Anti-pattern: Fetching too many objects at once with ray.get causes failure¶
TLDR: Avoid calling ray.get() on too many objects since this will lead to heap out-of-memory or object store out-of-space. Instead fetch and process one batch at a time.
If you have a large number of tasks that you want to run in parallel, trying to do
ray.get() on all of them at once could lead to failure with heap out-of-memory or object store out-of-space since Ray needs to fetch all the objects to the caller at the same time.
Instead you should get and process the results one batch at a time. Once a batch is processed, Ray will evict objects in that batch to make space for future batches.
import ray import numpy as np ray.init() def process_results(results): # custom process logic pass @ray.remote def return_big_object(): return np.zeros(1024 * 10) NUM_TASKS = 1000 object_refs = [return_big_object.remote() for _ in range(NUM_TASKS)] # This will fail with heap out-of-memory # or object store out-of-space if NUM_TASKS is large enough. results = ray.get(object_refs) process_results(results)
BATCH_SIZE = 100 while object_refs: # Process results in the finish order instead of the submission order. ready_object_refs, object_refs = ray.wait(object_refs, num_returns=BATCH_SIZE) # The node only needs enough space to store # a batch of objects instead of all objects. results = ray.get(ready_object_refs) process_results(results)
Here besides getting one batch at a time to avoid failure, we are also using
ray.wait() to process results in the finish order instead of the submission order to reduce the runtime. See Anti-pattern: Processing results in submission order using ray.get increases runtime for more details.