[Core] Faster logit_bias_logits_processor

[xu-song]

This PR changes python ops to tensor ops, which reduce time cost from 106ms to 0.01ms.

Before

    for token_id, bias in logit_bias.items():
        logits[token_id] += bias

The above approach is time consuming especially when len(logit_bias) is very large.

After

    logits.index_add_(0, logit_bias["index"], logit_bias["value"]) 

Time Cost

before -> v1 -> v2

len(logit_bias)	time cost (ms)
1	4.5 -> 0.3 -> 0.01
20	4.5 -> 0.3 -> 0.01
100	5.3 -> 0.3 -> 0.01
1000	14.4 -> 0.3 -> 0.01
10000	106 -> 0.4 -> 0.01

experiment settings:

GPU: A100 
model: Llama-3.2-1B-Instruct

impl history

• v1: no cache
  

  vllm/vllm/entrypoints/openai/logits_processors.py

  Lines 50 to 51 in a50b0ea

  	logits.index_add_(0, logit_bias["index"].to(logits.device),
  	logit_bias["value"].to(logits.device))

  
  106ms -> 0.4ms
• v2: cache in single request:

  vllm/vllm/entrypoints/openai/logits_processors.py

  Lines 50 to 53 in 62a74e3

  	if logit_bias["value"].device != logits.device:
  	logit_bias["index"] = logit_bias["index"].to(logits.device)
  	logit_bias["value"] = logit_bias["value"].to(logits.device)
  	logits.index_add_(0, logit_bias["index"], logit_bias["value"])

  
  106ms -> 0.01ms
• v3(current): cache across different requests
  

  vllm/vllm/entrypoints/openai/logits_processors.py

  Lines 70 to 74 in cd9f33f

  	@lru_cache(maxsize=32)
  	def _get_logit_bias_logits_processor(
  	logit_bias_index: Union[Tuple[int], Tuple[str]],
  	logit_bias_value: Tuple[float],
  	vocab_size: int,

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[imkero]

If len(logit_bias) is large, maybe we can keep the copy of logit_bias["index"] and logit_bias["value"] in the device memory ahead of time (or in the first sample step), and re-use it in the following sample steps, to avoid duplicated tensor copy?

[xu-song]

If len(logit_bias) is large, maybe we can keep the copy of logit_bias["index"] and logit_bias["value"] in the device memory ahead of time (or in the first sample step), and re-use it in the following sample steps, to avoid duplicated tensor copy?

@imkero Thanks for your suggestion, a new commit has been added, which avoid duplicated tensor copy.

After this change, the time_cost is reduced to 0.01ms

len(logit_bias)	time cost (ms)
1	4.5 -> 0.01
20	4.5 -> 0.01
100	5.3 -> 0.01
1000	14.4 -> 0.01
10000	106 -> 0.01