[UX] Replace VLLM_ALL2ALL_BACKEND with --all2all-backend (#26732)

Signed-off-by: mgoin <mgoin64@gmail.com>

docs/design/dbo.md

@@ -34,10 +34,10 @@ To enable the DBO system pass in the `--enable-dbo` argument to your vllm serve
 * `--dbo-decode-token-threshold` the minimum number of tokens in a decode-only batch required to enable DBO for that batch
 * `--dbo-prefill-token-threshold` the minimum number of tokens in a batch containing at least one prefill required to enable DBO for that batch
 
-Currently, DBO is only supported with DeepEP, so DeepEP must be installed and the `VLLM_ALL2ALL_BACKEND` environment variable must be set to `deepep_low_latency` if your workload is primarily decode requests, or `deepep_high_throughput` if your workload is primarily prefill requests.
+Currently, DBO is only supported with DeepEP, so DeepEP must be installed and the `--all2all-backend` argument must be set to `deepep_low_latency` if your workload is primarily decode requests, or `deepep_high_throughput` if your workload is primarily prefill requests.
 
 Below is a command that will spin up a two DP rank server with expert parallelism and DBO enabled.
-EX: `VLLM_ALL2ALL_BACKEND=deepep_low_latency vllm serve --model="deepseek-ai/DeepSeek-V2-Lite" --trust-remote-code --data-parallel-size 2 --enable-expert-parallel --enable-dbo`
+EX: `vllm serve deepseek-ai/DeepSeek-V2-Lite --trust-remote-code --data-parallel-size 2 --enable-expert-parallel --enable-dbo --all2all-backend deepep_low_latency`
 
 Note that there must be at least two GPUs visible in `CUDA_VISIBLE_DEVICES`
 

docs/serving/expert_parallel_deployment.md

@@ -14,13 +14,16 @@ Before using EP, you need to install the necessary dependencies. We are actively
 
 ### Backend Selection Guide
 
-vLLM provides three communication backends for EP:
+vLLM provides multiple communication backends for EP. Use `--all2all-backend` to select one:
 
 | Backend | Use Case | Features | Best For |
 |---------|----------|----------|----------|
-| `pplx` | Single node | Chunked prefill support | Development, best for intra-node deployments |
+| `allgather_reducescatter` | Default backend | Standard all2all using allgather/reducescatter primitives | General purpose, works with any EP+DP configuration |
-| `deepep_high_throughput` | Multi-node prefill | Grouped GEMM with continuous layout | High-throughput scenarios, prefill-dominated workloads |
+| `pplx` | Single node | Chunked prefill support, efficient intra-node communication | Single-node deployments, development |
-| `deepep_low_latency` | Multi-node decode | CUDA graph support, masked layout | Low-latency scenarios, decode-dominated workloads |
+| `deepep_high_throughput` | Multi-node prefill | Grouped GEMM with continuous layout, optimized for prefill | Prefill-dominated workloads, high-throughput scenarios |
+| `deepep_low_latency` | Multi-node decode | CUDA graph support, masked layout, optimized for decode | Decode-dominated workloads, low-latency scenarios |
+| `flashinfer_all2allv` | MNNVL systems | FlashInfer alltoallv kernels for multi-node NVLink | Systems with NVLink across nodes |
+| `naive` | Testing/debugging | Simple broadcast-based implementation | Debugging, not recommended for production |
 
 ## Single Node Deployment
 
@@ -47,11 +50,11 @@ The following command serves a `DeepSeek-V3-0324` model with 1-way tensor parall
 
 ```bash
 # Single node EP deployment with pplx backend
-VLLM_ALL2ALL_BACKEND=pplx VLLM_USE_DEEP_GEMM=1 \
+vllm serve deepseek-ai/DeepSeek-V3-0324 \
-    vllm serve deepseek-ai/DeepSeek-V3-0324 \
+    --tensor-parallel-size 1 \       # Tensor parallelism across 1 GPU
-    --tensor-parallel-size 1 \      # Tensor parallelism across 1 GPU
     --data-parallel-size 8 \         # Data parallelism across 8 processes
-    --enable-expert-parallel         # Enable expert parallelism
+    --enable-expert-parallel \       # Enable expert parallelism
+    --all2all-backend pplx           # Use pplx communication backend
 ```
 
 ## Multi-Node Deployment
@@ -70,8 +73,8 @@ The following example deploys `DeepSeek-V3-0324` across 2 nodes using `deepep_lo
 
 ```bash
 # Node 1 (Primary - handles incoming requests)
-VLLM_ALL2ALL_BACKEND=deepep_low_latency VLLM_USE_DEEP_GEMM=1 \
+vllm serve deepseek-ai/DeepSeek-V3-0324 \
-    vllm serve deepseek-ai/DeepSeek-V3-0324 \
+    --all2all-backend deepep_low_latency \
     --tensor-parallel-size 1 \               # TP size per node
     --enable-expert-parallel \               # Enable EP
     --data-parallel-size 16 \                # Total DP size across all nodes
@@ -81,8 +84,8 @@ VLLM_ALL2ALL_BACKEND=deepep_low_latency VLLM_USE_DEEP_GEMM=1 \
     --api-server-count=8                     # Number of API servers for load handling (scaling this out to total ranks are recommended)
 
 # Node 2 (Secondary - headless mode, no API server)
-VLLM_ALL2ALL_BACKEND=deepep_low_latency VLLM_USE_DEEP_GEMM=1 \
+vllm serve deepseek-ai/DeepSeek-V3-0324 \
-    vllm serve deepseek-ai/DeepSeek-V3-0324 \
+    --all2all-backend deepep_low_latency \
     --tensor-parallel-size 1 \               # TP size per node
     --enable-expert-parallel \               # Enable EP
     --data-parallel-size 16 \                # Total DP size across all nodes
@@ -169,11 +172,12 @@ Single node deployment with EPLB enabled:
 
 ```bash
 # Single node with EPLB load balancing
-VLLM_ALL2ALL_BACKEND=pplx VLLM_USE_DEEP_GEMM=1 vllm serve deepseek-ai/DeepSeek-V3-0324 \
+vllm serve deepseek-ai/DeepSeek-V3-0324 \
-    --tensor-parallel-size 1 \     # Tensor parallelism
+    --tensor-parallel-size 1 \       # Tensor parallelism
-    --data-parallel-size 8 \        # Data parallelism  
+    --data-parallel-size 8 \         # Data parallelism
-    --enable-expert-parallel \      # Enable EP
+    --enable-expert-parallel \       # Enable EP
-    --enable-eplb \                 # Enable load balancer
+    --all2all-backend pplx \         # Use pplx communication backend
+    --enable-eplb \                  # Enable load balancer
     --eplb-config '{"window_size":1000,"step_interval":3000,"num_redundant_experts":2,"log_balancedness":true}'
 ```
 

vllm/config/parallel.py

@@ -113,6 +113,25 @@ class ParallelConfig:
       with 4 experts and 2 ranks, rank 0 will have experts [0, 2] and rank 1
       will have experts [1, 3]. This strategy can help improve load balancing
       for grouped expert models with no redundant experts."""
+    all2all_backend: (
+        Literal[
+            "naive",
+            "pplx",
+            "deepep_high_throughput",
+            "deepep_low_latency",
+            "allgather_reducescatter",
+            "flashinfer_all2allv",
+        ]
+        | None
+    ) = None
+    """All2All backend for MoE expert parallel communication. If not set, uses
+    the value from VLLM_ALL2ALL_BACKEND environment variable. Available options:
+    - "naive": Naive all2all implementation using broadcasts
+    - "allgather_reducescatter": All2all based on allgather and reducescatter
+    - "pplx": Use pplx kernels
+    - "deepep_high_throughput": Use deepep high-throughput kernels
+    - "deepep_low_latency": Use deepep low-latency kernels
+    - "flashinfer_all2allv": Use flashinfer alltoallv kernels for mnnvl"""
     num_redundant_experts: int | None = None
     """`num_redundant_experts` is deprecated and has been replaced with
     `eplb_config.num_redundant_experts`. This will be removed in v0.12.0.
@@ -341,7 +360,7 @@ class ParallelConfig:
     @property
     def use_sequence_parallel_moe(self) -> bool:
         return (
-            envs.VLLM_ALL2ALL_BACKEND
+            self.all2all_backend
             in (
                 "allgather_reducescatter",
                 "naive",
@@ -390,7 +409,7 @@ class ParallelConfig:
         factors.append(self.tensor_parallel_size)
         factors.append(self.enable_expert_parallel)
         factors.append(self.data_parallel_size)
-        factors.append(envs.VLLM_ALL2ALL_BACKEND)
+        factors.append(self.all2all_backend)
         factors.append(self.enable_eplb)
         if self.enable_eplb:
             factors.append(self.eplb_config.log_balancedness)
@@ -400,6 +419,16 @@ class ParallelConfig:
         return hashlib.sha256(str(factors).encode()).hexdigest()
 
     def __post_init__(self) -> None:
+        # Set all2all_backend from env var if not specified, with deprecation warning
+        if self.all2all_backend is None:
+            self.all2all_backend = envs.VLLM_ALL2ALL_BACKEND
+            if envs.is_set("VLLM_ALL2ALL_BACKEND"):
+                logger.warning_once(
+                    "VLLM_ALL2ALL_BACKEND environment variable is deprecated and "
+                    "will be removed in a future release. Please use the "
+                    "--all2all-backend command-line argument instead."
+                )
+
         # Forward deprecated fields to their new location
         if self.num_redundant_experts is not None:
             self.eplb_config.num_redundant_experts = self.num_redundant_experts

vllm/config/vllm.py

@@ -523,13 +523,13 @@ class VllmConfig:
             )
 
         if self.parallel_config.enable_dbo:
-            a2a_backend = envs.VLLM_ALL2ALL_BACKEND
+            a2a_backend = self.parallel_config.all2all_backend
             assert a2a_backend in ["deepep_low_latency", "deepep_high_throughput"], (
                 "Microbatching currently only supports the deepep_low_latency and "
                 f"deepep_high_throughput all2all backend. {a2a_backend} is not "
-                "supported. To fix set the VLLM_ALL2ALL_BACKEND environment "
+                "supported. To fix use --all2all-backend=deepep_low_latency or "
-                "variable to deepep_low_latency or deepep_high_throughput and "
+                "--all2all-backend=deepep_high_throughput and install the DeepEP"
-                "install the DeepEP kernels."
+                " kernels."
             )
 
             if not self.model_config.disable_cascade_attn:

vllm/distributed/device_communicators/base_device_communicator.py

@@ -111,6 +111,7 @@ class DeviceCommunicatorBase:
         self.rank_in_group = dist.get_group_rank(self.cpu_group, self.global_rank)
 
         use_ep = False
+        all2all_backend = None
         from vllm.config import get_current_vllm_config
 
         config = get_current_vllm_config()
@@ -119,9 +120,11 @@ class DeviceCommunicatorBase:
             # where all data parallel ranks execute forward together),
             # we initialize the all2all manager used in expert parallel.
             use_ep = config.parallel_config.data_parallel_size > 1
+            all2all_backend = config.parallel_config.all2all_backend
 
         self.is_ep_communicator = "ep" in unique_name
         self.use_all2all = self.is_ep_communicator and use_ep
+        self.all2all_backend = all2all_backend
         self.all2all_manager: All2AllManagerBase | None = None
 
     def all_reduce(self, input_: torch.Tensor) -> torch.Tensor:

vllm/distributed/device_communicators/cuda_communicator.py

@@ -91,33 +91,32 @@ class CudaCommunicator(DeviceCommunicatorBase):
                 self.qr_comm = QuickAllReduce(group=self.cpu_group, device=self.device)
 
         if self.use_all2all:
-            all2all_backend = envs.VLLM_ALL2ALL_BACKEND
+            if self.all2all_backend == "naive":
-            if all2all_backend == "naive":
                 from .all2all import NaiveAll2AllManager
 
                 self.all2all_manager = NaiveAll2AllManager(self.cpu_group)
-            elif all2all_backend == "allgather_reducescatter":
+            elif self.all2all_backend == "allgather_reducescatter":
                 from .all2all import AgRsAll2AllManager
 
                 self.all2all_manager = AgRsAll2AllManager(self.cpu_group)
-            elif all2all_backend == "pplx":
+            elif self.all2all_backend == "pplx":
                 from .all2all import PPLXAll2AllManager
 
                 self.all2all_manager = PPLXAll2AllManager(self.cpu_group)
-            elif all2all_backend == "deepep_high_throughput":
+            elif self.all2all_backend == "deepep_high_throughput":
                 from .all2all import DeepEPHTAll2AllManager
 
                 self.all2all_manager = DeepEPHTAll2AllManager(self.cpu_group)
-            elif all2all_backend == "deepep_low_latency":
+            elif self.all2all_backend == "deepep_low_latency":
                 from .all2all import DeepEPLLAll2AllManager
 
                 self.all2all_manager = DeepEPLLAll2AllManager(self.cpu_group)
-            elif all2all_backend == "flashinfer_all2allv":
+            elif self.all2all_backend == "flashinfer_all2allv":
                 from .all2all import FlashInferAllToAllManager
 
                 self.all2all_manager = FlashInferAllToAllManager(self.cpu_group)
             else:
-                raise ValueError(f"Unknown all2all backend: {all2all_backend}")
+                raise ValueError(f"Unknown all2all backend: {self.all2all_backend}")
 
             if is_global_first_rank():
                 logger.info(

vllm/distributed/device_communicators/xpu_communicator.py

@@ -6,7 +6,6 @@ import torch
 import torch.distributed as dist
 from torch.distributed import ProcessGroup
 
-import vllm.envs as envs
 from vllm.logger import init_logger
 
 from .base_device_communicator import DeviceCommunicatorBase
@@ -24,15 +23,14 @@ class XpuCommunicator(DeviceCommunicatorBase):
     ):
         super().__init__(cpu_group, device, device_group, unique_name)
         if self.use_all2all:
-            all2all_backend = envs.VLLM_ALL2ALL_BACKEND
+            if self.all2all_backend != "naive":
-            if all2all_backend != "naive":
                 logger.warning(
-                    "`%s` all2all manager is not supported on XPU."
+                    "`%s` all2all manager is not supported on XPU. "
                     "Falling back to `naive` all2all manager for XPU.",
-                    all2all_backend,
+                    self.all2all_backend,
                 )
-                all2all_backend = "naive"
+                self.all2all_backend = "naive"
-            if all2all_backend == "naive":
+            if self.all2all_backend == "naive":
                 from .all2all import NaiveAll2AllManager
 
                 self.all2all_manager = NaiveAll2AllManager(self.cpu_group)

vllm/engine/arg_utils.py

@@ -371,6 +371,7 @@ class EngineArgs:
     data_parallel_hybrid_lb: bool = False
     data_parallel_backend: str = ParallelConfig.data_parallel_backend
     enable_expert_parallel: bool = ParallelConfig.enable_expert_parallel
+    all2all_backend: str | None = ParallelConfig.all2all_backend
     enable_dbo: bool = ParallelConfig.enable_dbo
     dbo_decode_token_threshold: int = ParallelConfig.dbo_decode_token_threshold
     dbo_prefill_token_threshold: int = ParallelConfig.dbo_prefill_token_threshold
@@ -763,6 +764,9 @@ class EngineArgs:
         parallel_group.add_argument(
             "--enable-expert-parallel", **parallel_kwargs["enable_expert_parallel"]
         )
+        parallel_group.add_argument(
+            "--all2all-backend", **parallel_kwargs["all2all_backend"]
+        )
         parallel_group.add_argument("--enable-dbo", **parallel_kwargs["enable_dbo"])
         parallel_group.add_argument(
             "--dbo-decode-token-threshold",
@@ -1461,6 +1465,7 @@ class EngineArgs:
             data_parallel_backend=self.data_parallel_backend,
             data_parallel_hybrid_lb=self.data_parallel_hybrid_lb,
             enable_expert_parallel=self.enable_expert_parallel,
+            all2all_backend=self.all2all_backend,
             enable_dbo=self.enable_dbo,
             dbo_decode_token_threshold=self.dbo_decode_token_threshold,
             dbo_prefill_token_threshold=self.dbo_prefill_token_threshold,

vllm/model_executor/layers/fused_moe/config.py

@@ -641,6 +641,7 @@ class FusedMoEParallelConfig:
     ep_rank: int
 
     use_ep: bool  # whether to use EP or not
+    all2all_backend: str  # all2all backend for MoE communication
 
     @property
     def use_all2all_kernels(self):
@@ -648,21 +649,18 @@ class FusedMoEParallelConfig:
 
     @property
     def use_pplx_kernels(self):
-        return self.use_all2all_kernels and envs.VLLM_ALL2ALL_BACKEND == "pplx"
+        return self.use_all2all_kernels and self.all2all_backend == "pplx"
 
     @property
     def use_deepep_ht_kernels(self):
         return (
             self.use_all2all_kernels
-            and envs.VLLM_ALL2ALL_BACKEND == "deepep_high_throughput"
+            and self.all2all_backend == "deepep_high_throughput"
         )
 
     @property
     def use_deepep_ll_kernels(self):
-        return (
+        return self.use_all2all_kernels and self.all2all_backend == "deepep_low_latency"
-            self.use_all2all_kernels
-            and envs.VLLM_ALL2ALL_BACKEND == "deepep_low_latency"
-        )
 
     @staticmethod
     def make(
@@ -762,6 +760,7 @@ class FusedMoEParallelConfig:
                 ep_size=1,
                 ep_rank=0,
                 use_ep=False,
+                all2all_backend=vllm_parallel_config.all2all_backend,
             )
         # DP + EP / TP + EP / DP + TP + EP
         assert use_ep
@@ -777,6 +776,7 @@ class FusedMoEParallelConfig:
             ep_size=ep_size,
             ep_rank=ep_rank,
             use_ep=True,
+            all2all_backend=vllm_parallel_config.all2all_backend,
         )
 
 

vllm/model_executor/layers/quantization/utils/flashinfer_fp4_moe.py

@@ -58,7 +58,7 @@ def build_flashinfer_fp4_cutlass_moe_prepare_finalize(
 ) -> mk.FusedMoEPrepareAndFinalize:
     """Create a FlashInfer CUTLASS fused-MoE prepare finalize kernel"""
     use_dp = moe.moe_parallel_config.dp_size > 1
-    enable_alltoallv = envs.VLLM_ALL2ALL_BACKEND == "flashinfer_all2allv"
+    enable_alltoallv = moe.moe_parallel_config.all2all_backend == "flashinfer_all2allv"
     return create_flashinfer_prepare_finalize(
         use_dp=use_dp, use_nvfp4=True, enable_alltoallv=enable_alltoallv
     )

vllm/platforms/cuda.py

@@ -192,7 +192,7 @@ class CudaPlatformBase(Platform):
 
         compilation_config = vllm_config.compilation_config
         if (
-            envs.VLLM_ALL2ALL_BACKEND == "deepep_high_throughput"
+            parallel_config.all2all_backend == "deepep_high_throughput"
             and parallel_config.data_parallel_size > 1
             and compilation_config.cudagraph_mode != CUDAGraphMode.NONE
         ):
@@ -204,7 +204,7 @@ class CudaPlatformBase(Platform):
                 "kernels are optimized for prefill and are incompatible with "
                 "CUDA Graphs. "
                 "In order to use CUDA Graphs for decode-optimized workloads, "
-                "set VLLM_ALL2ALL_BACKEND to another option, such as "
+                "use --all2all-backend with another option, such as "
                 "deepep_low_latency, pplx, or allgather_reducescatter."
             )
             compilation_config.cudagraph_mode = CUDAGraphMode.NONE

vllm/v1/engine/utils.py

@@ -356,9 +356,10 @@ class CoreEngineActorManager:
         )
         device_str = current_platform.ray_device_key
 
+        all2all_backend = vllm_config.parallel_config.all2all_backend
         if envs.VLLM_RAY_DP_PACK_STRATEGY == "fill" and (
-            envs.VLLM_ALL2ALL_BACKEND == "deepep_high_throughput"
+            all2all_backend == "deepep_high_throughput"
-            or envs.VLLM_ALL2ALL_BACKEND == "deepep_low_latency"
+            or all2all_backend == "deepep_low_latency"
         ):
             raise ValueError(
                 "DeepEP kernels require EP ranks [0,7] (same for [8,15], ...) "