Warn when using --update-freq on a single machine and --ddp-backend != no_c10d

Summary: Pull Request resolved: https://github.com/pytorch/fairseq/pull/400

Differential Revision: D13366996

Pulled By: myleott

fbshipit-source-id: b4907815e7cc1b4a2aceab11210bf64cb3d814c9

multiprocessing_train.py

@@ -23,6 +23,9 @@ def main(args):
     args.distributed_init_method = 'tcp://localhost:{port}'.format(port=port)
     args.distributed_init_host = 'localhost'
     args.distributed_port = port + 1
+    if max(args.update_freq) > 1 and args.ddp_backend != 'no_c10d':
+        print('| WARNING: when using --update-freq on a single machine, you '
+              'will get better performance with --ddp-backend=no_c10d')
 
     mp = torch.multiprocessing.get_context('spawn')