# Scheduler Overview

After we create a dask graph, we use a scheduler to run it. Dask currently implements a few different schedulers:

- `dask.threaded.get`: a scheduler backed by a thread pool

- `dask.multiprocessing.get`: a scheduler backed by a process pool

- `dask.get`: a synchronous scheduler, good for debugging

- `distributed.Client.get`: a distributed scheduler for executing graphs on multiple machines. This lives in the external distributed project.

## The `get` function

The entry point for all schedulers is a `get` function. This takes a dask graph, and a key or list of keys to compute:

    >>> from operator import add

    >>> dsk = {'a': 1,
    ...        'b': 2,
    ...        'c': (add, 'a', 'b'),
    ...        'd': (sum, ['a', 'b', 'c'])}

    >>> get(dsk, 'c')
    3

    >>> get(dsk, 'd')
    6

    >>> get(dsk, ['a', 'b', 'c'])
    [1, 2, 3]

## Using `compute` methods

When working with dask collections, you will rarely need to interact with scheduler `get` functions directly. Each collection has a default scheduler, and a built-in `compute` method that calculates the output of the collection:

    >>> import dask.array as da
    >>> x = da.arange(100, chunks=10)
    >>> x.sum().compute()
    4950

The compute method takes a number of keywords:

- `scheduler`: the name of the desired scheduler as a string (`"threads"`, `"processes"`, `"single-threaded"`, etc.), a `get` function, or a `dask.distributed.Client` object. Overrides the default for the collection.

- `**kwargs`: extra keywords to pass on to the scheduler `get` function.

See also: Configuring the schedulers.

## The `compute` function

You may wish to compute results from multiple dask collections at once. Similar to the `compute` method on each collection, there is a general `compute` function that takes multiple collections and returns multiple results. This merges the graphs from each collection, so intermediate results are shared:

    >>> y = (x + 1).sum()
    >>> z = (x + 1).mean()
    >>> da.compute(y, z)    # Compute y and z, sharing intermediate results
    (5050, 50.5)

Here the `x + 1` intermediate was only computed once, while calling `y.compute()` and `z.compute()` would compute it twice. For large graphs that share many intermediates, this can be a big performance gain.

The `compute` function works with any dask collection, and is found in `dask.base`. For convenience it has also been imported into the top level namespace of each collection.

    >>> from dask.base import compute
    >>> compute is da.compute
    True

## Configuring the schedulers

The dask collections each have a default scheduler:

- `dask.array` and `dask.dataframe` use the threaded scheduler by default

- `dask.bag` uses the multiprocessing scheduler by default.

For most cases, the default settings are good choices. However, sometimes you may want to use a different scheduler. There are two ways to do this.

1. Using the `scheduler` keyword in the `compute` method:

   >     >>> x.sum().compute(scheduler='processes')

2. Using `dask.config.set`. This can be used either as a context manager, or to set the scheduler globally:

   >     # As a context manager
   >     >>> with dask.config.set(scheduler='processes'):
   >     ...     x.sum().compute()

   >     # Set globally
   >     >>> dask.config.set(scheduler='processes')
   >     >>> x.sum().compute()

Additionally, each scheduler may take a few extra keywords specific to that scheduler. For example, the multiprocessing and threaded schedulers each take a `num_workers` keyword, which sets the number of processes or threads to use (defaults to number of cores). This can be set by passing the keyword when calling `compute`:

    # Compute with 4 threads
    >>> x.compute(num_workers=4)

Alternatively, the multiprocessing and threaded schedulers will check for a global pool set with `dask.config.set`:

    >>> from concurrent.futures import ThreadPoolExecutor
    >>> with dask.config.set(pool=ThreadPoolExecutor(4)):
    ...     x.compute()

The multiprocessing scheduler also supports different contexts (“spawn”, “forkserver”, “fork”) which you can set with `dask.config.set`. The default context is “spawn”, but you can set a different one:

    >>> with dask.config.set({"multiprocessing.context": "forkserver"}):
    ...     x.compute()

For more information on the individual options for each scheduler, see the docstrings for each scheduler `get` function.

## Debugging the schedulers

Debugging parallel code can be difficult, as conventional tools such as `pdb` don’t work well with multiple threads or processes. To get around this when debugging, we recommend using the synchronous scheduler found at `dask.get`. This runs everything serially, allowing it to work well with `pdb`:

    >>> dask.config.set(scheduler='single-threaded')
    >>> x.sum().compute()    # This computation runs serially instead of in parallel

The shared memory schedulers also provide a set of callbacks that can be used for diagnosing and profiling. You can learn more about scheduler callbacks and diagnostics here.

## More Information

- See Shared Memory for information on the design of the shared memory (threaded or multiprocessing) schedulers

- See distributed for information on the distributed memory scheduler