.. _nested-ray-get:

Anti-pattern: Calling ray.get on task arguments harms performance

TLDR: If possible, pass ObjectRefs as direct task arguments, instead of passing a list as the task argument and then calling :func:ray.get() <ray.get> inside the task.

When a task calls ray.get(), it must block until the value of the ObjectRef is ready. If all cores are already occupied, this situation can lead to a deadlock, as the task that produces the ObjectRef's value may need the caller task's resources in order to run. To handle this issue, if the caller task would block in ray.get(), Ray temporarily releases the caller's CPU resources to allow the pending task to run. This behavior can harm performance and stability because the caller continues to use a process and memory to hold its stack while other tasks run.

Therefore, it is always better to pass ObjectRefs as direct arguments to a task and avoid calling ray.get inside of the task, if possible.

For example, in the following code, prefer the latter method of invoking the dependent task.

.. literalinclude:: ../doc_code/anti_pattern_nested_ray_get.py :language: python :start-after: anti_pattern_start :end-before: anti_pattern_end

Avoiding ray.get in nested tasks may not always be possible. Some valid reasons to call ray.get include:

• :doc:nested-tasks
• If the nested task has multiple ObjectRefs to ray.get, and it wants to choose the order and number to get.