[Train] Don't use NCCL_BLOCKING_WAIT (#29562)

From the pytorch docs, we should use NCCL_ASYNC_ERROR_HANDLING instead.

Signed-off-by: amogkam <[email protected]>

python/ray/train/tests/test_gpu.py

@@ -1,5 +1,6 @@
 import os
 from collections import Counter
+import time
 
 from unittest.mock import patch
 import pytest
@@ -9,6 +10,7 @@
 from torch.utils.data import DataLoader, DistributedSampler
 
 import ray
+from ray.exceptions import RayTaskError
 from ray.air import session
 from ray import tune
 
@@ -307,6 +309,32 @@ def assert_env_var_set():
     worker_group.execute(assert_env_var_set)
 
 
+def test_torch_fail_on_nccl_timeout(ray_start_4_cpus_2_gpus):
+    """Tests that TorchTrainer raises exception on NCCL timeouts."""
+
+    def train_fn():
+        model = torch.nn.Linear(1, 1)
+        model = train.torch.prepare_model(model)
+
+        # Rank 0 worker will never reach the collective operation.
+        # NCCL should timeout.
+        if session.get_world_rank() == 0:
+            while True:
+                time.sleep(100)
+
+        torch.distributed.barrier()
+
+    trainer = TorchTrainer(
+        train_fn,
+        scaling_config=ScalingConfig(num_workers=2, use_gpu=True),
+        torch_config=TorchConfig(timeout_s=5),
+    )
+
+    # Training should fail and not hang.
+    with pytest.raises(RayTaskError):
+        trainer.fit()
+
+
 if __name__ == "__main__":
     import sys
 

python/ray/train/torch/config.py

@@ -94,12 +94,21 @@ def _setup_torch_process_group(
         )
     logger.debug(f"using {backend}")
 
-    if backend == "nccl" and "NCCL_BLOCKING_WAIT" not in os.environ:
+    # See the `timeout` arg in https://pytorch.org/docs/master/
+    # distributed.html#torch.distributed.init_process_group for description of
+    # NCCL_ASYNC_ERROR_HANDLING. We do not use NCCL_BLOCKING_WAIT due to performance
+    # overhead.
+    if (
+        backend == "nccl"
+        and "NCCL_ASYNC_ERROR_HANDLING" not in os.environ
+        and "NCCL_BLOCKING_WAIT" not in os.environ
+    ):
         logger.debug(
-            "Setting NCCL_BLOCKING_WAIT for detecting node failure. "
-            "To override this behavior, you can set NCCL_BLOCKING_WAIT=0."
+            "Setting NCCL_ASYNC_ERROR_HANDLING to fail if NCCL collective "
+            "communication operations are timing out. "
+            "To override this behavior, you can set NCCL_ASYNC_ERROR_HANDLING=0."
         )
-        os.environ["NCCL_BLOCKING_WAIT"] = "1"
+        os.environ["NCCL_ASYNC_ERROR_HANDLING"] = "1"
 
     dist.init_process_group(
         backend=backend,