Context manager to get temporary gloo pg (#902)

Summary: Pull Request resolved: #902

Reviewed By: JKSenthil

Differential Revision: D62414608

fbshipit-source-id: 2686f0a018a7ff06d0a989a8b6bfdcd4928c8b87

Author: diego-urgell

tests/utils/test_distributed.py

@@ -9,14 +9,15 @@
 
 import os
 import unittest
-from typing import Literal, Optional, Union
+from typing import Callable, Literal, Optional, Union
 from unittest.mock import MagicMock, patch
 from urllib.parse import parse_qs, urlparse
 
 import torch
 import torch.distributed as dist
 import torch.distributed.launcher as launcher
 from pyre_extensions import none_throws
+from torch.distributed import ProcessGroup
 from torchtnt.utils.distributed import (
     _validate_global_rank_world_size,
     all_gather_tensors,
@@ -25,6 +26,7 @@
     get_global_rank,
     get_local_rank,
     get_local_world_size,
+    get_or_create_gloo_pg,
     get_process_group_backend_from_device,
     get_tcp_init_method,
     get_world_size,
@@ -463,3 +465,94 @@ def _test_method_for_rank_zero() -> str:
         val_from_test_method = _test_method_for_rank_zero()
         tc = unittest.TestCase()
         tc.assertEqual(val_from_test_method, "foo")
+
+    @skip_if_not_distributed
+    def test_get_or_create_gloo_pg(self) -> None:
+        spawn_multi_process(2, "gloo", self._test_get_or_create_gloo_pg)
+
+    @staticmethod
+    @patch("torchtnt.utils.distributed.dist.destroy_process_group")
+    def _test_get_or_create_gloo_pg(mock_destroy_process_group: MagicMock) -> None:
+
+        # Get a side effect for the get_backend function that returns NCCL the first time it is called,
+        # and then will return GLOO for subsequent calls. For use with _test_get_or_create_gloo_pg.
+        def _get_backend_side_effect() -> Callable[[Optional[ProcessGroup]], str]:
+            called_get_backend = False
+
+            def get_backend(_) -> str:
+                # We just want to return NCCL the first time we call this function.
+                nonlocal called_get_backend
+                if not called_get_backend:
+                    called_get_backend = True
+                    return dist.Backend.NCCL
+                else:
+                    return dist.Backend.GLOO  # real PG
+
+            return get_backend
+
+        tc = unittest.TestCase()
+
+        # Test not distributed - no-op
+        with patch(
+            "torchtnt.utils.distributed.dist.is_initialized",
+            return_value=False,
+        ):
+            with get_or_create_gloo_pg() as pg:
+                tc.assertIsNone(pg)
+
+        mock_destroy_process_group.assert_not_called()
+
+        # Test no-op since gloo pg already exists
+        mock_destroy_process_group.reset_mock()
+        with get_or_create_gloo_pg() as pg:
+            tc.assertIs(pg, dist.group.WORLD)
+
+        mock_destroy_process_group.assert_not_called()
+
+        # Test creating new gloo candidate pg - no op
+        mock_destroy_process_group.reset_mock()
+        gloo_pg = dist.new_group(backend=dist.Backend.GLOO)
+        with get_or_create_gloo_pg(gloo_pg) as pg:
+            tc.assertIs(pg, gloo_pg)
+
+        mock_destroy_process_group.assert_not_called()
+
+        # Test with NCCL backend - should create a new gloo pg and destroy
+        mock_destroy_process_group.reset_mock()
+
+        with patch(
+            "torchtnt.utils.distributed.dist.get_backend",
+            side_effect=_get_backend_side_effect(),
+        ):
+            with get_or_create_gloo_pg() as pg:
+                pg = none_throws(pg)
+                tc.assertIsNot(pg, dist.group.WORLD)
+                tc.assertEqual(pg._get_backend_name(), dist.Backend.GLOO)
+
+        mock_destroy_process_group.assert_called_once_with(pg)
+
+        # Test exception handling with existing pg - forward exception, group should not be destroyed
+        mock_destroy_process_group.reset_mock()
+        with tc.assertRaisesRegex(Exception, "Test Exception"):
+            gloo_pg = dist.new_group(backend=dist.Backend.GLOO)
+            with get_or_create_gloo_pg(gloo_pg) as pg:
+                tc.assertIs(pg, gloo_pg)
+                raise Exception("Test Exception")
+
+        mock_destroy_process_group.assert_not_called()
+
+        # Test exception handling with new pg - forward exception, group should be destroyed
+        mock_destroy_process_group.reset_mock()
+        with tc.assertRaisesRegex(Exception, "Test Exception"):
+            with patch(
+                "torchtnt.utils.distributed.dist.get_backend",
+                side_effect=_get_backend_side_effect(),
+            ):
+                with get_or_create_gloo_pg() as pg:
+                    tc.assertIsNot(pg, dist.group.WORLD)
+                    tc.assertEqual(
+                        none_throws(pg)._get_backend_name(), dist.Backend.GLOO
+                    )
+                    raise Exception("Test Exception")
+
+        mock_destroy_process_group.assert_called_once_with(pg)

torchtnt/utils/distributed.py

@@ -8,12 +8,14 @@
 # pyre-strict
 
 
+import logging
 import os
 import tempfile
+from contextlib import contextmanager
 from dataclasses import dataclass
 from datetime import timedelta
 from functools import wraps
-from typing import Any, Callable, cast, Dict, List, Optional, TypeVar, Union
+from typing import Any, Callable, cast, Dict, Generator, List, Optional, TypeVar, Union
 
 import torch
 import torch.nn.functional as F
@@ -28,6 +30,8 @@
 TParams = ParameterSpecification("TParams")
 TReturn = TypeVar("TReturn")
 
+logger: logging.Logger = logging.getLogger(__name__)
+
 
 class PGWrapper:
     """
@@ -641,3 +645,43 @@ def wrapper(*args: Any, **kwargs: Any) -> T:
         return cast(T, val)
 
     return wrapper
+
+
+@contextmanager
+def get_or_create_gloo_pg(
+    candidate_pg: Optional[dist.ProcessGroup] = None,
+) -> Generator[Optional[dist.ProcessGroup], None, None]:
+    """
+    Context manager to ensure that a gloo process group is used for the contained operations. First checks if the
+    WORLD process group, or the provided candidate process group, is already gloo-based. In case it is, that is returned.
+    Otherwise, a new gloo process group will be created and returned. Upon exiting the context, if a new process group
+    was created, it will be destroyed.
+
+    Note: If the distributed environment is not initialized, this context manager will return None and will be no-op.
+
+    Args:
+        candidate_pg: Optional process group to check if it is gloo-based. If None, the WORLD process group will be checked.
+    """
+    gloo_pg_created = False
+
+    if not dist.is_initialized():
+        logger.info("Not in a distributed environment, gloo process group not created")
+        pg = None
+
+    else:
+        pg = candidate_pg or dist.group.WORLD
+        if dist.get_backend(pg) != dist.Backend.GLOO:
+            logger.info("Creating temporary gloo process group")
+            pg = dist.new_group(
+                timeout=timedelta(seconds=3600), backend=dist.Backend.GLOO
+            )
+            gloo_pg_created = True
+
+    try:
+        yield pg
+
+    finally:
+        # Cleanup temporary gloo pg if it was created
+        if gloo_pg_created:
+            dist.destroy_process_group(pg)
+            logger.info("Destroyed temporary gloo process group")