Horovod on Ray

horovod.ray allows users to leverage Horovod on a Ray cluster.

Currently, the Ray + Horovod integration provides a RayExecutor API.


The Ray + Horovod integration currently only supports a Gloo backend.


Use the extra [ray] option to install Ray along with Horovod.

$ HOROVOD_WITH_GLOO=1 ... pip install 'horovod[ray]'

See the Ray documentation for advanced installation instructions.

Horovod Ray Executor

The Horovod Ray integration offers a RayExecutor abstraction (docs), which is a wrapper over a group of Ray actors (stateful processes).

from horovod.ray import RayExecutor

# Start the Ray cluster or attach to an existing Ray cluster

# Start num_workers actors on the cluster
executor = RayExecutor(
    setting, num_workers=num_workers, use_gpu=True)

# This will launch `num_workers` actors on the Ray Cluster.

All actors will be part of the Horovod ring, so RayExecutor invocations will be able to support arbitrary Horovod collective operations.

Note that there is an implicit assumption on the cluster being homogenous in shape (i.e., all machines have the same number of slots available). This is simply an implementation detail and is not a fundamental limitation.

To actually execute a function, you can run the following:

# Using the stateless `run` method, a function can take in any args or kwargs
def simple_fn():
    print("hvd rank", hvd.rank())
    return hvd.rank()

# Execute the function on all workers at once
result = executor.run(simple_fn)
# Check that the rank of all workers is unique
assert len(set(result)) == hosts * num_slots


Stateful Execution

A unique feature of Ray is its support for stateful Actors. This means that you can start arbitrary Python classes on each worker, easily supporting operations and calls where data is cached in memory.

import torch
from horovod.torch import hvd
from horovod.ray import RayExecutor

class MyModel:
    def __init__(self, learning_rate):
        self.model = NeuralNet()
        optimizer = torch.optim.SGD(
        self.optimizer = hvd.DistributedOptimizer(optimizer)

    def get_weights(self):
        return dict(self.model.parameters())

    def train(self):
        return self._train(self.model, self.optimizer)

executor = RayExecutor(...)

# Run 5 training steps
for i in range(5):
    # Stateful `execute` method takes the current worker executable as a parameter
    executor.execute(lambda worker: worker.train())

# Obtain the trained weights from each model replica
result = executor.execute(lambda worker: worker.get_weights())

# `result` will be N copies of the model weights
assert all(isinstance(res, dict) for res in result)

Elastic Ray Executor

Ray also supports elastic execution via the RayExecutor. Similar to default Horovod, the difference between the non-elastic and elastic versions of Ray is that the hosts and number of workers is dynamically determined at runtime.

You must first set up a Ray cluster. Ray clusters can support autoscaling for any cloud provider (AWS, GCP, Azure).

# First, run `pip install boto3` and `aws configure`
# Create or update the cluster. When the command finishes, it will print
# out the command that can be used to SSH into the cluster head node.
$ ray up ray/python/ray/autoscaler/aws/example-full.yaml

After you have a Ray cluster setup, you will need to move parts of your existing elastic Horovod training script into a training function. Specifically, the instantiation of your model and the invocation of the hvd.elastic.run call should be done inside this function.

import horovod.torch as hvd

# Put the Horovod concepts into a single function
# This function will be serialized with Cloudpickle
def training_fn():
    model = Model()

    def train(state):
        for state.epoch in range(state.epoch, epochs):

    state = hvd.elastic.TorchState(model, optimizer, batch=0, epoch=0)

You can then attach to the underlying Ray cluster and execute the training function:

import ray
from horovod.ray import RayExecutor

ray.init(address="auto")  # attach to the Ray cluster
settings = RayExecutor.create_settings(verbose=True)
executor = RayExecutor(
    settings, min_workers=1, use_gpu=True, cpus_per_slot=2)

Ray will automatically start remote actors which execute training_fn on nodes as they become available. Note that executor.run call will terminate whenever any one of the training functions terminates successfully, or if all workers fail.

AWS: Cluster Launcher

You can also easily leverage the Ray cluster launcher to spin up cloud instances.

# Save as `ray_cluster.yaml`

cluster_name: horovod-cluster
provider: {type: aws, region: us-west-2}
auth: {ssh_user: ubuntu}
min_workers: 3
max_workers: 3

# Deep Learning AMI (Ubuntu) Version 21.0
head_node: {InstanceType: p3.2xlarge, ImageId: ami-0b294f219d14e6a82}
worker_nodes: {InstanceType: p3.2xlarge, ImageId: ami-0b294f219d14e6a82}
setup_commands: # Set up each node.
    - HOROVOD_WITH_GLOO=1 HOROVOD_GPU_OPERATIONS=NCCL pip install horovod[ray]

You can start the specified Ray cluster and monitor its status with:

$ ray up ray_cluster.yaml  # starts the head node
$ ray monitor ray_cluster.yaml  # wait for worker nodes

Then, in your python script, make sure you add ray.init(address="auto") to connect to the distributed Ray cluster.


Then you can execute Ray scripts on the cluster:

$ ray submit ray_cluster.yaml <your_script.py>

# the above is is equivalent to
$ ray attach ray_cluster.yaml  # ssh
ubuntu@ip-172-31-24-53:~$ python <your_script.py>