Antipattern: Unnecessary call of ray.get in a task

TLDR: Avoid calling ray.get too frequently/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 in a task, you probably don’t need to call ray.get on it!

Typically, it’s a best practice to wait as long as possible before calling ray.get, or even design your program to avoid having to call ray.get too soon.

Notes

Notice in the first example, we call ray.get which forces us to transfer the large rollout to the driver, then to reducer after that.

In the fixed version, we only pass the reference to the object to the reducer. The reducer automatically calls ray.get once, which means the data is passed directly from generate_rollout to reduce, avoiding the driver.

Code example

Antipattern:

@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))

Better approach:

# Don't need ray.get here.
rollout = generate_rollout.remote()
# Rollout object is passed by reference.
reduced = ray.get(reduce.remote(rollout))