use the same stream for cuda graph catpure and replay for NCCL (#29207)

Signed-off-by: Amir Samani <asamani@nvidia.com>
Signed-off-by: youkaichao <youkaichao@gmail.com>
Co-authored-by: youkaichao <youkaichao@gmail.com>

benchmarks/kernels/benchmark_device_communicators.py

@@ -293,7 +293,7 @@ class CommunicatorBenchmark:
                     graph = torch.cuda.CUDAGraph()
                     graph_pool = torch.cuda.graph_pool_handle()
                     set_graph_pool_id(graph_pool)
-                    with torch.cuda.graph(graph, pool=graph_pool):
+                    with torch.cuda.graph(graph, pool=graph_pool, stream=stream):
                         for _ in range(CUDA_GRAPH_CAPTURE_CYCLES):
                             allreduce_fn(graph_input)
 

tests/utils_/test_torch_utils.py

@@ -99,30 +99,18 @@ def _test_stream_thread(main_expected_stream: torch.cuda.Stream):
 
 
 def test_current_stream_multithread():
-    from vllm.platforms import current_platform
-
     if not torch.cuda.is_available():
         pytest.skip("CUDA not available")
 
-    if current_platform.is_rocm():
-        main_dedicated_stream = current_stream()
+    main_dedicated_stream = current_stream()
 
-        assert main_dedicated_stream.cuda_stream != 0, (
-            "ROCm should create a dedicated stream, not use default stream (0x0)"
-        )
+    assert main_dedicated_stream.cuda_stream != 0, (
+        "ROCm/CUDA should create a dedicated stream, not use default stream (0x0)"
+    )
 
-        main_stream_again = current_stream()
-        assert main_stream_again == main_dedicated_stream, (
-            "Multiple calls to current_stream should return the same dedicated stream"
-        )
+    main_stream_again = current_stream()
+    assert main_stream_again == main_dedicated_stream, (
+        "Multiple calls to current_stream should return the same dedicated stream"
+    )
 
-        _test_stream_thread(main_dedicated_stream)
-    else:
-        main_default_stream = torch.cuda.default_stream()
-        main_initial_stream = current_stream()
-
-        assert main_initial_stream == main_default_stream, (
-            "First call to current_stream should return default stream on CUDA"
-        )
-
-        _test_stream_thread(main_default_stream)
+    _test_stream_thread(main_dedicated_stream)

vllm/compilation/cuda_graph.py

@@ -18,7 +18,7 @@ from vllm.distributed.device_communicators.pynccl_allocator import set_graph_poo
 from vllm.forward_context import BatchDescriptor, get_forward_context
 from vllm.logger import init_logger
 from vllm.platforms import current_platform
-from vllm.utils.torch_utils import weak_ref_tensors
+from vllm.utils.torch_utils import current_stream, weak_ref_tensors
 
 logger = init_logger(__name__)
 
@@ -263,7 +263,11 @@ class CUDAGraphWrapper:
                 else:
                     set_graph_pool_id(current_platform.graph_pool_handle())
                 # mind-exploding: carefully manage the reference and memory.
-                with torch.cuda.graph(cudagraph, pool=self.graph_pool):
+                with torch.cuda.graph(
+                    cudagraph,
+                    pool=self.graph_pool,
+                    stream=current_stream(),
+                ):
                     # `output` is managed by pytorch's cudagraph pool
                     output = self.runnable(*args, **kwargs)
                     if self.cudagraph_options.weak_ref_output:

vllm/utils/torch_utils.py

@@ -465,9 +465,13 @@ def current_stream() -> torch.cuda.Stream:
         # when this function is called before any stream is set,
         # we return the default stream.
         # On ROCm using the default 0 stream in combination with RCCL
-        # is hurting performance. Therefore creating a dedicated stream
-        # per process
-        if current_platform.is_rocm():
+        # is hurting performance.
+        # On CUDA, we capture and replay cudagraph on the same stream,
+        # so we need to avoid using the default stream as well. The default
+        # stream cannot be used for cudagraph capture, see
+        # https://github.com/pytorch/pytorch/blob/42ad9edfb754743fdae3276ade43de000beb4f60/aten/src/ATen/cuda/CUDAGraph.cpp#L77
+        # for more details. Therefore, we create a dedicated stream per process.
+        if current_platform.is_rocm() or current_platform.is_cuda():
             # torch.cuda.set_stream here is the alias of _pathed_set_stream
             torch.cuda.set_stream(torch.cuda.Stream())
         elif current_platform.is_cpu():