Return from mp.spawn()

mrshenli · August 27, 2020, 2:28pm

Is there a better way to return values from spawned functions?

If you want to pass the result from spawned processes back to the parent process, you can let the parent process create multiprocessing queues, pass it to children processes, and let children processes send result back through the queue. See the following code:

github.com

pytorch/pytorch/blob/cb26661fe4faf26386703180a9045e6ac6d157df/test/test_multiprocessing.py#L577-L600


      
          @unittest.skipIf(NO_MULTIPROCESSING_SPAWN, "Disabled for environments that \
                           don't support multiprocessing with spawn start method")
          @unittest.skipIf(not TEST_CUDA_IPC, 'CUDA IPC not available')
          def test_event_multiprocess(self):
              event = torch.cuda.Event(enable_timing=False, interprocess=True)
              self.assertTrue(event.query())
          
              ctx = mp.get_context('spawn')
              p2c = ctx.SimpleQueue()
              c2p = ctx.SimpleQueue()
              p = ctx.Process(
                  target=TestMultiprocessing._test_event_multiprocess_child,
                  args=(event, p2c, c2p))
              p.start()
          
              c2p.get()  # wait for until child process is ready
              torch.cuda._sleep(50000000)  # spin for about 50 ms
              event.record()
              p2c.put(0)  # notify child event is recorded

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If the result does not have to go back to the parent process, you can use gather or allgather to communicate the result across children processes.