[BugFix][Core] Prefix caching: Postpone prefill scheduling for prompts with the same prefix

[zhengy001]

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prompts = [ "Hello, my name is", "Hello, my name is", ]

Identical slot_mapping for these two prompts during prefill and will update the same kv cache slots simultaneously.

Propose scheduling the 2nd prompt which has the same prefix with previous prompt in the budget. BlockSpaceManagerv2 has the same issue, I only change the v1 and would like to have some feedback before looking at v2. Ideally, there should be a dedupe logic to filter out other identical prompts or have the same prefix.

FIX: #7414

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[zhengy001]

BlockSpaceManagerV1 got just removed. :(

[comaniac]

Thanks for the PR. Unfortunately I don't think this is the strategy we want to have in vLLM core. Although we indeed have this issue, we attempt to solve it by improving prefix sharing performance of sequences the same batch in model runner instead of in the scheduler. There are a few reasons:

1. This approach increases TTFT of leftover sequences.
2. We attempt to move parallel sampling (e.g. best_of) out of core, meaning that it will be multiple duplicated sequences from the core's perspective. The also increases TTFT for all requests with parallel sampling.
3. It increases the complexity of scheduler which seems not necessary, because if users are aware of this case, they could arrange the requests to avoid in-batch prefix sharing.

[zhengy001]

Thanks for the PR. Unfortunately I don't think this is the strategy we want to have in vLLM core. Although we indeed have this issue, we attempt to solve it by improving prefix sharing performance of sequences the same batch in model runner instead of in the scheduler. There are a few reasons:

1. This approach increases TTFT of leftover sequences.
2. We attempt to move parallel sampling (e.g. best_of) out of core, meaning that it will be multiple duplicated sequences from the core's perspective. The also increases TTFT for all requests with parallel sampling.
3. It increases the complexity of scheduler which seems not necessary, because if users are aware of this case, they could arrange the requests to avoid in-batch prefix sharing.

Thank for explanation.

[zhengy001]

Thanks for the PR. Unfortunately I don't think this is the strategy we want to have in vLLM core. Although we indeed have this issue, we attempt to solve it by improving prefix sharing performance of sequences the same batch in model runner instead of in the scheduler. There are a few reasons:

1. This approach increases TTFT of leftover sequences.
2. We attempt to move parallel sampling (e.g. best_of) out of core, meaning that it will be multiple duplicated sequences from the core's perspective. The also increases TTFT for all requests with parallel sampling.
3. It increases the complexity of scheduler which seems not necessary, because if users are aware of this case, they could arrange the requests to avoid in-batch prefix sharing.

Thanks for explanation.