[skipCI][Docs] Add expert_parallel.md (#2471)

Signed-off-by: skf1999 <13234016272@163.com> Co-authored-by: Canlin Guo <canlinguosdu@gmail.com>
2026-04-08 02:35:14 +00:00
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@@ -98,6 +98,7 @@ nav:
      - design/feature/ray_based_execution.md
      - design/feature/omni_connectors/
      - design/feature/cfg_parallel.md
+      - design/feature/expert_parallel.md
      - design/feature/sequence_parallel.md
      - design/feature/tensor_parallel.md
      - design/feature/vae_parallel.md
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+# Expert Parallel
+
+This section describes how to add Expert Parallel (EP) to a diffusion transformer that uses Mixture-of-Experts (MoE) layers.
+We use **HunyuanImage3.0** as the reference implementation.
+
+---
+
+## Table of Contents
+
+- [Overview](#overview)
+- [Step-by-Step Implementation](#step-by-step-implementation)
+- [Testing](#testing)
+- [Reference Implementations](#reference-implementations)
+- [Summary](#summary)
+
+---
+
+## Overview
+
+### What is Expert Parallel?
+
+**Expert Parallel** is a parallelism strategy in Mixture-of-Experts (MoE) models that distributes different expert networks across distinct computational devices. Each device holds and computes only a subset of experts (local experts), with tokens dispatched to and gathered from remote devices via collective communication operations (e.g., All-to-All, All-Gather).
+
+| Backend | Description |
+|---------|-------------|
+| `allgather_reducescatter` | Default backend based on allgather/reducescatter primitives, suitable for general EP+DP deployments.|
+
+## Configuration
+
+Enable EP by setting the `--enable-expert-parallel` flag. The EP size is automatically calculated as:
+
+```text
+EP_SIZE = TP_SIZE × SP_SIZE × CFG_SIZE × DP_SIZE
+```
+
+
+Where:
+
+- `TP_SIZE`: Tensor parallel size
+- `SP_SIZE`: Sequence parallel size
+- `CFG_SIZE`: Classifier-free guidance parallel size
+- `DP_SIZE`: Data parallel size
+- `EP_SIZE`: Expert parallel size (computed automatically)
+
+Note:
+- Expert parallelism is only applicable to Mixture-of-Experts (MoE) models.
+- The EP group is created **per pipeline stage**, meaning it includes all ranks that participate in model parallelism except pipeline parallelism.
+- The underlying communication pattern for expert parallelism is **All-to-All** among the ranks in the EP group.
+
+For example, consider a configuration with `TP=2`, `SP=1`, `CFG=2`, and `DP=4` (total 2×1×2×4 = 16 GPUs).
+
+- Expert layers are handled by an EP group of size 16.
+
+- Attention layers use tensor parallelism of size 2 within each of the 8 DP groups (because `DP×CFG×SP = 4×2×1 = 8` groups, each containing the 2 TP ranks). Inside each such group, the attention weights are sharded across the 2 GPUs.
+
+
+## Step-by-Step Implementation
+
+### Step 1: Configure Expert Parallelism Settings
+
+Calculate local experts per rank:
+
+```
+ep_size = 8  # Expert Parallel size (typically equals TP size)
+num_experts = 64
+num_local_experts = num_experts // ep_size  # 8 experts per card
+
+# Check divisibility
+assert num_experts % ep_size == 0, "Experts must be divisible by EP size"
+```
+
+### Step 2: Use Sparse MoE Block to enable EP routing.
+
+Example:
+```
+from vllm.model_executor.layers.linear import ReplicatedLinear
+class HunYuanSparseMoeBlock(nn.Module):
+    def __init__(
+        self,
+        config: PretrainedConfig,
+        layer_id: int = -1,
+        prefix: str = "",
+    ):
+        super().__init__()
+        self.tp_size = get_tensor_model_parallel_world_size()
+        self.n_routed_experts = config.num_experts  # 64
+
+        # Calculate local experts per rank (key for EP)
+        if self.tp_size > self.n_routed_experts:
+            raise ValueError(f"TP size {self.tp_size} > experts {self.n_routed_experts}")
+
+        # Routing gate (replicated on all ranks, computes scores for all tokens to all experts)
+        self.gate = ReplicatedLinear(
+            config.hidden_size,
+            config.num_experts,
+            bias=False,
+            quant_config=None,
+            prefix=f"{prefix}.gate",
+        )
+
+        # EP expert layer (factory loads platform-specific implementation)
+        self.experts = HunyuanFusedMoE(...)
+```
+**Key Points:**
+- gate is **ReplicatedLinear** (replicated on all ranks)
+- experts is created via **HunyuanFusedMoE factory**, which automatically handles EP dispatch
+
+### Step 3: Initialize EP Runtime
+
+Initialize the EP communication context before model loading.
+```
+from vllm.utils.import_utils import resolve_obj_by_qualname
+# Call during __init__ or model loading
+op_name = "hunyuan_fused_moe"
+
+# Prepare EP runtime: establish communication groups, assign local expert indices, init _expert_map
+current_omni_platform.prepare_diffusion_op_runtime(op_name)
+
+# Factory automatically resolves platform implementation (GPU: FusedMoE / NPU: AscendFusedMoE)
+impl = resolve_obj_by_qualname(
+    current_omni_platform.get_diffusion_model_impl_qualname(op_name)
+)
+```
+
+### Step 4: Expert Weight Mapping & Loading
+
+Each rank loads only the expert weights assigned to its local allocation.
+```
+# Get expert parameter mapping (different per rank)
+expert_mapping = HunyuanFusedMoE.make_expert_params_mapping(
+    model=self,
+    ckpt_gate_proj_name="gate_proj",
+    ckpt_down_proj_name="down_proj",
+    ckpt_up_proj_name="up_proj",
+    num_experts=64,
+    num_redundant_experts=0,
+)
+# Returns: [(param_name, weight_name, expert_id, shard_id), ...]
+# Note: Each rank only contains mappings for its local expert_ids
+
+# Filter non-local experts during loading
+for name, loaded_weight in weights:
+    if "mlp.experts" in name:
+        # Parse expert_id from weight name (implementation needed)
+        expert_id = parse_expert_id_from_name(name)
+        local_expert_start = (ep_rank) * num_local_experts
+        local_expert_end = (ep_rank + 1) * num_local_experts
+
+        if not (local_expert_start <= expert_id < local_expert_end):
+            continue  # Skip non-local expert weights
+```
+### Step 5: Forward Pass with EP
+
+Example (MoE Forward):
+```
+def forward(self, hidden_states: torch.Tensor) -> torch.Tensor:
+    orig_shape = hidden_states.shape
+    hidden_states = hidden_states.view(-1, hidden_states.shape[-1])
+
+    # 1. Global routing computation (all tokens, all expert scores)
+    # hidden_states: [num_tokens, hidden_dim] (full tensor)
+    router_logits, _ = self.gate(hidden_states)  # [num_tokens, num_experts]
+
+    # 2. EP dispatch and compute (HunyuanFusedMoE handles all_to_all internally)
+    # - Dispatch: Send tokens to target ranks based on router_logits
+    # - Local Compute: Each rank processes only its num_local_experts
+    # - Combine: Results returned to original token positions
+    final_hidden_states = self.experts(
+        hidden_states=hidden_states,
+        router_logits=router_logits,
+    )
+
+    # 3. Add shared expert output (not EP, computed on all ranks)
+    if self.shared_mlp is not None:
+        shared_out = self.shared_mlp(hidden_states)
+        final_hidden_states = final_hidden_states + shared_out
+
+    # 4. Tensor Parallel All-Reduce (synchronize across TP group)
+    if self.tp_size > 1:
+        final_hidden_states = self.experts.maybe_all_reduce_tensor_model_parallel(
+            final_hidden_states
+        )
+
+    return final_hidden_states.view(orig_shape)
+```
+
+## Testing
+After adding Expert Parallel support, test via command line:
+```bash
+cd examples/offline_inference/text_to_image
+python text_to_image.py \
+    --model Your-org/your-model \
+    --prompt "a cup of coffee on the table" \
+    --output "ep_enabled.png" \
+    --num-inference-steps 50 \
+    --guidance-scale 5.0 \
+    --tensor-parallel-size 8 \
+    --seed 1234 \
+    --enable-expert-parallel
+```
+
+vLLM‑Omni currently focuses on core diffusion model inference acceleration, so the Expert Parallel implementation includes only the basic multi‑GPU expert sharding functionality (enabled via --enable-expert-parallel). Advanced features such as communication backend selection (--all2all-backend), load balancing (--enable-eplb and its configuration), and multi‑node deployment belong to the extended capabilities of the main vLLM project and have not yet been integrated into Omni.
+
+## Reference Implementations
+
+Complete examples in the codebase:
+
+| Model | Path | Pattern | Notes |
+|-------|------|---------|-------|
+| **HunyuanImage3.0** | `vllm_omni/diffusion/models/hunyuan_image_3/hunyuan_image_3_transformer.py` | Standard EP | Full implementation with validation |
+| **EP Tests** | `vllm-omni/tests/e2e/offline_inference/test_expert_parallel.py` | E2E testing | EP correctness and performance |
+| **Constraint Tests** | `vllm-omni/tests/diffusion/models/hunyuan_image_3/test_hunyuan_fused_moe.py` | Unit testing | Validation logic |
+
+---
+## Summary
+
+Adding Expert Parallel support to diffusion model:
+
+1. **Identify MoE layers** - Locate the router and expert networks in each transformer block.
+2. **Validate EP constraints** – Ensure num_experts is divisible by expert_parallel_size.
+3. **Test** - Run with enable-expert-parallel, check memory reduction, speedup, and output quality against single‑GPU baseline.