[docs] Add antipattern for nested ray.get (#43184)

Signed-off-by: Stephanie Wang <[email protected]>
Signed-off-by: Stephanie wang <[email protected]>
Signed-off-by: Jiajun Yao <[email protected]>
Co-authored-by: angelinalg <[email protected]>
Co-authored-by: Jiajun Yao <[email protected]>

Author: 3 people

doc/source/ray-core/doc_code/anti_pattern_nested_ray_get.py

@@ -0,0 +1,27 @@
+# __anti_pattern_start__
+import ray
+import time
+
+
+@ray.remote
+def f():
+    return 1
+
+
+@ray.remote
+def pass_via_nested_ref(refs):
+    print(sum(ray.get(refs)))
+
+
+@ray.remote
+def pass_via_direct_arg(*args):
+    print(sum(args))
+
+
+# Anti-pattern: Passing nested refs requires `ray.get` in a nested task.
+ray.get(pass_via_nested_ref.remote([f.remote() for _ in range(3)]))
+
+# Better approach: Pass refs as direct arguments. Use *args syntax to unpack
+# multiple arguments.
+ray.get(pass_via_direct_arg.remote(*[f.remote() for _ in range(3)]))
+# __anti_pattern_end__

doc/source/ray-core/patterns/index.rst

@@ -17,6 +17,7 @@ This section is a collection of common design patterns and anti-patterns for wri
     tree-of-actors
     pipelining
     return-ray-put
+    nested-ray-get
     ray-get-loop
     unnecessary-ray-get
     ray-get-submission-order

doc/source/ray-core/patterns/nested-ray-get.rst

@@ -0,0 +1,26 @@
+.. _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.

doc/source/ray-core/patterns/nested-tasks.rst

@@ -1,4 +1,4 @@
-.. _task-pattern-nested-tasks:
+.. _nested-tasks:
 
 Pattern: Using nested tasks to achieve nested parallelism
 =========================================================

doc/source/ray-core/patterns/ray-get-loop.rst

@@ -28,5 +28,7 @@ Instead, we should first schedule all remote calls, which are then processed in
 
 Other ``ray.get()`` related anti-patterns are:
 
+- :doc:`nested-ray-get`
 - :doc:`unnecessary-ray-get`
 - :doc:`ray-get-submission-order`
+- :doc:`ray-get-too-many-objects`

doc/source/ray-core/patterns/ray-get-too-many-objects.rst

@@ -1,3 +1,5 @@
+.. _ray-get-too-many-objects:
+
 Anti-pattern: Fetching too many objects at once with ray.get causes failure
 ===========================================================================