[diffusion] fix: stop reserving nccl device buffers for single-rank groups (#35538)

python/sglang/multimodal_gen/runtime/cache/cache_dit_integration.py

@@ -77,7 +77,9 @@ def _patch_cache_dit_similarity():
         tp_sp_group = getattr(self, "_sglang_tp_sp_group", None)
         target_group = tp_sp_group or sp_group or tp_group
 
-        if target_group is None:
+        # Averaging over a one-rank group returns the input, so skip the
+        # collective rather than pay for a round trip that cannot change it.
+        if target_group is None or dist.get_world_size(target_group) == 1:
             return _original_similarity(
                 self,
                 t1,

python/sglang/multimodal_gen/runtime/distributed/group_coordinator.py

@@ -123,6 +123,18 @@ class GraphCaptureContext:
     stream: torch.cuda.Stream | None
 
 
+def new_device_group(ranks, backend=None):
+    """Create a process group for device collectives.
+
+    A single-rank group never runs one: every collective short-circuits on
+    world_size == 1. NCCL would still allocate its per-channel device buffers
+    for it, which costs ~390 MiB a group.
+    """
+    return torch.distributed.new_group(
+        ranks, backend="gloo" if len(ranks) == 1 else backend
+    )
+
+
 class GroupCoordinator:
     """
     PyTorch ProcessGroup wrapper for a group of processes.
@@ -169,9 +181,7 @@ class GroupCoordinator:
         self.cpu_group = None
 
         for ranks in group_ranks:
-            device_group = torch.distributed.new_group(
-                ranks, backend=torch_distributed_backend
-            )
+            device_group = new_device_group(ranks, torch_distributed_backend)
             # a group with `gloo` backend, to allow direct coordination between
             # processes through the CPU.
             with suppress_stdout():
@@ -863,9 +873,7 @@ class PipelineGroupCoordinator(GroupCoordinator):
         self.device_groups = []
         if len(group_ranks[0]) > 2 or len(group_ranks[0]) == 1:
             for ranks in group_ranks:
-                device_group = torch.distributed.new_group(
-                    ranks, backend=torch_distributed_backend
-                )
+                device_group = new_device_group(ranks, torch_distributed_backend)
                 # a group with `gloo` backend, to allow direct coordination between
                 # processes through the CPU.
                 with suppress_stdout():
@@ -927,9 +935,7 @@ class PipelineGroupCoordinator(GroupCoordinator):
         ] = None
         self.skip_device_group = None
         for ranks in group_ranks:
-            skip_device_group = torch.distributed.new_group(
-                ranks, backend=torch_distributed_backend
-            )
+            skip_device_group = new_device_group(ranks, torch_distributed_backend)
             if self.rank in ranks:
                 self.skip_device_group = skip_device_group
         assert self.skip_device_group is not None

python/sglang/multimodal_gen/runtime/distributed/parallel_groups.py

@@ -1,7 +1,7 @@
 # Reference: https://github.com/feifeibear/long-context-attention/blob/main/yunchang/globals.py
 
 
-import torch
+from .group_coordinator import new_device_group
 
 
 class Singleton:
@@ -56,34 +56,34 @@ def set_seq_parallel_pg_by_sp_groups(
     def _map_indices_to_ranks(ranks: list[int], indices: list[int]) -> list[int]:
         return [ranks[i] for i in indices]
 
-    # Important: call torch.distributed.new_group in the same order on all ranks.
+    # Important: create the groups in the same order on all ranks.
     for sp_ranks in sp_groups:
         if use_ulysses_low:
             for i in range(num_ulysses_pgs):
                 idx = list(range(i * sp_ulysses_degree, (i + 1) * sp_ulysses_degree))
                 ulysses_ranks = _map_indices_to_ranks(sp_ranks, idx)
-                group = torch.distributed.new_group(ulysses_ranks)
+                group = new_device_group(ulysses_ranks)
                 if rank in ulysses_ranks:
                     ulyssess_pg = group
 
             for i in range(num_ring_pgs):
                 idx = list(range(i, sp_degree, num_ring_pgs))
                 ring_ranks = _map_indices_to_ranks(sp_ranks, idx)
-                group = torch.distributed.new_group(ring_ranks)
+                group = new_device_group(ring_ranks)
                 if rank in ring_ranks:
                     ring_pg = group
         else:
             for i in range(num_ring_pgs):
                 idx = list(range(i * sp_ring_degree, (i + 1) * sp_ring_degree))
                 ring_ranks = _map_indices_to_ranks(sp_ranks, idx)
-                group = torch.distributed.new_group(ring_ranks)
+                group = new_device_group(ring_ranks)
                 if rank in ring_ranks:
                     ring_pg = group
 
             for i in range(num_ulysses_pgs):
                 idx = list(range(i, sp_degree, num_ulysses_pgs))
                 ulysses_ranks = _map_indices_to_ranks(sp_ranks, idx)
-                group = torch.distributed.new_group(ulysses_ranks)
+                group = new_device_group(ulysses_ranks)
                 if rank in ulysses_ranks:
                     ulyssess_pg = group
 

python/sglang/multimodal_gen/runtime/distributed/parallel_state.py

@@ -56,6 +56,7 @@ from .group_coordinator import (
     PipelineGroupCoordinator,
     SequenceParallelGroupCoordinator,
     get_local_torch_device,
+    new_device_group,
 )
 
 logger = init_logger(__name__)
@@ -999,9 +1000,7 @@ def init_dit_group(
 ) -> None:
     global _DIT
     assert _DIT is None, "DIT group is already initialized"
-    _DIT = torch.distributed.new_group(
-        ranks=list(range(dit_parallel_size)), backend=backend
-    )
+    _DIT = new_device_group(list(range(dit_parallel_size)), backend)
 
 
 def get_dit_group() -> ProcessGroup:
@@ -1018,7 +1017,7 @@ def init_vae_group(
     global _VAE
     assert _VAE is None, "VAE parallel group is already initialized"
     vae_ranks = list(range(dit_parallel_size, dit_parallel_size + vae_parallel_size))
-    _VAE = torch.distributed.new_group(ranks=vae_ranks, backend=backend)
+    _VAE = new_device_group(vae_ranks, backend)
 
 
 def destroy_model_parallel() -> None:

python/sglang/multimodal_gen/test/unit/test_single_rank_device_group.py

@@ -0,0 +1,35 @@
+"""Single-rank groups get gloo, so NCCL does not reserve device buffers for them."""
+
+import unittest
+from unittest.mock import patch
+
+from sglang.multimodal_gen.runtime.distributed.group_coordinator import (
+    new_device_group,
+)
+
+NEW_GROUP_PATH = "torch.distributed.new_group"
+
+
+class TestSingleRankDeviceGroup(unittest.TestCase):
+    def test_single_rank_group_avoids_the_device_backend(self):
+        for ranks, requested in [([0], "nccl"), ([3], "hccl"), ([0], None)]:
+            with self.subTest(ranks=ranks, requested=requested):
+                with patch(NEW_GROUP_PATH) as new_group:
+                    new_device_group(ranks, requested)
+                new_group.assert_called_once_with(ranks, backend="gloo")
+
+    def test_multi_rank_group_keeps_the_requested_backend(self):
+        for ranks, requested in [([0, 1], "nccl"), ([0, 1, 2, 3], None)]:
+            with self.subTest(ranks=ranks, requested=requested):
+                with patch(NEW_GROUP_PATH) as new_group:
+                    new_device_group(ranks, requested)
+                new_group.assert_called_once_with(ranks, backend=requested)
+
+    def test_backend_defaults_to_none_for_multi_rank(self):
+        with patch(NEW_GROUP_PATH) as new_group:
+            new_device_group([0, 1])
+        new_group.assert_called_once_with([0, 1], backend=None)
+
+
+if __name__ == "__main__":
+    unittest.main()