Anti-pattern: Calling ray.get unnecessarily harms performance

TLDR: Avoid calling ray.get() unnecessarily for intermediate steps. Work with object references directly, and only call ray.get() at the end to get the final result.

When ray.get() is called, objects must be transferred to the worker/node that calls ray.get(). If you don’t need to manipulate the object, you probably don’t need to call ray.get() on it!

Typically, it’s best practice to wait as long as possible before calling ray.get(), or even design your program to avoid having to call ray.get() at all.

Code example

Anti-pattern:

import ray
import numpy as np

ray.init()


@ray.remote
def generate_rollout():
    return np.ones((10000, 10000))


@ray.remote
def reduce(rollout):
    return np.sum(rollout)


# `ray.get()` downloads the result here.
rollout = ray.get(generate_rollout.remote())
# Now we have to reupload `rollout`
reduced = ray.get(reduce.remote(rollout))
../../_images/unnecessary-ray-get-anti.svg

Better approach:

# Don't need ray.get here.
rollout_obj_ref = generate_rollout.remote()
# Rollout object is passed by reference.
reduced = ray.get(reduce.remote(rollout_obj_ref))
../../_images/unnecessary-ray-get-better.svg

Notice in the anti-pattern example, we call ray.get() which forces us to transfer the large rollout to the driver, then again to the reduce worker.

In the fixed version, we only pass the reference to the object to the reduce task. The reduce worker will implicitly call ray.get() to fetch the actual rollout data directly from the generate_rollout worker, avoiding the extra copy to the driver.

Other ray.get() related anti-patterns are: