optimize.rst

Optimization

Performance can be significantly improved in different contexts by making small optimizations on the dask graph before calling the scheduler.

The dask.optimization module contains several functions to transform graphs in a variety of useful ways. In most cases, users won't need to interact with these functions directly, as specialized subsets of these transforms are done automatically in the dask collections (dask.array, dask.bag, and dask.dataframe). However, users working with custom graphs or computations may find that applying these methods results in substantial speedups.

In general, there are two goals when doing graph optimizations:

1. Simplify computation
2. Improve parallelism.

Simplifying computation can be done on a graph level by removing unnecessary tasks (cull), or on a task level by replacing expensive operations with cheaper ones (RewriteRule).

Parallelism can be improved by reducing inter-task communication, whether by fusing many tasks into one (fuse), or by inlining cheap operations (inline, inline_functions).

Below, we show an example walking through the use of some of these to optimize a task graph.

Example

Suppose you had a custom dask graph for doing a word counting task:

>>> from __future__ import print_function

>>> def print_and_return(string):
...     print(string)
...     return string

>>> def format_str(count, val, nwords):
...     return ('word list has {0} occurrences of {1}, '
...             'out of {2} words').format(count, val, nwords)

>>> dsk = {'words': 'apple orange apple pear orange pear pear',
...        'nwords': (len, (str.split, 'words')),
...        'val1': 'orange',
...        'val2': 'apple',
...        'val3': 'pear',
...        'count1': (str.count, 'words', 'val1'),
...        'count2': (str.count, 'words', 'val2'),
...        'count3': (str.count, 'words', 'val3'),
...        'out1': (format_str, 'count1', 'val1', 'nwords'),
...        'out2': (format_str, 'count2', 'val2', 'nwords'),
...        'out3': (format_str, 'count3', 'val3', 'nwords'),
...        'print1': (print_and_return, 'out1'),
...        'print2': (print_and_return, 'out2'),
...        'print3': (print_and_return, 'out3')}

Here we're counting the occurrence of the words 'orange, 'apple', and 'pear' in the list of words, formatting an output string reporting the results, printing the output, then returning the output string.

To perform the computation, we pass the dask graph and the desired output keys to a scheduler get function:

>>> from dask.threaded import get

>>> outputs = ['print1', 'print2']
>>> results = get(dsk, outputs)
word list has 2 occurrences of apple, out of 7 words
word list has 2 occurrences of orange, out of 7 words

>>> results
('word list has 2 occurrences of orange, out of 7 words',
 'word list has 2 occurrences of apple, out of 7 words')

As can be seen above, the scheduler computed only the requested outputs ('print3' was never computed). This is because the scheduler internally calls cull, which removes the unnecessary tasks from the graph. Even though this is done internally in the scheduler, it can be beneficial to call it at the start of a series of optimizations to reduce the amount of work done in later steps:

>>> from dask.optimization import cull
>>> dsk1, dependencies = cull(dsk, outputs)

Looking at the task graph above, there are multiple accesses to constants such as 'val1' or 'val2' in the dask graph. These can be inlined into the tasks to improve efficiency using the inline function. For example:

>>> from dask.optimization import inline
>>> dsk2 = inline(dsk1, dependencies=dependencies)
>>> results = get(dsk2, outputs)
word list has 2 occurrences of apple, out of 7 words
word list has 2 occurrences of orange, out of 7 words

Now we have two sets of almost linear task chains. The only link between them is the word counting function. For cheap operations like this, the serialization cost may be larger than the actual computation, so it may be faster to do the computation more than once, rather than passing the results to all nodes. To perform this function inlining, the inline_functions function can be used:

>>> from dask.optimization import inline_functions
>>> dsk3 = inline_functions(dsk2, outputs, [len, str.split],
...                         dependencies=dependencies)
>>> results = get(dsk3, outputs)
word list has 2 occurrences of apple, out of 7 words
word list has 2 occurrences of orange, out of 7 words

Now we have a set of purely linear tasks. We'd like to have the scheduler run all of these on the same worker to reduce data serialization between workers. One option is just to merge these linear chains into one big task using the fuse function:

>>> from dask.optimization import fuse
>>> dsk4, dependencies = fuse(dsk3)
>>> results = get(dsk4, outputs)
word list has 2 occurrences of apple, out of 7 words
word list has 2 occurrences of orange, out of 7 words

Putting it all together:

>>> def optimize_and_get(dsk, keys):
...     dsk1, deps = cull(dsk, keys)
...     dsk2 = inline(dsk1, dependencies=deps)
...     dsk3 = inline_functions(dsk2, keys, [len, str.split],
...                             dependencies=deps)
...     dsk4, deps = fuse(dsk3)
...     return get(dsk4, keys)

>>> optimize_and_get(dsk, outputs)
word list has 2 occurrences of apple, out of 7 words
word list has 2 occurrences of orange, out of 7 words

In summary, the above operations accomplish the following:

1. Removed tasks unnecessary for the desired output using cull.
2. Inlined constants using inline.
3. Inlined cheap computations using inline_functions, improving parallelism.
4. Fused linear tasks together to ensure they run on the same worker using fuse.

As stated previously, these optimizations are already performed automatically in the dask collections. Users not working with custom graphs or computations should rarely need to directly interact with them.

These are just a few of the optimizations provided in dask.optimization. For more information, see the API below.

Rewrite Rules

For context based optimizations, dask.rewrite provides functionality for pattern matching and term rewriting. This is useful for replacing expensive computations with equivalent, cheaper computations. For example, dask.array uses the rewrite functionality to replace series of array slicing operations with a more efficient single slice.

The interface to the rewrite system consists of two classes:

1. RewriteRule(lhs, rhs, vars)

   Given a left-hand-side (lhs), a right-hand-side (rhs), and a set of variables (vars), a rewrite rule declaratively encodes the following operation:

   lhs -> rhs if task matches lhs over variables

2. RuleSet(*rules)

   A collection of rewrite rules. The design of RuleSet class allows for efficient "many-to-one" pattern matching, meaning that there is minimal overhead for rewriting with multiple rules in a rule set.

Example

Here we create two rewrite rules expressing the following mathematical transformations:

1. a + a -> 2*a
2. a * a -> a**2

where 'a' is a variable:

>>> from dask.rewrite import RewriteRule, RuleSet
>>> from operator import add, mul, pow

>>> variables = ('a',)

>>> rule1 = RewriteRule((add, 'a', 'a'), (mul, 'a', 2), variables)

>>> rule2 = RewriteRule((mul, 'a', 'a'), (pow, 'a', 2), variables)

>>> rs = RuleSet(rule1, rule2)

The RewriteRule objects describe the desired transformations in a declarative way, and the RuleSet builds an efficient automata for applying that transformation. Rewriting can then be done using the rewrite method:

>>> rs.rewrite((add, 5, 5))
(mul, 5, 2)

>>> rs.rewrite((mul, 5, 5))
(pow, 5, 2)

>>> rs.rewrite((mul, (add, 3, 3), (add, 3, 3)))
(pow, (mul, 3, 2), 2)

The whole task is traversed by default. If you only want to apply a transform to the top-level of the task, you can pass in strategy='top_level' as shown:

# Transforms whole task
>>> rs.rewrite((sum, [(add, 3, 3), (mul, 3, 3)]))
(sum, [(mul, 3, 2), (pow, 3, 2)])

# Only applies to top level, no transform occurs
>>> rs.rewrite((sum, [(add, 3, 3), (mul, 3, 3)]), strategy='top_level')
(sum, [(add, 3, 3), (mul, 3, 3)])

The rewriting system provides a powerful abstraction for transforming computations at a task level. Again, for many users, directly interacting with these transformations will be unnecessary.

Keyword Arguments

Some optimizations take optional keyword arguments. To pass keywords from the compute call down to the right optimization, prepend the keyword with the name of the optimization. For example to send a keys= keyword argument to the fuse optimization from a compute call, use the fuse_keys= keyword:

def fuse(dsk, keys=None):
    ...

x.compute(fuse_keys=['x', 'y', 'z'])

Customizing Optimization

Dask defines a default optimization strategy for each collection type (Array, Bag, DataFrame, Delayed). However different applications may have different needs. To address this variability of needs, you can construct your own custom optimization function and use it instead of the default. An optimization function takes in a task graph and list of desired keys and returns a new task graph.

def my_optimize_function(dsk, keys):
    new_dsk = {...}
    return new_dsk

You can then register this optimization class against whichever collection type you prefer and it will be used instead of the default scheme.

with dask.config.set(array_optimize=my_optimize_function):
    x, y = dask.compute(x, y)

You can register separate optimization functions for different collections, or you can register None if you do not want particular types of collections to be optimized.

with dask.config.set(array_optimize=my_optimize_function,
                     dataframe_optimize=None,
                     delayed_optimize=my_other_optimize_function):
    ...

You need not specify all collections. Collections will default to their standard optimization scheme (which is usually a good choice).

API

.. currentmodule:: dask.optimization

Top level optimizations

.. autosummary::
   cull
   fuse
   inline
   inline_functions

Utility functions

.. autosummary::
   functions_of

Rewrite Rules

.. currentmodule:: dask.rewrite

.. autosummary::
    RewriteRule
    RuleSet

Definitions

.. currentmodule:: dask.optimization

.. autofunction:: cull

.. autofunction:: fuse

.. autofunction:: inline

.. autofunction:: inline_functions

.. autofunction:: functions_of

.. currentmodule:: dask.rewrite

.. autofunction:: RewriteRule

.. autofunction:: RuleSet