[Perf] Optimize hidden state extraction logic (#37374)

Signed-off-by: Benjamin Chislett <bchislett@nvidia.com>
Signed-off-by: Benjamin Chislett <chislett.ben@gmail.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

docs/features/speculative_decoding/README.md

@@ -15,6 +15,7 @@ vLLM supports a variety of methods of speculative decoding. Model-based methods
 - [Multi-Layer Perceptron](mlp.md)
 - [N-Gram](n_gram.md)
 - [Suffix Decoding](suffix.md)
+- [Hidden State Extraction](extract_hidden_states.md)
 - [Custom Proposer Backend (Experimental)](#custom-proposer-backend-experimental)
 
 ## Method Selection at a Glance

docs/features/speculative_decoding/extract_hidden_states.md

@@ -0,0 +1,86 @@
+# Hidden State Extraction
+
+The Hidden State Extraction feature allows vLLM to save intermediate layer activations from a target model during inference. This is useful for training [EAGLE](eagle.md)-style draft models, knowledge distillation, or offline analysis of model internals.
+
+!!! note
+    It is possible to save the last-layer's output hidden states by passing `num_hidden_layers` as a layer id. Note that these are _not_ normalized using the output norm.
+
+## Offline Example
+
+```python
+import tempfile
+
+from vllm import LLM, SamplingParams
+from vllm.config.kv_transfer import KVTransferConfig
+from vllm.distributed.kv_transfer.kv_connector.v1 import (
+    example_hidden_states_connector,
+)
+
+with tempfile.TemporaryDirectory() as tmpdir:
+    llm = LLM(
+        model="Qwen/Qwen3-8B",
+        enable_chunked_prefill=False,
+        speculative_config={
+            "method": "extract_hidden_states",
+            "num_speculative_tokens": 1,
+            "draft_model_config": {
+                "hf_config": {
+                    "eagle_aux_hidden_state_layer_ids": [1, 2, 3, 4],
+                },
+            },
+        },
+        kv_transfer_config=KVTransferConfig(
+            kv_connector="ExampleHiddenStatesConnector",
+            kv_role="kv_producer",
+            kv_connector_extra_config={
+                "shared_storage_path": tmpdir,
+            },
+        ),
+    )
+
+    outputs = llm.generate(
+        ["The future of AI is"],
+        SamplingParams(max_tokens=1),
+    )
+
+    for output in outputs:
+        path = output.kv_transfer_params["hidden_states_path"]
+        obj = example_hidden_states_connector.load_hidden_states(path)
+        print(f"token_ids: {obj['token_ids'].shape}")
+        print(f"hidden_states: {obj['hidden_states'].shape}")
+```
+
+A complete example is available at [`examples/features/speculative_decoding/extract_hidden_states_offline.py`](../../../examples/features/speculative_decoding/extract_hidden_states_offline.py).
+
+## Online Example
+
+For improved performance, it is recommended to use a RAM-mounted file system such as `/dev/shm/` for online usage in which the client cleans up the files soon after they are generated.
+
+```bash
+vllm serve Qwen/Qwen3-8B \
+    --speculative_config '{"method": "extract_hidden_states", "num_speculative_tokens": 1, "draft_model_config": {"hf_config": {"eagle_aux_hidden_state_layer_ids": [1, 2, 3, 4]}}}' \
+    --kv_transfer_config '{"kv_connector": "ExampleHiddenStatesConnector", "kv_role": "kv_producer", "kv_connector_extra_config": {"shared_storage_path": "/dev/shm/hidden_states"}}' \
+    --no-enable-chunked-prefill
+```
+
+## Configuration
+
+The `kv_connector_extra_config` dict accepts these options:
+
+| Parameter | Default | Description |
+| --- | --- | --- |
+| `shared_storage_path` | `/tmp` | Directory where hidden state files are saved |
+| `num_writer_threads` | `8` | Thread pool size for async disk writes |
+| `use_synchronization_lock` | `True` | Use file locks so concurrent readers block until writes complete. Can be disabled for batch generation where synchronization is not needed. |
+
+## Output Format
+
+Each request produces a `.safetensors` file containing:
+
+- **`hidden_states`** — shape `[num_tokens, num_extracted_layers, hidden_size]`
+- **`token_ids`** — shape `[num_tokens]`
+
+The file path is returned in `output.kv_transfer_params["hidden_states_path"]`. Use `load_hidden_states()` from the connector module to read the file with proper synchronization.
+
+!!! note
+    Chunked prefill is not compatible with this feature and must be disabled.