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 Job

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_hosts * num_slots actors on the cluster
executor = RayExecutor(
    setting, num_hosts=num_hosts, num_slots=num_slots, use_gpu=True)

# Launch the Ray actors on each machine
# This will launch `num_slots` actors on each machine

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)

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>