Distributed multiprocessing.Pool#

Ray supports running distributed python programs with the multiprocessing.Pool API using Ray Actors instead of local processes. This makes it easy to scale existing applications that use multiprocessing.Pool from a single node to a cluster.


To get started, first install Ray, then use ray.util.multiprocessing.Pool in place of multiprocessing.Pool. This will start a local Ray cluster the first time you create a Pool and distribute your tasks across it. See the Run on a Cluster section below for instructions to run on a multi-node Ray cluster instead.

from ray.util.multiprocessing import Pool

def f(index):
    return index

pool = Pool()
for result in pool.map(f, range(100)):

The full multiprocessing.Pool API is currently supported. Please see the multiprocessing documentation for details.


The context argument in the Pool constructor is ignored when using Ray.

Run on a Cluster#

This section assumes that you have a running Ray cluster. To start a Ray cluster, please refer to the cluster setup instructions.

To connect a Pool to a running Ray cluster, you can specify the address of the head node in one of two ways:

  • By setting the RAY_ADDRESS environment variable.

  • By passing the ray_address keyword argument to the Pool constructor.

from ray.util.multiprocessing import Pool

# Starts a new local Ray cluster.
pool = Pool()

# Connects to a running Ray cluster, with the current node as the head node.
# Alternatively, set the environment variable RAY_ADDRESS="auto".
pool = Pool(ray_address="auto")

# Connects to a running Ray cluster, with a remote node as the head node.
# Alternatively, set the environment variable RAY_ADDRESS="<ip_address>:<port>".
pool = Pool(ray_address="<ip_address>:<port>")

You can also start Ray manually by calling ray.init() (with any of its supported configuration options) before creating a Pool.