How do I wait for ray on Actor class?

Question

I am developing Actor class and ray.wait() to collect the results.

Below is the code and console outputs which is collecting the result for only 2 Actors when there are 3 Actors.

import time
import ray


@ray.remote
class Tester:
    def __init__(self, param):
        self.param = param

    def run(self):
        return self.param

params = [0,1,2]
testers = []
for p in params:
    tester = Tester.remote(p)
    testers.append(tester)

runs = []
for i, tester in enumerate(testers):
    runs.append(tester.run.remote())

while len(runs):
    done_id, result_ids = ray.wait(runs)
    #runs size is not decreasing

    result = ray.get(done_id[0])
    print('result:{}'.format(result))
    time.sleep(1)

result:2
(pid=819202) 
(pid=819200) 
(pid=819198) 
result:1
result:0
result:0
result:0
result:0
result:0
...
...
...

The console is printing out forever because the runs variable's size is not reduced.

When I call ray.wait(runs) and get the done_id, runs's element with the done_id should be removed, but it is not removed.

I want the console output to be like below.

result:2
(pid=819202) 
(pid=819200) 
(pid=819198) 
result:1
result:0

Answer 1

The script you provided is using ray.wait incorrectly. The following code does what you want:

import time
import ray

@ray.remote
class Tester:
    def __init__(self, param):
        self.param = param

    def run(self):
        return self.param

params = [0, 1, 2]

# I use list comprehensions instead of for loops for terseness.
testers = [Tester.remote(p) for p in params]
not_done_ids = [tester.run.remote() for tester in testers]

# len() is not required to check that the list is empty.
while not_done_ids:
    
    # Replace not_done_ids with the list of object references that aren't
    # ready. Store the list of object references that are ready in done_ids.
    # timeout=1 means sleep at most 1 second, do not sleep if there are
    # new object references that are ready.
    done_ids, not_done_ids = ray.wait(not_done_ids, timeout=1)
    
    # ray.get can take an iterable of object references.
    done_return_values = ray.get(done_ids)

    # Process each result.
    for result in done_return_values:
        print(f'result: {result}')

I added the following fixes:

• ray.wait returns two lists, a list of objects that are ready, and a list of objects that may or may not be ready. You should iterate over the first list to get all object references that are ready.
• Your while loop goes forever until the list is empty. I simply replaced the runs list with not_done_ids so that once all object references are ready, the while loop breaks.
• ray.wait supports sleeping, with timeout. I removed your sleep and added timeout=1, which enables the program to run more efficiently (there is no sleep if another object is ready!).