7  Dask delayed

“Slidified” from the original dask documentation

Author
Affiliation

The dask Team

Paul-Valéry Montpellier 3 University

8 Dask Delayed

Sometimes problems don't fit into one of the collections like dask.array or dask.dataframe. In these cases, users can parallelize custom algorithms using the simpler dask.delayed interface. This allows you to create graphs directly with a light annotation of normal python code:

>>> x = dask.delayed(inc)(1)
>>> y = dask.delayed(inc)(2)
>>> z = dask.delayed(add)(x, y)
>>> z.compute()
5
>>> z.visualize()

Dask Delayed

A Dask Delayed task graph with two "inc" functions combined using an "add" function resulting in an output node.

8.1 Example

Visit https://examples.dask.org/delayed.html to see and run examples using Dask Delayed.

Sometimes we face problems that are parallelizable, but don't fit into high-level abstractions like Dask Array or Dask DataFrame. Consider the following example:

def inc(x):
    return x + 1

def double(x):
    return x * 2

def add(x, y):
    return x + y

data = [1, 2, 3, 4, 5]

output = []
for x in data:
    a = inc(x)
    b = double(x)
    c = add(a, b)
    output.append(c)

total = sum(output)

Example

There is clearly parallelism in this problem (many of the inc, double, and add functions can be evaluated independently), but it's not clear how to convert this to an array or DataFrame computation. As written, this code runs sequentially in a single thread. However, we see that a lot of this could be executed in parallel.

The Dask delayed function decorates your functions so that they operate lazily. Rather than executing your function immediately, it will defer execution, placing the function and its arguments into a task graph.

dask.delayed

delayed

Example

We slightly modify our code by wrapping functions in delayed. This delays the execution of the function and generates a Dask graph instead:

import dask

output = []
for x in data:
    a = dask.delayed(inc)(x)
    b = dask.delayed(double)(x)
    c = dask.delayed(add)(a, b)
    output.append(c)

total = dask.delayed(sum)(output)

Example

We used the dask.delayed function to wrap the function calls that we want to turn into tasks. None of the inc, double, add, or sum calls have happened yet. Instead, the object total is a Delayed result that contains a task graph of the entire computation. Looking at the graph we see clear opportunities for parallel execution. The Dask schedulers <scheduling> will exploit this parallelism, generally improving performance (although not in this example, because these functions are already very small and fast.)

Example

total.visualize()  # see image to the right

A task graph with many nodes for "inc" and "double" that combine with "add" nodes. The output of the "add" nodes finally aggregate with a "sum" node.

Example

We can now compute this lazy result to execute the graph in parallel:

>>> total.compute()
45

8.2 Decorator

It is also common to see the delayed function used as a decorator. Here is a reproduction of our original problem as a parallel code:

import dask

@dask.delayed
def inc(x):
    return x + 1

@dask.delayed
def double(x):
    return x * 2

@dask.delayed
def add(x, y):
    return x + y

data = [1, 2, 3, 4, 5]

output = []
for x in data:
    a = inc(x)
    b = double(x)
    c = add(a, b)
    output.append(c)

total = dask.delayed(sum)(output)

8.3 Real time

Sometimes you want to create and destroy work during execution, launch tasks from other tasks, etc. For this, see the Futures <futures> interface.

8.4 Best Practices

For a list of common problems and recommendations see Delayed Best Practices <delayed-best-practices>.

8.5 Indirect Dependencies

Sometimes you might find yourself wanting to add a dependency to a task that does not take the result of that dependency as an input. For example when a task depends on the side-effect of another task. In these cases you can use dask.graph_manipulation.bind.

import dask
from dask.graph_manipulation import bind

DATA = []

@dask.delayed
def inc(x):
    return x + 1

@dask.delayed
def add_data(x):
    DATA.append(x)

@dask.delayed
def sum_data(x):
    return sum(DATA) + x

a = inc(1)
b = add_data(a)
c = inc(3)
d = add_data(c)
e = inc(5)
f = bind(sum_data, [b, d])(e)
f.compute()

sum_data will operate on DATA only after both the expected items have been appended to it. bind can also be used along with direct dependencies passed through the function arguments.

9 Best Practices

It is easy to get started with Dask delayed, but using it well does require some experience. This page contains suggestions for best practices, and includes solutions to common problems.

9.1 Call delayed on the function, not the result

Dask delayed operates on functions like dask.delayed(f)(x, y), not on their results like dask.delayed(f(x, y)). When you do the latter, Python first calculates f(x, y) before Dask has a chance to step in.

Don't Do 
# This executes immediately

dask.delayed(f(x, y))
 
# This ma
kes a delayed function, acting lazily

dask.delayed(f)(x, y)

9.2 Compute on lots of computation at once

To improve parallelism, you want to include lots of computation in each compute call. Ideally, you want to make many dask.delayed calls to define your computation and then call dask.compute only at the end. It is ok to call dask.compute in the middle of your computation as well, but everything will stop there as Dask computes those results before moving forward with your code.

Don't Do 
# Avoid calling compute repeatedly

results = []
for x in L:
    y = dask.delayed(f)(x)
    results.append(y.compute())

results
 
# Collec
t many calls for one compute

results = []
for x in L:
    y = dask.delayed(f)(x)
    results.append(y)

resu
lts = dask.compute(*results)

Calling y.compute() within the loop would await the result of the computation every time, and so inhibit parallelism.

9.3 Don't mutate inputs

Your functions should not change the inputs directly.

Don't Do 
# Mutate inputs in functions

@dask.delayed
def f(x):
    x += 1
    return x
 
# Return new values or copies

@dask.delayed
def f(x):
    x = x + 1
    return x

If you need to use a mutable operation, then make a copy within your function first:

@dask.delayed
def f(x):
    x = copy(x)
    x += 1
    return x

9.4 Avoid global state

Ideally, your operations shouldn't rely on global state. Using global state might work if you only use threads, but when you move to multiprocessing or distributed computing then you will likely encounter confusing errors.

Don't 
L = []

# This references global variable L

@dask.delayed
def f(x):
    L.append(x)

9.5 Don't rely on side effects

Delayed functions only do something if they are computed. You will always need to pass the output to something that eventually calls compute.

Don't Do 
# Forget to call compute

dask.delayed(f)(1, 2, 3)

...
 
# Ensure delayed tasks are computed

x = dask.delayed(f)(1, 2, 3)
...
dask.compute(x, ...)

In the first case here, nothing happens, because compute() is never called.

9.6 Break up computations into many pieces

Every dask.delayed function call is a single operation from Dask's perspective. You achieve parallelism by having many delayed calls, not by using only a single one: Dask will not look inside a function decorated with @dask.delayed and parallelize that code internally. To accomplish that, it needs your help to find good places to break up a computation.

Don't Do 
# One giant task


def load(filename):
    ...


def process(data):
    ...


def save(data):
    ...

@dask.delayed
def f(filenames):
    results = []
    for filename in filenames:
        data = load(filename)
        data = process(data)
        result = save(data)
        results.append(result)

    return results

dask.compute(f(filenames))
 
# Break up into many tasks

@dask.delayed
def load(filename):
    ...

@dask.delayed
def process(data):
    ...

@dask.delayed
def save(data):
    ...


def f(filenames):
    results = []
    for filename in filenames:
        data = load(filename)
        data = process(data)
        result = save(data)
        results.append(result)

    return results

dask.compute(f(filenames))

The first version only has one delayed task, and so cannot parallelize.

9.7 Avoid too many tasks

Every delayed task has an overhead of a few hundred microseconds. Usually this is ok, but it can become a problem if you apply dask.delayed too finely. In this case, it's often best to break up your many tasks into batches or use one of the Dask collections to help you.

Don't Do 
# Too many tasks

results = []
fo
r x in range(10000000):

 y = dask.delayed(f)(x)
    results.append(y)
 
# Use collections

import dask.bag as db
b = db.from_s
equence(range(10000000), npartitions=1000)
b = b.map(f)
...

Avoid too many tasks

Here we use dask.bag to automatically batch applying our function. We could also have constructed our own batching as follows

def batch(seq):
    sub_results = []
    for x in seq:
        sub_results.append(f(x))
    return sub_results

 batches = []
 for i in range(0, 10000000, 10000):
     result_batch = dask.delayed(batch)(range(i, i + 10000))
     batches.append(result_batch)

Here we construct batches where each delayed function call computes for many data points from the original input.

9.8 Avoid calling delayed within delayed functions

Often, if you are new to using Dask delayed, you place dask.delayed calls everywhere and hope for the best. While this may actually work, it's usually slow and results in hard-to-understand solutions.

Usually you never call dask.delayed within dask.delayed functions.

Don't Do 
# Delayed function calls delayed

@dask.delayed
def process_all(L):
    result = []
    for x in L:
        y = dask.delayed(f)(x)
        result.append(y)
    return result
 
# Normal function calls delayed


def process_all(L):
    result = []
    for x in L:
        y = dask.delayed(f)(x)
        result.append(y)
    return result

Because the normal function only does delayed work it is very fast and so there is no reason to delay it.

9.9 Don't call dask.delayed on other Dask collections

When you place a Dask array or Dask DataFrame into a delayed call, that function will receive the NumPy or Pandas equivalent. Beware that if your array is large, then this might crash your workers.

Instead, it's more common to use methods like da.map_blocks

Don't Do 
# Call del
ayed functions on Dask collections

import dask.dataframe as dd
df = dd.read_csv('/path/to/*.csv')

dask.delayed(train)(df)
 
# Us
e mapping methods if applicable

import dask.dataframe as dd
df
= dd.read_csv('/path/to/*.csv')

df.map_partitions(train)

Don't call dask.delayed on other Dask collections

Alternatively, if the procedure doesn't fit into a mapping, you can always turn your arrays or dataframes into many delayed objects, for example

partitions = df.to_delayed()
delayed_values = [dask.delayed(train)(part)
                  for part in partitions]

However, if you don't mind turning your Dask array/DataFrame into a single chunk, then this is ok.

dask.delayed(train)(..., y=df.sum())

9.10 Avoid repeatedly putting large inputs into delayed calls

Every time you pass a concrete result (anything that isn't delayed) Dask will hash it by default to give it a name. This is fairly fast (around 500 MB/s) but can be slow if you do it over and over again. Instead, it is better to delay your data as well.

This is especially important when using a distributed cluster to avoid sending your data separately for each function call.

Don't Do 
x = np.arr
ay(...)  # some large array

results =
 [dask.delayed(train)(x, i)

      for i in range(1000)]
 
x
 = np.array(...)    # some large array
x =
dask.delayed(x)  # delay the data once
results = [dask.delayed(train)(x, i)
           for i in range(1000)]

Every call to dask.delayed(train)(x, ...) has to hash the NumPy array x, which slows things down.

10 Working with Collections

Often we want to do a bit of custom work with dask.delayed (for example, for complex data ingest), then leverage the algorithms in dask.array or dask.dataframe, and then switch back to custom work. To this end, all collections support from_delayed functions and to_delayed methods.

Working with Collections

As an example, consider the case where we store tabular data in a custom format not known by Dask DataFrame. This format is naturally broken apart into pieces and we have a function that reads one piece into a Pandas DataFrame. We use dask.delayed to lazily read these files into Pandas DataFrames, use dd.from_delayed to wrap these pieces up into a single Dask DataFrame, use the complex algorithms within the DataFrame (groupby, join, etc.), and then switch back to dask.delayed to save our results back to the custom format:

Working with Collections

import dask.dataframe as dd
from dask.delayed import delayed

from my_custom_library import load, save

filenames = ...
dfs = [delayed(load)(fn) for fn in filenames]

df = dd.from_delayed(dfs)
df = ... # do work with dask.dataframe

dfs = df.to_delayed()
writes = [delayed(save)(df, fn) for df, fn in zip(dfs, filenames)]

dd.compute(*writes)

Working with Collections

Data science is often complex, and dask.delayed provides a release valve for users to manage this complexity on their own, and solve the last mile problem for custom formats and complex situations.