), 2)]
  • Fetch final result:
>>> xs[0].result()
array([  2.069694  ,   9.01727599,   5.42682524, ...,   0.42372487,
         1.50364966,  13.48674896])

This entire computation, from writing the code to receiving the answer takes well under a second on a small cluster. This isn't surprising, it's a very small computation. What's notable though is the very small startup and overhead time. It's rare to find distributed computing systems that can spin up and finish small computations this quickly.


Various other Python frameworks provide distributed function evaluation. A few are listed here . Notably we're stepping on the toes of SCOOP, an excellent library that also provides a distributed concurrent.futures interface.

The distributed project could use a more distinct name. Any suggestions?

For more information see the following links:

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