[EPLB] Add alternative communication for EPLB weight exchange (#33176)

Signed-off-by: ilmarkov <markovilya197@gmail.com>
Signed-off-by: Markov Ilya <markovilya19@gmail.com>
Co-authored-by: Markov Ilya <markovilya19@gmail.com>

.buildkite/test_areas/expert_parallelism.yaml

@@ -13,8 +13,8 @@ steps:
   - pytest -v -s distributed/test_eplb_algo.py
   - pytest -v -s distributed/test_eplb_utils.py
 
-- label: EPLB Execution
-  timeout_in_minutes: 20
+- label: EPLB Execution # 17min
+  timeout_in_minutes: 27
   working_dir: "/vllm-workspace/tests"
   num_devices: 4
   source_file_dependencies:

tests/distributed/eplb_utils.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+import atexit
 import os
 import random
 
+import pytest
 import torch
 import torch.multiprocessing as mp
 
@@ -16,9 +18,20 @@ from vllm.utils.system_utils import update_environment_variables
 mp.set_start_method("spawn", force=True)
 
 
+def _distributed_worker_wrapper(fn, env, world_size, args, rank, skip_queue):
+    try:
+        fn(env, world_size, *args)
+    except BaseException as exc:
+        if isinstance(exc, pytest.skip.Exception):
+            skip_queue.put((rank, str(exc)))
+            return
+        raise
+
+
 def distributed_run(fn, world_size, *args):
     number_of_processes = world_size
     processes: list[mp.Process] = []
+    skip_queue: mp.SimpleQueue = mp.SimpleQueue()
     for i in range(number_of_processes):
         env: dict[str, str] = {}
         env["RANK"] = str(i)
@@ -27,13 +40,32 @@ def distributed_run(fn, world_size, *args):
         env["LOCAL_WORLD_SIZE"] = str(number_of_processes)
         env["MASTER_ADDR"] = "localhost"
         env["MASTER_PORT"] = "12345"
-        p = mp.Process(target=fn, args=(env, world_size, *args))
+        p = mp.Process(
+            target=_distributed_worker_wrapper,
+            args=(fn, env, world_size, args, i, skip_queue),
+        )
         processes.append(p)
         p.start()
 
     for p in processes:
         p.join()
 
+    skipped: list[tuple[int, str]] = []
+    while not skip_queue.empty():
+        rank, reason = skip_queue.get()
+        skipped.append((rank, reason))
+
+    if len(skipped) == number_of_processes:
+        reason = skipped[0][1]
+        pytest.skip(reason)
+    if 0 < len(skipped) < number_of_processes:
+        skipped_ranks = sorted(rank for rank, _ in skipped)
+        raise AssertionError(
+            "Distributed test had partial skips; expected either all ranks "
+            f"to skip or none. Skipped ranks: {skipped_ranks}, "
+            f"total ranks: {number_of_processes}"
+        )
+
     for p in processes:
         assert p.exitcode == 0
 
@@ -48,7 +80,12 @@ def set_env_vars_and_device(env: dict[str, str]) -> None:
     vllm_config = VllmConfig()
     with set_current_vllm_config(vllm_config):
         init_distributed_environment()
-
+    atexit.register(_destroy_process_group_if_initialized)
     # Ensure each worker process has the same random seed
     random.seed(42)
     torch.manual_seed(42)
+
+
+def _destroy_process_group_if_initialized() -> None:
+    if torch.distributed.is_available() and torch.distributed.is_initialized():
+        torch.distributed.destroy_process_group()

tests/distributed/test_eplb_execute.py

@@ -9,6 +9,7 @@ import torch
 import torch.distributed
 
 from vllm.config import VllmConfig, set_current_vllm_config
+from vllm.distributed.eplb.eplb_communicator import create_eplb_communicator
 from vllm.distributed.eplb.rebalance_execute import (
     move_from_buffer,
     rearrange_expert_weights_inplace,
@@ -130,9 +131,10 @@ def verify_expert_weights_after_shuffle(
     hidden_sizes: list[int],
     ep_rank: int,
     num_local_experts: int,
-):
+) -> bool:
     """Verify the weights after shuffling are correct."""
     num_layers = len(expert_weights)
+    ok = True
 
     for layer in range(num_layers):
         for weight_idx, hidden_size in enumerate(hidden_sizes):
@@ -155,29 +157,38 @@ def verify_expert_weights_after_shuffle(
                     dtype=actual_weights.dtype,
                 )
 
-                torch.testing.assert_close(
-                    actual_weights,
-                    expected_weights,
-                    msg=f"Layer {layer}, weight {weight_idx},"
-                    f"local expert {local_expert}: "
-                    f"weights do not match. "
-                    f"Expected logical expert {expected_logical_expert}",
+                if not torch.equal(actual_weights, expected_weights):
+                    ok = False
+                    actual_head = actual_weights[:8].detach().cpu().tolist()
+                    expected_head = expected_weights[:8].detach().cpu().tolist()
+                    print(
+                        "verify_expert_weights_after_shuffle failed: "
+                        f"rank={ep_rank}, "
+                        f"layer={layer}, weight_idx={weight_idx}, "
+                        f"local_expert={local_expert}, "
+                        f"expected_logical_expert={expected_logical_expert}, "
+                        f"actual_head={actual_head}, expected_head={expected_head}",
+                        flush=True,
                     )
 
+    return ok
+
 
 def verify_redundant_experts_have_same_weights(
     expert_weights: list[list[torch.Tensor]],
     indices: torch.Tensor,
     hidden_sizes: list[int],
+    ep_rank: int,
     world_size: int,
     num_local_experts: int,
-):
+) -> bool:
     """
     Verify that all replicas of the same logical expert have the same weights.
     """
     num_layers = len(expert_weights)
     total_physical_experts = world_size * num_local_experts
 
+    ok = True
     for layer in range(num_layers):
         # Collect weights for all physical experts for each weight matrix
         all_weights: list[torch.Tensor] = []
@@ -227,14 +238,54 @@ def verify_redundant_experts_have_same_weights(
                 # Verify that current physical expert's weights match the
                 # previously saved logical expert weights
                 for weight_idx in range(len(hidden_sizes)):
-                    torch.testing.assert_close(
+                    if not torch.equal(
                         all_weights[weight_idx][physical_pos],
                         logical_expert_weights[logical_expert_id][weight_idx],
-                        msg=f"Layer {layer}, weight {weight_idx},"
-                        f"logical expert {logical_expert_id}: "
-                        f"Physical expert {physical_pos} has different weights"
-                        f"than expected",
+                    ):
+                        ok = False
+                        actual_head = (
+                            all_weights[weight_idx][physical_pos][:8]
+                            .detach()
+                            .cpu()
+                            .tolist()
+                        )
+                        reference_head = (
+                            logical_expert_weights[logical_expert_id][weight_idx][:8]
+                            .detach()
+                            .cpu()
+                            .tolist()
+                        )
+                        print(
+                            "verify_redundant_experts_have_same_weights failed: "
+                            f"rank={ep_rank}, "
+                            f"layer={layer}, weight_idx={weight_idx}, "
+                            f"logical_expert={logical_expert_id}, "
+                            f"physical_pos={physical_pos}, "
+                            f"actual_head={actual_head}, "
+                            f"reference_head={reference_head}",
+                            flush=True,
+                        )
+
+    return ok
+
+
+def assert_verification_synced(local_ok: bool, msg: str) -> None:
+    ok_tensor = torch.tensor([1 if local_ok else 0], device="cuda", dtype=torch.int32)
+    torch.distributed.all_reduce(ok_tensor, op=torch.distributed.ReduceOp.MIN)
+    assert bool(ok_tensor.item()), msg
+
+
+def create_eplb_communicator_or_raise(*, group_coordinator, backend, expert_weights):
+    try:
+        return create_eplb_communicator(
+            group_coordinator=group_coordinator,
+            backend=backend,
+            expert_weights=expert_weights,
         )
+    except Exception as exc:
+        raise RuntimeError(
+            f"Failed to create EPLB communicator for backend={backend}: {exc}"
+        ) from exc
 
 
 def _test_async_transfer_layer_without_mtp_worker(
@@ -243,6 +294,7 @@ def _test_async_transfer_layer_without_mtp_worker(
     num_layers: int,
     num_local_experts: int,
     num_logical_experts: int,
+    eplb_communicator: str,
 ) -> None:
     set_env_vars_and_device(env)
 
@@ -254,8 +306,8 @@ def _test_async_transfer_layer_without_mtp_worker(
             tensor_model_parallel_size=world_size, pipeline_model_parallel_size=1
         )
 
-        tp_group = get_tp_group()
-        ep_group = tp_group.device_group
+        ep_group_coordinator = get_tp_group()
+        ep_group = ep_group_coordinator.device_group
         ep_rank = torch.distributed.get_rank()
         device = torch.device(f"cuda:{ep_rank}")
 
@@ -298,6 +350,13 @@ def _test_async_transfer_layer_without_mtp_worker(
         expert_buffer = [torch.empty_like(w) for w in expert_weights[0]]
         cuda_stream = torch.cuda.Stream(device=device)
 
+        communicator = create_eplb_communicator_or_raise(
+            group_coordinator=ep_group_coordinator,
+            backend=eplb_communicator,
+            expert_weights=expert_weights[0],
+        )
+        communicator.set_stream(cuda_stream)
+
         for layer_idx in range(num_layers):
             is_unchanged, is_received_locally, recv_metadata = asyncio.run(
                 transfer_layer(
@@ -306,6 +365,7 @@ def _test_async_transfer_layer_without_mtp_worker(
                     expert_weights=expert_weights[layer_idx],
                     expert_weights_buffer=expert_buffer,
                     ep_group=ep_group,
+                    communicator=communicator,
                     cuda_stream=cuda_stream,
                 )
             )
@@ -320,24 +380,38 @@ def _test_async_transfer_layer_without_mtp_worker(
                 ep_rank=ep_rank,
             )
 
-        verify_expert_weights_after_shuffle(
+    local_ok = verify_expert_weights_after_shuffle(
         expert_weights,
         new_indices,
         hidden_sizes,
         ep_rank,
         num_local_experts,
     )
+    local_ok = (
         verify_redundant_experts_have_same_weights(
             expert_weights,
             new_indices,
             hidden_sizes,
+            ep_rank,
             world_size,
             num_local_experts,
         )
+        and local_ok
+    )
+    assert_verification_synced(
+        local_ok,
+        "Async transfer verification failed on at least one rank. "
+        "See logs for details.",
+    )
 
 
 def _test_rearrange_expert_weights_with_redundancy(
-    env, world_size, num_layers, num_local_experts, num_logical_experts
+    env,
+    world_size,
+    num_layers,
+    num_local_experts,
+    num_logical_experts,
+    eplb_communicator: str,
 ) -> None:
     # Initialize model parallel (using tensor parallel as an entrypoint
     # to expert parallel)
@@ -351,7 +425,8 @@ def _test_rearrange_expert_weights_with_redundancy(
             tensor_model_parallel_size=world_size, pipeline_model_parallel_size=1
         )
 
-        ep_group = get_tp_group().cpu_group
+        ep_group_coordinator = get_tp_group()
+        ep_group = ep_group_coordinator.cpu_group
         ep_rank = torch.distributed.get_rank()
         device = torch.device(f"cuda:{ep_rank}")
 
@@ -387,6 +462,12 @@ def _test_rearrange_expert_weights_with_redundancy(
             num_layers, num_local_experts, hidden_sizes, ep_rank, device, old_indices
         )
 
+        communicator = create_eplb_communicator_or_raise(
+            group_coordinator=ep_group_coordinator,
+            backend=eplb_communicator,
+            expert_weights=expert_weights[0],
+        )
+
         # Execute weight rearrangement
         rearrange_expert_weights_inplace(
             old_indices,
@@ -394,10 +475,11 @@ def _test_rearrange_expert_weights_with_redundancy(
             expert_weights,
             ep_group,
             is_profile=False,
+            communicator=communicator,
         )
 
     # Verify the rearrangement result
-        verify_expert_weights_after_shuffle(
+    local_ok = verify_expert_weights_after_shuffle(
         expert_weights,
         new_indices,
         hidden_sizes,
@@ -405,13 +487,21 @@ def _test_rearrange_expert_weights_with_redundancy(
         num_local_experts,
     )
 
+    local_ok = (
         verify_redundant_experts_have_same_weights(
             expert_weights,
             new_indices,
             hidden_sizes,
+            ep_rank,
             world_size,
             num_local_experts,
         )
+        and local_ok
+    )
+    assert_verification_synced(
+        local_ok,
+        "Rearrange verification failed on at least one rank. See logs for details.",
+    )
 
 
 @pytest.mark.parametrize(
@@ -437,8 +527,13 @@ def _test_rearrange_expert_weights_with_redundancy(
         (4, 8, 8, 16),
     ],
 )
+@pytest.mark.parametrize("eplb_communicator", ["torch_nccl", "torch_gloo", "pynccl"])
 def test_rearrange_expert_weights_with_redundancy(
-    world_size, num_layers, num_local_experts, num_logical_experts
+    world_size,
+    num_layers,
+    num_local_experts,
+    num_logical_experts,
+    eplb_communicator,
 ):
     """Test the functionality of rearranging expert weights with redundancy."""
 
@@ -450,6 +545,7 @@ def test_rearrange_expert_weights_with_redundancy(
         num_layers,
         num_local_experts,
         num_logical_experts,
+        eplb_communicator,
     )
 
 
@@ -464,7 +560,8 @@ def _test_rearrange_expert_weights_no_change(env, world_size) -> None:
             tensor_model_parallel_size=world_size, pipeline_model_parallel_size=1
         )
 
-        ep_group = get_tp_group().cpu_group
+        ep_group_coordinator = get_tp_group()
+        ep_group = ep_group_coordinator.cpu_group
         ep_rank = torch.distributed.get_rank()
         device = torch.device(f"cuda:{ep_rank}")
 
@@ -494,23 +591,39 @@ def _test_rearrange_expert_weights_no_change(env, world_size) -> None:
                 layer_copy.append(weight.clone())
             original_weights.append(layer_copy)
 
+        communicator = create_eplb_communicator_or_raise(
+            group_coordinator=ep_group_coordinator,
+            backend="torch_nccl",
+            expert_weights=expert_weights[0],
+        )
+
         # Execute rearrangement (should be no change)
         rearrange_expert_weights_inplace(
             indices,
             indices,  # Same indices
             expert_weights,
             ep_group,
+            communicator,
             is_profile=False,
         )
 
     # Verify that the weights have not changed
+    local_ok = True
     for layer in range(num_layers):
         for weight_idx in range(len(hidden_sizes)):
-                torch.testing.assert_close(
+            if not torch.equal(
                 expert_weights[layer][weight_idx],
                 original_weights[layer][weight_idx],
-                    msg=f"""Layer {layer}, weight {weight_idx}
- should remain unchanged""",
+            ):
+                local_ok = False
+                print(
+                    "test_rearrange_expert_weights_no_change failed: "
+                    f"layer={layer}, weight_idx={weight_idx}",
+                    flush=True,
+                )
+    assert_verification_synced(
+        local_ok,
+        "No-change EPLB verification failed on at least one rank.",
     )
 
 
@@ -520,11 +633,13 @@ def _test_rearrange_expert_weights_no_change(env, world_size) -> None:
         (2, 2, 2, 3),
     ],
 )
+@pytest.mark.parametrize("eplb_communicator", ["torch_nccl", "torch_gloo", "pynccl"])
 def test_async_transfer_layer_without_mtp(
     world_size: int,
     num_layers: int,
     num_local_experts: int,
     num_logical_experts: int,
+    eplb_communicator: str,
 ):
     """Exercise async EPLB transfer path without MTP/spec decode."""
 
@@ -537,6 +652,7 @@ def test_async_transfer_layer_without_mtp(
         num_layers,
         num_local_experts,
         num_logical_experts,
+        eplb_communicator,
     )
 
 
@@ -549,7 +665,10 @@ def test_rearrange_expert_weights_no_change(world_size):
 
     if torch.accelerator.device_count() < world_size:
         pytest.skip(f"Need at least {world_size} GPUs to run the test")
-    distributed_run(_test_rearrange_expert_weights_no_change, world_size)
+    distributed_run(
+        _test_rearrange_expert_weights_no_change,
+        world_size,
+    )
 
 
 def _test_rearrange_expert_weights_profile_mode(env, world_size) -> None:
@@ -563,7 +682,8 @@ def _test_rearrange_expert_weights_profile_mode(env, world_size) -> None:
             tensor_model_parallel_size=world_size, pipeline_model_parallel_size=1
         )
 
-        ep_group = get_tp_group().cpu_group
+        ep_group_coordinator = get_tp_group()
+        ep_group = ep_group_coordinator.cpu_group
         ep_rank = torch.distributed.get_rank()
         device = torch.device(f"cuda:{ep_rank}")
 
@@ -600,22 +720,39 @@ def _test_rearrange_expert_weights_profile_mode(env, world_size) -> None:
                 layer_copy.append(weight.clone())
             original_weights.append(layer_copy)
 
+        communicator = create_eplb_communicator_or_raise(
+            group_coordinator=ep_group_coordinator,
+            backend="torch_nccl",
+            expert_weights=expert_weights[0],
+        )
+
         # Execute profile mode rearrangement
         rearrange_expert_weights_inplace(
             old_indices,
             new_indices,
             expert_weights,
             ep_group,
+            communicator,
             is_profile=True,  # Profile mode
         )
 
     # In profile mode, the weights should remain unchanged
+    local_ok = True
     for layer in range(num_layers):
         for weight_idx in range(len(hidden_sizes)):
-                torch.testing.assert_close(
+            if not torch.equal(
                 expert_weights[layer][weight_idx],
                 original_weights[layer][weight_idx],
-                    msg="In profile mode, the weights should remain unchanged",
+            ):
+                local_ok = False
+                print(
+                    "test_rearrange_expert_weights_profile_mode failed: "
+                    f"layer={layer}, weight_idx={weight_idx}",
+                    flush=True,
+                )
+    assert_verification_synced(
+        local_ok,
+        "Profile-mode EPLB verification failed on at least one rank.",
     )
 
 
@@ -625,4 +762,7 @@ def test_rearrange_expert_weights_profile_mode(world_size):
 
     if torch.accelerator.device_count() < world_size:
         pytest.skip(f"Need at least {world_size} GPUs to run the test")
-    distributed_run(_test_rearrange_expert_weights_profile_mode, world_size)
+    distributed_run(
+        _test_rearrange_expert_weights_profile_mode,
+        world_size,
+    )

vllm/config/parallel.py

@@ -35,6 +35,7 @@ DistributedExecutorBackend = Literal["ray", "mp", "uni", "external_launcher"]
 DataParallelBackend = Literal["ray", "mp"]
 EPLBPolicyOption = Literal["default"]
 DCPCommBackend = Literal["ag_rs", "a2a"]
+EPLBCommunicatorBackend = Literal["torch_nccl", "torch_gloo", "pynccl"]
 All2AllBackend = Literal[
     "naive",
     "pplx",
@@ -83,6 +84,15 @@ class EPLBConfig:
     policy: EPLBPolicyOption = "default"
     """The policy type for expert parallel load balancing (EPLB)."""
 
+    communicator: EPLBCommunicatorBackend | None = None
+    """
+    Backend for EPLB expert weight communication:
+    - "torch_nccl": Use torch.distributed on the device process group
+    - "torch_gloo": Use torch.distributed gloo with CPU staging
+    - "pynccl": Use PyNccl send/recv
+    - None: Auto-select backend ("torch_gloo" for async, "torch_nccl" for sync)
+    """
+
     @model_validator(mode="after")
     def _validate_eplb_config(self) -> Self:
         if self.use_async and self.policy != "default":
@@ -764,16 +774,18 @@ class ParallelConfig:
                 "backend is mp, uni or external_launcher."
             )
 
-        if (
-            self.all2all_backend in ("allgather_reducescatter")
-            and self.eplb_config.use_async
-        ):
-            logger.warning(
-                "Async EPLB causes hangs with the '%s' all2all backend. "
-                "Forcing synchronous EPLB.",
-                self.all2all_backend,
-            )
-            self.eplb_config.use_async = False
+        if self.enable_eplb and self.eplb_config.communicator is None:
+            if self.enable_elastic_ep:
+                # Elastic EP requires stateless mode
+                # (torch.distributed.batch_isend_irecv doesn't
+                # support stateless mode), so we use PyNCCL backend
+                self.eplb_config.communicator = "pynccl"
+            elif self.eplb_config.use_async:
+                # Torch Gloo is a backend that allows avoiding hangs
+                # due to NCCL multi-thread conflicts in async EPLB
+                self.eplb_config.communicator = "torch_gloo"
+            else:
+                self.eplb_config.communicator = "torch_nccl"
 
     @property
     def use_ray(self) -> bool:

vllm/distributed/device_communicators/pynccl_wrapper.py

@@ -23,6 +23,7 @@
 # variable in the code.
 
 import ctypes
+import functools
 import platform
 from dataclasses import dataclass
 from typing import Any
@@ -75,26 +76,34 @@ class ncclDataTypeEnum:
     ncclFloat8e4m3 = 10
     ncclNumTypes = 11
 
+    @classmethod
+    @functools.lru_cache(maxsize=1)
+    def _torch_to_nccl_map(cls) -> dict[torch.dtype, int]:
+        return {
+            torch.int8: cls.ncclInt8,
+            torch.uint8: cls.ncclUint8,
+            torch.int32: cls.ncclInt32,
+            torch.int64: cls.ncclInt64,
+            torch.float16: cls.ncclFloat16,
+            torch.float32: cls.ncclFloat32,
+            torch.float64: cls.ncclFloat64,
+            torch.bfloat16: cls.ncclBfloat16,
+            current_platform.fp8_dtype(): cls.ncclFloat8e4m3,
+        }
+
+    @classmethod
+    def supports_torch_dtype(cls, dtype: torch.dtype) -> bool:
+        return dtype in cls._torch_to_nccl_map()
+
+    @classmethod
+    def try_from_torch(cls, dtype: torch.dtype) -> int | None:
+        return cls._torch_to_nccl_map().get(dtype)
+
     @classmethod
     def from_torch(cls, dtype: torch.dtype) -> int:
-        if dtype == torch.int8:
-            return cls.ncclInt8
-        if dtype == torch.uint8:
-            return cls.ncclUint8
-        if dtype == torch.int32:
-            return cls.ncclInt32
-        if dtype == torch.int64:
-            return cls.ncclInt64
-        if dtype == torch.float16:
-            return cls.ncclFloat16
-        if dtype == torch.float32:
-            return cls.ncclFloat32
-        if dtype == torch.float64:
-            return cls.ncclFloat64
-        if dtype == torch.bfloat16:
-            return cls.ncclBfloat16
-        if dtype == current_platform.fp8_dtype():
-            return cls.ncclFloat8e4m3
+        nccl_dtype = cls.try_from_torch(dtype)
+        if nccl_dtype is not None:
+            return nccl_dtype
         raise ValueError(
             f"Unsupported dtype {dtype}: should be one of "
             f"int8, uint8, int32, int64, float16, float32, float64, bfloat16,"

vllm/distributed/elastic_ep/elastic_execute.py

@@ -29,8 +29,10 @@ from vllm.distributed.elastic_ep.standby_state import (
     get_standby_ep_group,
     pop_standby_groups,
 )
+from vllm.distributed.eplb.eplb_communicator import create_eplb_communicator
 from vllm.distributed.parallel_state import (
     _replace_active_groups,
+    get_eplb_group,
     prepare_communication_buffer_for_model,
 )
 from vllm.distributed.stateless_coordinator import StatelessGroupCoordinator
@@ -411,6 +413,13 @@ class ElasticEPScalingExecutor:
                     module.quant_method = module.quant_method.old_quant_method
                     module.runner = module._init_runner()
             prepare_communication_buffer_for_model(self.worker.model_runner.model)
+
+        eplb_model_state.communicator = create_eplb_communicator(
+            group_coordinator=get_eplb_group(),
+            backend=parallel_config.eplb_config.communicator,
+            expert_weights=model.expert_weights[0],
+        )
+
         if (
             self.worker.vllm_config.compilation_config.mode
             == CompilationMode.STOCK_TORCH_COMPILE

vllm/distributed/eplb/async_worker.py

@@ -98,6 +98,8 @@ async def transfer_run_periodically(
 
         assert state.is_async
         for model_state in state.model_states.values():
+            # Set the async worker's CUDA stream on the communicator
+            model_state.communicator.set_stream(cuda_stream)
             rebalancing_algorithm_executed = False
             physical_to_logical_map_cpu = None
             current_num_layers = model_state.model.num_moe_layers
@@ -157,6 +159,7 @@ async def transfer_run_periodically(
                             expert_weights=model_state.model.expert_weights[layer_idx],
                             expert_weights_buffer=model_state.expert_buffer,
                             ep_group=eplb_group,
+                            communicator=model_state.communicator,
                             is_profile=is_profile,
                             cuda_stream=cuda_stream,
                         )

vllm/distributed/eplb/eplb_communicator.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""
+EPLB communicator implementations and factory.
+"""
+
+from abc import ABC, abstractmethod
+from collections.abc import Sequence
+
+import torch
+from torch.distributed import (
+    P2POp,
+    ProcessGroup,
+    batch_isend_irecv,
+)
+
+from vllm.distributed.device_communicators.pynccl import PyNcclCommunicator
+from vllm.distributed.device_communicators.pynccl_wrapper import (
+    ncclDataTypeEnum,
+)
+from vllm.distributed.parallel_state import GroupCoordinator, is_local_first_rank
+from vllm.distributed.stateless_coordinator import StatelessGroupCoordinator
+from vllm.logger import init_logger
+
+logger = init_logger(__name__)
+
+
+class EplbCommunicator(ABC):
+    """Abstract EPLB communicator for expert weight transfers."""
+
+    @abstractmethod
+    def add_send(self, tensor: torch.Tensor, dst_rank: int) -> None:
+        pass
+
+    @abstractmethod
+    def add_recv(self, tensor: torch.Tensor, src_rank: int) -> None:
+        pass
+
+    @abstractmethod
+    def execute(self) -> None:
+        pass
+
+    def set_stream(self, cuda_stream: torch.cuda.Stream | None) -> None:
+        self._cuda_stream = cuda_stream
+
+    def _log_initialized(self) -> None:
+        if is_local_first_rank():
+            logger.info("Initialized EPLB communicator: %s.", self.__class__.__name__)
+
+
+class TorchDistNcclEplbCommunicator(EplbCommunicator):
+    """EPLB communicator backed by torch.distributed isend/irecv."""
+
+    def __init__(
+        self,
+        ep_group: ProcessGroup,
+        cuda_stream: torch.cuda.Stream | None = None,
+    ) -> None:
+        self._ep_group = ep_group
+        self._cuda_stream = cuda_stream
+        self._p2p_ops: list[P2POp] = []
+        self._log_initialized()
+
+    def add_send(self, tensor: torch.Tensor, dst_rank: int) -> None:
+        self._p2p_ops.append(
+            P2POp(
+                torch.distributed.isend,
+                tensor,
+                dst_rank,
+                self._ep_group,
+            )
+        )
+
+    def add_recv(self, tensor: torch.Tensor, src_rank: int) -> None:
+        self._p2p_ops.append(
+            P2POp(
+                torch.distributed.irecv,
+                tensor,
+                src_rank,
+                self._ep_group,
+            )
+        )
+
+    def execute(self) -> None:
+        if not self._p2p_ops:
+            return
+        try:
+            with torch.cuda.stream(self._cuda_stream):
+                reqs = batch_isend_irecv(self._p2p_ops)
+                for req in reqs:
+                    req.wait()
+        finally:
+            self._p2p_ops.clear()
+
+
+class TorchDistGlooStagedEplbCommunicator(EplbCommunicator):
+    """EPLB communicator using gloo P2P with CPU staging."""
+
+    def __init__(
+        self,
+        cpu_group: ProcessGroup,
+        cuda_stream: torch.cuda.Stream | None = None,
+    ) -> None:
+        self._cpu_group = cpu_group
+        self._cuda_stream = cuda_stream
+        self._ops: list[tuple[str, torch.Tensor, int]] = []
+        self._log_initialized()
+
+    def add_send(self, tensor: torch.Tensor, dst_rank: int) -> None:
+        self._ops.append(("send", tensor, dst_rank))
+
+    def add_recv(self, tensor: torch.Tensor, src_rank: int) -> None:
+        self._ops.append(("recv", tensor, src_rank))
+
+    def execute(self) -> None:
+        if not self._ops:
+            return
+
+        p2p_ops: list[P2POp] = []
+        recv_staging: list[tuple[torch.Tensor, torch.Tensor]] = []
+
+        def build_ops() -> None:
+            for op, tensor, peer_rank in self._ops:
+                if op == "send":
+                    cpu_tensor = tensor.to(device="cpu", non_blocking=True)
+                    p2p_ops.append(
+                        P2POp(
+                            torch.distributed.isend,
+                            cpu_tensor,
+                            peer_rank,
+                            self._cpu_group,
+                        )
+                    )
+                    continue
+                cpu_tensor = torch.empty_like(tensor, device="cpu")
+                p2p_ops.append(
+                    P2POp(
+                        torch.distributed.irecv,
+                        cpu_tensor,
+                        peer_rank,
+                        self._cpu_group,
+                    )
+                )
+                recv_staging.append((tensor, cpu_tensor))
+
+        try:
+            with torch.cuda.stream(self._cuda_stream):
+                build_ops()
+        finally:
+            self._ops.clear()
+
+        # Wait for all D2H copies to finish
+        # before issuing gloo batch_isend_irecv operations.
+        if self._cuda_stream is not None:
+            self._cuda_stream.synchronize()
+        else:
+            torch.cuda.current_stream().synchronize()
+
+        reqs = batch_isend_irecv(p2p_ops)
+        for req in reqs:
+            req.wait()
+
+        if not recv_staging:
+            return
+        with torch.cuda.stream(self._cuda_stream):
+            for dst_tensor, cpu_tensor in recv_staging:
+                dst_tensor.copy_(cpu_tensor, non_blocking=True)
+
+
+class PyNcclEplbCommunicator(EplbCommunicator):
+    """EPLB communicator backed by PyNcclCommunicator using ncclSend/ncclRecv."""
+
+    def __init__(
+        self,
+        pynccl_comm: PyNcclCommunicator,
+        cuda_stream: torch.cuda.Stream | None = None,
+    ) -> None:
+        self._pynccl_comm = pynccl_comm
+        self._cuda_stream = cuda_stream
+        self._group_started = False
+        self._log_initialized()
+
+    def _ensure_group_started(self) -> None:
+        if not self._group_started:
+            self._pynccl_comm.group_start()
+            self._group_started = True
+
+    def add_send(self, tensor: torch.Tensor, dst_rank: int) -> None:
+        self._ensure_group_started()
+        self._pynccl_comm.send(tensor, dst_rank, stream=self._cuda_stream)
+
+    def add_recv(self, tensor: torch.Tensor, src_rank: int) -> None:
+        self._ensure_group_started()
+        self._pynccl_comm.recv(tensor, src_rank, stream=self._cuda_stream)
+
+    def execute(self) -> None:
+        if self._group_started:
+            self._pynccl_comm.group_end()
+            self._group_started = False
+
+
+def create_eplb_communicator(
+    group_coordinator: GroupCoordinator,
+    backend: str | None,
+    expert_weights: Sequence[torch.Tensor],
+) -> EplbCommunicator:
+    # Keep a safe default for callers that have not resolved communicator yet.
+    if backend is None:
+        backend = "torch_nccl"
+
+    tensor_device_type = expert_weights[0].device.type if expert_weights else "cpu"
+    torch_group = (
+        group_coordinator.cpu_group
+        if tensor_device_type == "cpu"
+        else group_coordinator.device_group
+    )
+
+    def _create_pynccl() -> EplbCommunicator:
+        if tensor_device_type == "cpu":
+            raise RuntimeError(
+                "EPLB communicator 'pynccl' supports only cuda-like devices "
+                f"(got {tensor_device_type})."
+            )
+        unsupported_dtypes = sorted(
+            {
+                tensor.dtype
+                for tensor in expert_weights
+                if not ncclDataTypeEnum.supports_torch_dtype(tensor.dtype)
+            },
+            key=str,
+        )
+        if unsupported_dtypes:
+            raise RuntimeError(
+                "EPLB communicator 'pynccl' requested but expert weights contain "
+                "unsupported dtypes: "
+                f"({', '.join(str(dtype) for dtype in unsupported_dtypes)})."
+            )
+
+        device_comm = group_coordinator.device_communicator
+        pynccl_comm = (
+            getattr(device_comm, "pynccl_comm", None)
+            if device_comm is not None
+            else None
+        )
+        if pynccl_comm is None or pynccl_comm.disabled or not pynccl_comm.available:
+            raise RuntimeError("EPLB communicator 'pynccl' requested but unavailable.")
+        try:
+            return PyNcclEplbCommunicator(pynccl_comm=pynccl_comm)
+        except Exception as exc:
+            raise RuntimeError(
+                f"Failed to initialize PyNcclEplbCommunicator ({exc})."
+            ) from exc
+
+    is_stateless = isinstance(group_coordinator, StatelessGroupCoordinator)
+    if is_stateless:
+        if backend not in ("torch_nccl", "pynccl"):
+            raise ValueError(
+                f"Elastic EP requires 'torch_nccl' or 'pynccl' EPLB communicator "
+                f"(got '{backend}'). torch_gloo is not supported with stateless groups."
+            )
+        if backend == "torch_nccl":
+            logger.warning(
+                "Stateless elastic EP requires PyNCCL backend. "
+                "Forcing EPLB communicator to 'pynccl'."
+            )
+            backend = "pynccl"
+        return _create_pynccl()
+
+    if backend == "torch_gloo":
+        return TorchDistGlooStagedEplbCommunicator(
+            cpu_group=group_coordinator.cpu_group,
+        )
+    elif backend == "torch_nccl":
+        return TorchDistNcclEplbCommunicator(ep_group=torch_group)
+    elif backend == "pynccl":
+        return _create_pynccl()
+    raise ValueError(f"Unknown EPLB communicator backend: {backend}")

vllm/distributed/eplb/eplb_state.py

@@ -37,6 +37,7 @@ from torch.distributed import ProcessGroup, all_reduce
 from vllm.config import ModelConfig, ParallelConfig
 from vllm.distributed.parallel_state import (
     get_ep_group,
+    get_eplb_group,
     get_node_count,
     in_the_same_node_as,
 )
@@ -46,6 +47,7 @@ from vllm.logger import init_logger
 from vllm.model_executor.models.interfaces import MixtureOfExperts
 
 from .async_worker import start_async_worker
+from .eplb_communicator import EplbCommunicator, create_eplb_communicator
 from .policy import EPLB_POLICIES, AbstractEplbPolicy, DefaultEplbPolicy
 from .rebalance_execute import (
     RecvMetadata,
@@ -225,6 +227,10 @@ class EplbModelState:
     """
     CUDA device index for the async EPLB worker thread.
     """
+    communicator: EplbCommunicator
+    """
+    The communicator for expert weight transfers.
+    """
     new_physical_to_logical_map: torch.Tensor | None = None
     """
     intermediate variable between `move_to_buffer` and `move_to_workspace`.
@@ -472,6 +478,12 @@ class EplbState:
         self._init_should_record_tensor(model)
         expert_buffer = [torch.empty_like(w) for w in model.expert_weights[0]]
 
+        communicator = create_eplb_communicator(
+            group_coordinator=get_eplb_group(),
+            backend=self.parallel_config.eplb_config.communicator,
+            expert_weights=model.expert_weights[0],
+        )
+
         model_state = EplbModelState(
             physical_to_logical_map=physical_to_logical_map,
             logical_to_physical_map=logical_to_physical_map,
@@ -498,6 +510,7 @@ class EplbState:
                 recv_dst_rows=np.array([]),
             ),
             cuda_device_index=self.cuda_device_index,
+            communicator=communicator,
             new_physical_to_logical_map=None,
         )
         self.model_states[model_config.compute_hash()] = model_state
@@ -800,6 +813,7 @@ class EplbState:
                     new_physical_to_logical_map,
                     eplb_model_state.model.expert_weights,
                     ep_group,
+                    eplb_model_state.communicator,
                     is_profile,
                     rank_mapping,
                 )
@@ -923,11 +937,8 @@ class EplbState:
                 new_indices=new_indices,
                 ep_rank=ep_group.rank(),
             )
-            # Record event after consuming buffer to signal async thread
-            # that it's safe to overwrite the intermediate buffer
-            consumed_event = torch.cuda.Event()
-            consumed_event.record()
-            model_state.buffer_consumed_event = consumed_event
+
+            transferred_layer = model_state.layer_to_transfer
 
             transferred_layer = model_state.layer_to_transfer
             assert model_state.new_physical_to_logical_map is not None
@@ -936,6 +947,13 @@ class EplbState:
                 new_physical_to_logical_map=model_state.new_physical_to_logical_map,
                 layer=transferred_layer,
             )
+
+            # Record event after consuming buffer to signal async thread
+            # that it's safe to overwrite the intermediate buffer
+            consumed_event = torch.cuda.Event()
+            consumed_event.record()
+            model_state.buffer_consumed_event = consumed_event
+
             # After the main thread consumes, advance layer_to_transfer
             model_state.layer_to_transfer += 1
             model_state.ep_buffer_ready = 0

vllm/distributed/eplb/eplb_utils.py

@@ -21,6 +21,10 @@ def override_envs_for_eplb(parallel_config: ParallelConfig) -> None:
     is_eplb_enabled = parallel_config.enable_eplb
     async_eplb = parallel_config.eplb_config.use_async
     is_deepep_ll = parallel_config.all2all_backend == "deepep_low_latency"
+    is_nccl_based_eplb_communicator = parallel_config.eplb_config.communicator in (
+        "torch_nccl",
+        "pynccl",
+    )
 
     # Override NCCL_MAX_CTAS to avoid hangs when using async EPLB with the
     # DeepEP low-latency backend.
@@ -39,7 +43,13 @@ def override_envs_for_eplb(parallel_config: ParallelConfig) -> None:
     # Limiting NCCL occupancy via NCCL_MAX_CTAS leaves space for the DeepEP
     # cooperative kernel to launch and complete, breaking the deadlock.
     # See: https://github.com/deepseek-ai/DeepEP/issues/496
-    if is_data_parallel and is_eplb_enabled and is_deepep_ll and async_eplb:
+    if (
+        is_data_parallel
+        and is_eplb_enabled
+        and is_deepep_ll
+        and async_eplb
+        and is_nccl_based_eplb_communicator
+    ):
         current_value_str = os.getenv("NCCL_MAX_CTAS")
 
         if current_value_str and current_value_str.isdigit():
@@ -49,6 +59,7 @@ def override_envs_for_eplb(parallel_config: ParallelConfig) -> None:
         os.environ["NCCL_MAX_CTAS"] = str(override_value)
         logger.info_once(
             f"EPLB: Setting NCCL_MAX_CTAS={override_value} "
-            "for expert parallel with EPLB and deepep_low_latency backend",
+            "for expert parallel with NCCL-based EPLB communicator and "
+            "deepep_low_latency backend",
             scope="global",
         )

vllm/distributed/eplb/rebalance_execute.py

@@ -11,19 +11,9 @@ from dataclasses import dataclass
 
 import numpy as np
 import torch
-from torch.distributed import (
-    P2POp,
-    ProcessGroup,
-    all_gather,
-    batch_isend_irecv,
-    get_global_rank,
-)
+from torch.distributed import ProcessGroup, all_gather
 
-from vllm.distributed.parallel_state import get_ep_group
-from vllm.distributed.stateless_coordinator import StatelessGroupCoordinator
-from vllm.logger import init_logger
-
-logger = init_logger(__name__)
+from .eplb_communicator import EplbCommunicator
 
 
 @dataclass
@@ -158,7 +148,8 @@ def move_to_buffer(
     expert_weights: Sequence[torch.Tensor],
     expert_weights_buffers: Sequence[torch.Tensor],
     cuda_stream: torch.cuda.Stream | None,
-    ep_group: ProcessGroup,
+    ep_rank: int,
+    communicator: EplbCommunicator,
 ) -> MoveToBufferResult:
     """
     Rearranges expert weights during EPLB rebalancing.
@@ -172,7 +163,8 @@ def move_to_buffer(
         expert_weights: Original expert weights for the layer.
         expert_weights_buffers: Intermediate buffers (one per tensor).
         cuda_stream: CUDA stream for async copies (can be None for sync mode).
-        ep_group: Distributed process group for expert parallel comms.
+        ep_rank: Rank of this process in expert parallel group.
+        communicator: EplbCommunicator instance for P2P communication.
 
     Returns:
         is_unchanged (np.ndarray): (num_local_experts,), True where an expert row
@@ -182,8 +174,6 @@ def move_to_buffer(
         RecvMetadata: Metadata needed for completing remote weight transfers.
     """
     assert old_indices.shape == new_indices.shape
-    ep_rank = ep_group.rank()
-
     recv_primary_mask = np.zeros((num_local_experts,), dtype=np.bool_)
     send_expert_ids = np.full((num_local_experts,), -1, dtype=np.int64)
     send_src_rows = np.full((num_local_experts,), -1, dtype=np.int32)
@@ -247,23 +237,10 @@ def move_to_buffer(
             expert = new_local_expert_ids[dst]
             src_local = expert_to_src_map.get(expert, -1)
             if src_local != -1:
+                with torch.cuda.stream(cuda_stream):
                     for w, b in zip(expert_weights, expert_weights_buffers):
                         b[dst].copy_(w[src_local], non_blocking=True)
 
-    p2p_ops: list[P2POp] = []
-    if isinstance(get_ep_group(), StatelessGroupCoordinator):
-        ep_group = get_ep_group()
-        is_stateless = True
-    else:
-        is_stateless = False
-
-    # Pre-compute global ranks mapping (only needed for non-stateless groups)
-    ep_size = ep_group.size()
-    if not is_stateless:
-        rank_to_global = {
-            rank: get_global_rank(ep_group, rank) for rank in range(ep_size)
-        }
-
     # 2. Post sends
     if send_count > 0:
         experts = send_expert_ids[:send_count]
@@ -294,23 +271,8 @@ def move_to_buffer(
             if recver_pos < len(ranks_to_recv):
                 recv_ranks.append(ranks_to_recv[recver_pos])
             for dst in recv_ranks:
-                if is_stateless:
                 for w in expert_weights:
-                        op = object.__new__(P2POp)
-                        op.op = torch.distributed.isend
-                        op.tensor = w[src]
-                        op.group_peer = dst
-                        p2p_ops.append(op)
-                else:
-                    dst_global = rank_to_global[dst]
-                    p2p_ops += [
-                        P2POp(
-                            torch.distributed.isend,
-                            w[src],
-                            dst_global,
-                        )
-                        for w in expert_weights
-                    ]
+                    communicator.add_send(w[src], dst)
 
     # 3. Post recvs
     if recv_count > 0:
@@ -339,40 +301,11 @@ def move_to_buffer(
                 src = ranks_to_send[recver_pos // num_dst_per_sender]
             else:
                 src = ranks_to_send[recver_pos - remainder_start]
-            if is_stateless:
             for b in expert_weights_buffers:
-                    op = object.__new__(P2POp)
-                    op.op = torch.distributed.irecv
-                    op.tensor = b[dst]
-                    op.group_peer = src
-                    p2p_ops.append(op)
-            else:
-                src_global = rank_to_global[src]
-                p2p_ops += [
-                    P2POp(
-                        torch.distributed.irecv,
-                        b[dst],
-                        src_global,
-                    )
-                    for b in expert_weights_buffers
-                ]
+                communicator.add_recv(b[dst], src)
 
     # 4. Execute the P2P operations. The real communication happens here.
-    if p2p_ops and cuda_stream is not None:
-        with torch.cuda.stream(cuda_stream):
-            if is_stateless:
-                ep_group.device_communicator.batch_isend_irecv(p2p_ops)
-            else:
-                reqs = batch_isend_irecv(p2p_ops)
-                for req in reqs:
-                    req.wait()
-    elif p2p_ops:
-        if is_stateless:
-            ep_group.device_communicator.batch_isend_irecv(p2p_ops)
-        else:
-            reqs = batch_isend_irecv(p2p_ops)
-            for req in reqs:
-                req.wait()
+    communicator.execute()
     # wait for the communication to finish
     return (
         is_unchanged,
@@ -471,6 +404,7 @@ async def transfer_layer(
     expert_weights: Sequence[torch.Tensor],
     expert_weights_buffer: Sequence[torch.Tensor],
     ep_group: ProcessGroup,
+    communicator: EplbCommunicator,
     is_profile: bool = False,
     cuda_stream: torch.cuda.Stream | None = None,
     rank_mapping: dict[int, int] | None = None,
@@ -489,6 +423,7 @@ async def transfer_layer(
             For example, a linear layer may have up and down projection.
         expert_weights_buffer: Intermediate buffers (one per weight tensor).
         ep_group: The device process group for expert parallelism.
+        communicator: EplbCommunicator instance for P2P communication.
         is_profile (bool): If `True`, do not perform any actual weight copy.
             This is used during profile run, where we only perform dummy
             communications to reserve enough memory for the buffers.
@@ -542,7 +477,8 @@ async def transfer_layer(
         expert_weights=expert_weights,
         expert_weights_buffers=expert_weights_buffer,
         cuda_stream=cuda_stream,
-        ep_group=ep_group,
+        ep_rank=ep_group.rank(),
+        communicator=communicator,
     )
     return is_unchanged, is_received_locally, recv_metadata
 
@@ -552,6 +488,7 @@ def rearrange_expert_weights_inplace(
     new_global_expert_indices: torch.Tensor,
     expert_weights: Sequence[Sequence[torch.Tensor]],
     ep_group: ProcessGroup,
+    communicator: EplbCommunicator,
     is_profile: bool = False,
     rank_mapping: dict[int, int] | None = None,
 ) -> None:
@@ -569,6 +506,7 @@ def rearrange_expert_weights_inplace(
             For example, a linear layer may have up and down projection,
             so weight_count = 2. Each weight's hidden size can be different.
         ep_group: The device process group for expert parallelism.
+        communicator: EplbCommunicator instance for P2P communication.
         is_profile (bool): If `True`, do not perform any actual weight copy.
             This is used during profile run, where we only perform dummy
             communications to reserve enough memory for the buffers.
@@ -599,6 +537,7 @@ def rearrange_expert_weights_inplace(
     assert new_global_expert_indices.shape == (num_moe_layers, num_physical_experts)
 
     ep_size = ep_group.size()
+    ep_rank = ep_group.rank()
     assert num_physical_experts == ep_size * num_local_physical_experts
 
     first_layer_weights = list(expert_weights[0])
@@ -635,7 +574,8 @@ def rearrange_expert_weights_inplace(
             expert_weights=expert_weights[layer_idx],
             expert_weights_buffers=weights_buffer,
             cuda_stream=None,
-            ep_group=ep_group,
+            ep_rank=ep_rank,
+            communicator=communicator,
         )
 
         move_from_buffer(
@@ -645,7 +585,7 @@ def rearrange_expert_weights_inplace(
             is_received_locally=is_received_locally,
             recv_metadata=recv_metadata,
             new_indices=new_global_expert_indices_cpu[layer_idx],
-            ep_rank=ep_group.rank(),
+            ep_rank=ep_rank,
         )
 
 

vllm/distributed/parallel_state.py

@@ -1690,11 +1690,7 @@ def initialize_model_parallel(
         # using torch.distributed in execution with torch.distributed in EPLB.
         global _EPLB
         assert _EPLB is None, "EPLB group is already initialized"
-        if (
-            config is not None
-            and config.parallel_config is not None
-            and config.parallel_config.enable_eplb
-        ):
+        if config.parallel_config.enable_eplb:
             if enable_elastic_ep:
                 _EPLB = _init_stateless_group(
                     group_ranks,