[Docs] Clarify Expert Parallel behavior for attention and MoE layers (vllm-project#30615)

Signed-off-by: majiayu000 <[email protected]>

Author: majiayu000

docs/serving/data_parallel_deployment.md

@@ -8,11 +8,11 @@ For MoE models, particularly those like DeepSeek that employ MLA (Multi-head Lat
 
 In these cases, the data parallel ranks are not completely independent. Forward passes must be aligned, and expert layers across all ranks are required to synchronize during every forward pass, even when there are fewer requests to be processed than DP ranks.
 
-The expert layers will by default form a (DP x TP) sized tensor parallel group. To enable expert parallelism, include the `--enable-expert-parallel` CLI arg (on all nodes in the multi-node case).
+By default, expert layers form a tensor parallel group of size `DP × TP`. To use expert parallelism instead, include the `--enable-expert-parallel` CLI arg (on all nodes in the multi-node case). See [Expert Parallel Deployment](expert_parallel_deployment.md) for details on how attention and expert layers behave differently with EP enabled.
 
 In vLLM, each DP rank is deployed as a separate "core engine" process that communicates with front-end process(es) via ZMQ sockets. Data Parallel attention can be combined with Tensor Parallel attention, in which case each DP engine owns a number of per-GPU worker processes equal to the configured TP size.
 
-For MoE models, when any requests are in progress in any rank, we must ensure that empty "dummy" forward passes are performed in all ranks that don't currently have any requests scheduled. This is handled via a separate DP Coordinator process that communicates with all ranks, and a collective operation performed every N steps to determine when all ranks become idle and can be paused. When TP is used in conjunction with DP, expert layers form an EP or TP group of size (DP x TP).
+For MoE models, when any requests are in progress in any rank, we must ensure that empty "dummy" forward passes are performed in all ranks that don't currently have any requests scheduled. This is handled via a separate DP Coordinator process that communicates with all ranks, and a collective operation performed every N steps to determine when all ranks become idle and can be paused. When TP is used in conjunction with DP, expert layers form a group of size `DP × TP` (using either tensor parallelism by default, or expert parallelism if `--enable-expert-parallel` is set).
 
 In all cases, it is beneficial to load-balance requests between DP ranks. For online deployments, this balancing can be optimized by taking into account the state of each DP engine - in particular its currently scheduled and waiting (queued) requests, and KV cache state. Each DP engine has an independent KV cache, and the benefit of prefix caching can be maximized by directing prompts intelligently.
 

docs/serving/expert_parallel_deployment.md

@@ -44,7 +44,27 @@ Where:
 - `DP_SIZE`: Data parallel size
 - `EP_SIZE`: Expert parallel size (computed automatically)
 
-When EP is enabled, MoE layers use expert parallelism instead of tensor parallelism, while attention layers continue to use tensor parallelism if `TP_SIZE > 1`.
+### Layer Behavior with EP Enabled
+
+When EP is enabled, different layers in MoE models behave differently:
+
+| Layer Type | Behavior | Parallelism Used |
+|------------|----------|------------------|
+| **Expert (MoE) Layers** | Sharded across all EP ranks | Expert Parallel (EP) of size `TP × DP` |
+| **Attention Layers** | Behavior depends on TP size | See below |
+
+**Attention layer parallelism:**
+
+- **When `TP = 1`**: Attention weights are **replicated** across all DP ranks (data parallelism)
+- **When `TP > 1`**: Attention weights are **sharded** using tensor parallelism across TP ranks within each DP group
+
+For example, with `TP=2, DP=4` (8 GPUs total):
+
+- Expert layers form an EP group of size 8, with experts distributed across all GPUs
+- Attention layers use TP=2 within each of the 4 DP groups
+
+!!! note "Key Difference from Data Parallel Deployment"
+    Without `--enable-expert-parallel`, MoE layers would use tensor parallelism (forming a TP group of size `TP × DP`), similar to dense models. With EP enabled, expert layers switch to expert parallelism, which can provide better efficiency and locality for MoE models.
 
 ### Example Command