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Draft of updated doc for sampling params. (#3260)
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Co-authored-by: shuaills <[email protected]>
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# Sampling Parameters in SGLang Runtime

This doc describes the sampling parameters of the SGLang Runtime.
It is the low-level endpoint of the runtime.
If you want a high-level endpoint that can automatically handle chat templates, consider using the [OpenAI Compatible API](../backend/openai_api_completions.ipynb).

The `/generate` endpoint accepts the following arguments in the JSON format.

```python
@dataclass
class GenerateReqInput:
# The input prompt. It can be a single prompt or a batch of prompts.
text: Optional[Union[List[str], str]] = None
# The token ids for text; one can specify either text or input_ids
input_ids: Optional[Union[List[List[int]], List[int]]] = None
# The embeddings for input_ids; one can specify either text or input_ids or input_embeds.
input_embeds: Optional[Union[List[List[List[float]]], List[List[float]]]] = None
# The image input. It can be a file name, a url, or base64 encoded string.
# See also python/sglang/srt/utils.py:load_image.
image_data: Optional[Union[List[str], str]] = None
# The sampling_params. See descriptions below.
sampling_params: Optional[Union[List[Dict], Dict]] = None
# The request id.
rid: Optional[Union[List[str], str]] = None
# Whether to return logprobs.
return_logprob: Optional[Union[List[bool], bool]] = None
# If return logprobs, the start location in the prompt for returning logprobs.
# By default, this value is "-1", which means it will only return logprobs for output tokens.
logprob_start_len: Optional[Union[List[int], int]] = None
# If return logprobs, the number of top logprobs to return at each position.
top_logprobs_num: Optional[Union[List[int], int]] = None
# Whether to detokenize tokens in text in the returned logprobs.
return_text_in_logprobs: bool = False
# Whether to stream output.
stream: bool = False
# Whether to log metrics for this request (e.g. health_generate calls do not log metrics)
log_metrics: bool = True

# The modalities of the image data [image, multi-images, video]
modalities: Optional[List[str]] = None
# LoRA related
lora_path: Optional[Union[List[Optional[str]], Optional[str]]] = None

# Session info for continual prompting
session_params: Optional[Union[List[Dict], Dict]] = None
# Custom logit processor for advanced sampling control. Must be a serialized instance
# of `CustomLogitProcessor` in python/sglang/srt/sampling/custom_logit_processor.py
# Use the processor's `to_str()` method to generate the serialized string.
custom_logit_processor: Optional[Union[List[Optional[str]], str]] = None
```

The `sampling_params` follows this format

```python
# The maximum number of output tokens
max_new_tokens: int = 128,
# Stop when hitting any of the strings in this list
stop: Optional[Union[str, List[str]]] = None,
# Stop when hitting any of the token_ids in this list
stop_token_ids: Optional[List[int]] = [],
# Sampling temperature
temperature: float = 1.0,
# Top-p sampling
top_p: float = 1.0,
# Top-k sampling
top_k: int = -1,
# Min-p sampling
min_p: float = 0.0,
# Whether to ignore EOS token
ignore_eos: bool = False,
# Whether to skip the special tokens during detokenization
skip_special_tokens: bool = True,
# Whether to add spaces between special tokens during detokenization
spaces_between_special_tokens: bool = True,
# Do parallel sampling and return `n` outputs.
n: int = 1,

## Structured Outputs
# Only one of the below three can be set for a request.

# Constrain the output to follow a given JSON schema.
json_schema: Optional[str] = None,
# Constrain the output to follow a given regular expression.
regex: Optional[str] = None,
# Constrain the output to follow a given EBNF grammar.
ebnf: Optional[str] = None,

## Penalties.

# Float that penalizes new tokens based on their frequency in the generated text so far.
# Values > 0 encourage the model to use new tokens, while values < 0 encourage the model to
# repeat tokens. Must be -2 <= value <= 2. Setting to 0 (default) will disable this penalty.
frequency_penalty: float = 0.0,
# Float that penalizes new tokens based on whether they appear in the generated text so far.
# Values > 0 encourage the model to use new tokens, while values < 0 encourage the model to repeat
# tokens. Must be -2 <= value <= 2. Setting to 0 (default) will disable this penalty.
presence_penalty: float = 0.0,
# Float that penalizes new tokens based on whether they appear in the prompt and the generated text
# so far. Values > 1 encourage the model to use new tokens, while values < 1 encourage the model to
# repeat tokens. Must be 0 <= value <= 2. Setting to 1 (default) will disable this penalty.
repetition_penalty: float = 1.0,
# Guides inference to generate at least this number of tokens by penalizing logits of tokenizer's
# EOS token and `stop_token_ids` to -inf, until the output token reaches given length.
# Note that any of the `stop` string can be generated before reaching `min_new_tokens`, as it is
# difficult to infer the correct token ID by given `stop` strings.
# Must be 0 <= value < max_new_tokens. Setting to 0 (default) will disable this penalty.
min_new_tokens: int = 0,


## Custom Parameters for Custom Logit Processor.
# A dictionary of custom parameters for the custom logit processor.
# The custom logit processor takes a list of dictionaries as input, where each
# dictionary is the custom parameters for one token in a batch of the input.
# See also python/sglang/srt/sampling/custom_logit_processor.py
custom_params: Optional[Dict[str, Any]] = None,
```

## Examples

### Normal
Launch a server
```
python -m sglang.launch_server --model-path meta-llama/Meta-Llama-3-8B-Instruct --port 30000
```

Send a request
```python
import requests

response = requests.post(
"http://localhost:30000/generate",
json={
"text": "The capital of France is",
"sampling_params": {
"temperature": 0,
"max_new_tokens": 32,
},
},
)
print(response.json())
```

### Streaming
Send a request and stream the output
```python
import requests, json

response = requests.post(
"http://localhost:30000/generate",
json={
"text": "The capital of France is",
"sampling_params": {
"temperature": 0,
"max_new_tokens": 32,
},
"stream": True,
},
stream=True,
)
If you want a high-level endpoint that can automatically handle chat templates, consider using the [OpenAI Compatible API](https://docs.sglang.ai/backend/openai_api_completions.html).

prev = 0
for chunk in response.iter_lines(decode_unicode=False):
chunk = chunk.decode("utf-8")
if chunk and chunk.startswith("data:"):
if chunk == "data: [DONE]":
break
data = json.loads(chunk[5:].strip("\n"))
output = data["text"].strip()
print(output[prev:], end="", flush=True)
prev = len(output)
print("")
```
## `/generate` Endpoint

### Multi modal
The `/generate` endpoint accepts the following parameters in JSON format. For in detail usage see the [native api doc](https://docs.sglang.ai/backend/native_api.html).

Launch a server
```
python3 -m sglang.launch_server --model-path lmms-lab/llava-onevision-qwen2-7b-ov --chat-template chatml-llava
```
* `prompt`: The input prompt. Can be a single prompt or a batch of prompts.
* `input_ids`: Alternative to `text`. Specify the input as token IDs instead of text.
* `sampling_params`: The sampling parameters as described in the sections below.
* `return_logprob`: Whether to return log probabilities for tokens.
* `logprob_start_len`: If returning log probabilities, specifies the start position in the prompt. Default is "-1" which returns logprobs only for output tokens.
* `top_logprobs_num`: If returning log probabilities, specifies the number of top logprobs to return at each position.
* `stream`: Whether to stream the output.
* `lora_path`: Path to LoRA weights.
* `custom_logit_processor`: Custom logit processor for advanced sampling control. For usage see below.

Download an image
```
curl -o example_image.png -L https://github.com/sgl-project/sglang/blob/main/test/lang/example_image.png?raw=true
```
## Sampling params

Send a request
```python
import requests
### Core Parameters

response = requests.post(
"http://localhost:30000/generate",
json={
"text": "<|im_start|>system\nYou are a helpful assistant.<|im_end|>\n"
"<|im_start|>user\n<image>\nDescribe this image in a very short sentence.<|im_end|>\n"
"<|im_start|>assistant\n",
"image_data": "example_image.png",
"sampling_params": {
"temperature": 0,
"max_new_tokens": 32,
},
},
)
print(response.json())
```
* `max_new_tokens`: The maximum output length measured in tokens.
* `stop`: One or multiple [stop words](https://developer.nvidia.com/blog/how-to-get-better-outputs-from-your-large-language-model/#let_the_model_know_when_to_stop). Generation will stop if one of these words is sampled.
* `stop_token_ids`: Provide stop words in form of token ids. Generation will stop if one of these token ids is sampled.
* `temperature`: [Temperature](https://developer.nvidia.com/blog/how-to-get-better-outputs-from-your-large-language-model/#predictability_vs_creativity) when sampling the next token. `temperature = 0` corresponds to greedy sampling, higher temperature leads to more diversity.
* `top_p`: [Top-p](https://developer.nvidia.com/blog/how-to-get-better-outputs-from-your-large-language-model/#predictability_vs_creativity) selects tokens from the smallest sorted set whose cumulative probability exceeds `top_p`. When `top_p = 1`, this reduces to unrestricted sampling from all tokens.
* `top_k`: [Top-k](https://developer.nvidia.com/blog/how-to-get-better-outputs-from-your-large-language-model/#predictability_vs_creativity) randomly selects from the `k` highest-probability tokens.
* `min_p`: [Min-p](https://github.com/huggingface/transformers/issues/27670) samples from tokens with probability larger than `min_p * highest_token_probability`.

The `image_data` can be a file name, a URL, or a base64 encoded string. See also `python/sglang/srt/utils.py:load_image`.
Streaming is supported in a similar manner as [above](#streaming).
### Penalizers

### Structured Outputs (JSON, Regex, EBNF)
You can specify a JSON schema, regular expression or [EBNF](https://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_form) to constrain the model output. The model output will be guaranteed to follow the given constraints. Only one constraint parameter (`json_schema`, `regex`, or `ebnf`) can be specified for a request.
To use penalizers you will need to `--disable-overlap`. Please note that this might degrade performance.

SGLang supports two grammar backends:
* `frequency_penalty`: Penalizes tokens based on their frequency in generation so far. Must be between `-2` and `2` where negative numbers encourage repeatment of tokens and positive number encourages sampling of new tokens. The scaling of penalization grows linearly with each appearance of a token.
* `presence_penalty`: Penalizes tokens if they appeared in the generation so far. Must be between `-2` and `2` where negative numbers encourage repeatment of tokens and positive number encourages sampling of new tokens. The scaling of the penalization is constant if a token occured.
* `repetition_penalty`: Penalizes tokens if they appeared in prompt or generation so far. Must be between `0` and `2` where numbers smaller than `1` encourage repeatment of tokens and numbers larger than `2` encourages sampling of new tokens. The penalization scales multiplicatively.
* `min_new_tokens`: Forces the model to generate at least `min_new_tokens` until a stop word or EOS token is sampled. Note that this might lead to unintended behavior for example if the distribution is highly skewed towards these tokens.

- [Outlines](https://github.com/dottxt-ai/outlines) (default): Supports JSON schema and regular expression constraints.
- [XGrammar](https://github.com/mlc-ai/xgrammar): Supports JSON schema, regular expression, and EBNF constraints.
- XGrammar currently uses the [GGML BNF format](https://github.com/ggerganov/llama.cpp/blob/master/grammars/README.md)
### Constrained decoding

Initialize the XGrammar backend using `--grammar-backend xgrammar` flag
```bash
python -m sglang.launch_server --model-path meta-llama/Meta-Llama-3.1-8B-Instruct \
--port 30000 --host 0.0.0.0 --grammar-backend [xgrammar|outlines] # xgrammar or outlines (default: outlines)
```
Please refer to our dedicated guide on [constrained decoding](https://docs.sglang.ai/backend/structured_outputs.html#Native-API-and-SGLang-Runtime-(SRT)) for the following parameters.

```python
import json
import requests
* `json_schema`
* `regex`
* `ebnf`

json_schema = json.dumps({
"type": "object",
"properties": {
"name": {"type": "string", "pattern": "^[\\w]+$"},
"population": {"type": "integer"},
},
"required": ["name", "population"],
})
### Other options

# JSON (works with both Outlines and XGrammar)
response = requests.post(
"http://localhost:30000/generate",
json={
"text": "Here is the information of the capital of France in the JSON format.\n",
"sampling_params": {
"temperature": 0,
"max_new_tokens": 64,
"json_schema": json_schema,
},
},
)
print(response.json())
* `n`: ?
* `spaces_between_special_tokens`: Whether or not to add spaces between special tokens during detokenization.
* `no_stop_trim`: Don't trim stop words or EOS token from the generated text.
* `ignore_eos`: Don't stop generation when EOS token is sampled.
* `skip_special_tokens`: Remove special tokens during decoding.
* `custom_params`: Used when employing `CustomLogitProcessor`. For usage see below.

# Regular expression (Outlines backend only)
response = requests.post(
"http://localhost:30000/generate",
json={
"text": "Paris is the capital of",
"sampling_params": {
"temperature": 0,
"max_new_tokens": 64,
"regex": "(France|England)",
},
},
)
print(response.json())

# EBNF (XGrammar backend only)
response = requests.post(
"http://localhost:30000/generate",
json={
"text": "Write a greeting.",
"sampling_params": {
"temperature": 0,
"max_new_tokens": 64,
"ebnf": 'root ::= "Hello" | "Hi" | "Hey"',
},
},
)
print(response.json())
```
### Custom Logit Processor
Launch a server with `--enable-custom-logit-processor` flag on.
```
python -m sglang.launch_server --model-path meta-llama/Meta-Llama-3-8B-Instruct --port 30000 --enable-custom-logit-processor
```

Define a custom logit processor that will always sample a specific token id.

```python
from sglang.srt.sampling.custom_logit_processor import CustomLogitProcessor

Expand All @@ -304,6 +87,7 @@ class DeterministicLogitProcessor(CustomLogitProcessor):
```

Send a request

```python
import requests

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