Allow actors to pin at most 1000 dummy objects at a time. (#1241)

* Allow actors to pin at most 1000 dummy objects at a time.

* Fix linting.

python/ray/actor.py

@@ -134,7 +134,10 @@ def put_dummy_object(worker, dummy_object_id):
     # actor, to prevent eviction from the object store.
     dummy_object = worker.get_object([dummy_object_id])
     dummy_object = dummy_object[0]
-    worker.actor_pinned_objects[dummy_object_id] = dummy_object
+    worker.actor_pinned_objects.append(dummy_object)
+    if (len(worker.actor_pinned_objects) >
+            ray._config.actor_max_dummy_objects()):
+        worker.actor_pinned_objects.pop(0)
 
 
 def make_actor_method_executor(worker, method_name, method):

python/ray/worker.py

@@ -1836,7 +1836,7 @@ def connect(info, object_id_seed=None, mode=WORKER_MODE, worker=global_worker,
         worker.class_id = class_id
         # Store a list of the dummy outputs produced by actor tasks, to pin the
         # dummy outputs in the object store.
-        worker.actor_pinned_objects = {}
+        worker.actor_pinned_objects = []
 
     # Initialize the serialization library. This registers some classes, and so
     # it must be run before we export all of the cached remote functions.