[Misc][Doc] Add note regarding loading generation_config by default (#15281)

Signed-off-by: Roger Wang <ywang@roblox.com>

docs/source/getting_started/quickstart.md

@@ -58,6 +58,11 @@ from vllm import LLM, SamplingParams
 ```
 
 The next section defines a list of input prompts and sampling parameters for text generation. The [sampling temperature](https://arxiv.org/html/2402.05201v1) is set to `0.8` and the [nucleus sampling probability](https://en.wikipedia.org/wiki/Top-p_sampling) is set to `0.95`. You can find more information about the sampling parameters [here](#sampling-params).
+:::{important}
+By default, vLLM will use sampling parameters recommended by model creator by applying the `generation_config.json` from the Hugging Face model repository if it exists. In most cases, this will provide you with the best results by default if {class}`~vllm.SamplingParams` is not specified.
+
+However, if vLLM's default sampling parameters are preferred, please set `generation_config="vllm"` when creating the {class}`~vllm.LLM` instance.
+:::
 
 ```python
 prompts = [
@@ -76,7 +81,7 @@ llm = LLM(model="facebook/opt-125m")
 ```
 
 :::{note}
-By default, vLLM downloads models from [HuggingFace](https://huggingface.co/). If you would like to use models from [ModelScope](https://www.modelscope.cn), set the environment variable `VLLM_USE_MODELSCOPE` before initializing the engine.
+By default, vLLM downloads models from [Hugging Face](https://huggingface.co/). If you would like to use models from [ModelScope](https://www.modelscope.cn), set the environment variable `VLLM_USE_MODELSCOPE` before initializing the engine.
 :::
 
 Now, the fun part! The outputs are generated using `llm.generate`. It adds the input prompts to the vLLM engine's waiting queue and executes the vLLM engine to generate the outputs with high throughput. The outputs are returned as a list of `RequestOutput` objects, which include all of the output tokens.
@@ -107,6 +112,11 @@ vllm serve Qwen/Qwen2.5-1.5B-Instruct
 By default, the server uses a predefined chat template stored in the tokenizer.
 You can learn about overriding it [here](#chat-template).
 :::
+:::{important}
+By default, the server applies `generation_config.json` from the huggingface model repository if it exists. This means the default values of certain sampling parameters can be overridden by those recommended by the model creator.
+
+To disable this behavior, please pass `--generation-config vllm` when launching the server.
+:::
 
 This server can be queried in the same format as OpenAI API. For example, to list the models:
 

docs/source/models/generative_models.md

@@ -46,6 +46,11 @@ for output in outputs:
     print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")
 ```
 
+:::{important}
+By default, vLLM will use sampling parameters recommended by model creator by applying the `generation_config.json` from the huggingface model repository if it exists. In most cases, this will provide you with the best results by default if {class}`~vllm.SamplingParams` is not specified.
+
+However, if vLLM's default sampling parameters are preferred, please pass `generation_config="vllm"` when creating the {class}`~vllm.LLM` instance.
+:::
 A code example can be found here: <gh-file:examples/offline_inference/basic/basic.py>
 
 ### `LLM.beam_search`

docs/source/serving/openai_compatible_server.md

@@ -33,7 +33,11 @@ print(completion.choices[0].message)
 vLLM supports some parameters that are not supported by OpenAI, `top_k` for example.
 You can pass these parameters to vLLM using the OpenAI client in the `extra_body` parameter of your requests, i.e. `extra_body={"top_k": 50}` for `top_k`.
 :::
+:::{important}
+By default, the server applies `generation_config.json` from the Hugging Face model repository if it exists. This means the default values of certain sampling parameters can be overridden by those recommended by the model creator.
 
+To disable this behavior, please pass `--generation-config vllm` when launching the server.
+:::
 ## Supported APIs
 
 We currently support the following OpenAI APIs:

vllm/config.py

@@ -1023,6 +1023,13 @@ class ModelConfig:
                     "max_new_tokens")
         else:
             diff_sampling_param = {}
+
+        if diff_sampling_param:
+            logger.warning_once(
+                "Default sampling parameters have been overridden by the "
+                "model's Hugging Face generation config recommended from the "
+                "model creator. If this is not intended, please relaunch "
+                "vLLM instance with `--generation-config vllm`.")
         return diff_sampling_param
 
     @property