[Misc]allow disable pynccl (#25421)

Signed-off-by: Lu Fang <fanglu@fb.com>
Co-authored-by: Lucia (Lu) Fang <fanglu@meta.com>

vllm/distributed/device_communicators/cuda_communicator.py

@@ -147,6 +147,10 @@ class CudaCommunicator(DeviceCommunicatorBase):
             assert out is not None
             return out
         pynccl_comm = self.pynccl_comm
+        if pynccl_comm is None or pynccl_comm.disabled:
+            out = input_.clone()
+            torch.distributed.all_reduce(out, group=self.device_group)
+            return out
         assert pynccl_comm is not None
         out = pynccl_comm.all_reduce(input_)
         if out is None:

vllm/distributed/device_communicators/pynccl.py

@@ -8,6 +8,7 @@ import torch
 import torch.distributed as dist
 from torch.distributed import ProcessGroup, ReduceOp
 
+import vllm.envs as envs
 from vllm.distributed.device_communicators.pynccl_wrapper import (
     NCCLLibrary, buffer_type, cudaStream_t, ncclComm_t, ncclDataTypeEnum,
     ncclRedOpTypeEnum, ncclUniqueId)
@@ -83,7 +84,7 @@ class PyNcclCommunicator:
         self.group = group
 
         # if world_size == 1, no need to create communicator
-        if self.world_size == 1:
+        if self.world_size == 1 or envs.VLLM_DISABLE_PYNCCL:
             self.available = False
             self.disabled = True
             return

vllm/envs.py

@@ -98,6 +98,7 @@ if TYPE_CHECKING:
     VLLM_SKIP_P2P_CHECK: bool = False
     VLLM_DISABLED_KERNELS: list[str] = []
     VLLM_DISABLE_NCCL_FOR_DP_SYNCHRONIZATION: bool = False
+    VLLM_DISABLE_PYNCCL: bool = False
     VLLM_USE_V1: bool = True
     VLLM_ROCM_USE_AITER: bool = False
     VLLM_ROCM_USE_AITER_PAGED_ATTN: bool = False
@@ -897,6 +898,11 @@ environment_variables: dict[str, Callable[[], Any]] = {
     (os.getenv("VLLM_DISABLE_NCCL_FOR_DP_SYNCHRONIZATION", "False").lower() in
              ("true", "1")),
 
+    # Disable pynccl (using torch.distributed instead)
+    "VLLM_DISABLE_PYNCCL":
+    lambda:
+    (os.getenv("VLLM_DISABLE_PYNCCL", "False").lower() in ("true", "1")),
+
     # If set, use the V1 code path.
     "VLLM_USE_V1":
     lambda: bool(int(os.getenv("VLLM_USE_V1", "1"))),