[Core][Speculative Decoding] Add multi-query verifier for speculative decoding without batch expansion

[sighingnow]

FIX P1 part of #4630 (link existing issues this PR will resolve)

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[mpjlu]

Have you tested this PR:https://github.com/vllm-project/vllm/pull/5691, which performance is better?

[sighingnow]

Have you tested this PR:#5691, which performance is better?

These two PRs shouldn't have too many differences in implementation, so the performance should be roughly the same.

One comment for #5691: it must be done carefully for prompt tokens processing in sampling_metadata.py, as the prompt tokens of sequences would affect the applying penalities logic in sampler.

[mpjlu]

@sighingnow Thanks for your kindly reply. Does this PR support cuda graph now.

[sighingnow]

@sighingnow Thanks for your kindly reply. Does this PR support cuda graph now.

~~Cuda graph has been supported and tested.~~

Hi @mpjlu I have just tried some correctness test and found that this PR cannot work as expected with cuda graph.

Apologize for the misinformation.

[cadedaniel]

I will take a pass this week. also cc @LiuXiaoxuanPKU

one major challenge is cuda graph support. it will be necessary since the pytorch dispatch and scheduling overhead is large for small batch sizes.

btw if you have performance numbers to prove ^ wrong, please share

[sighingnow]

@cadedaniel @mpjlu @LiuXiaoxuanPKU The cuda graph of both draft model and target model (with multi-query verifier) has been added.

[cadedaniel]

Hi @sighingnow , thanks for adding this. Feedback:

• The MultiQueryTop1Scorer is really good
• For the cuda graph support, can we instead integrate with @LiuXiaoxuanPKU 's work in https://github.com/vllm-project/vllm/pull/6052 ? More details here. The reason is that we can use the same backend for chunked prefill / prefix caching / other MQA applications. With this PR, the speculation logic must spill into the model runner, which couples the two together.

One way forward is to merge the MultiQueryTop1Scorer without cuda graph support, then once https://github.com/vllm-project/vllm/pull/6052 is done we can integrate the two.

What do you think?

[sighingnow]

Hi @cadedaniel,

If the PR #6052 will be merged soon, I prefer waiting it been merged then rebasing this PR, to avoid conflicts and merging effort.

[cadedaniel]

I am not sure how soon it will be merged. cc @LiuXiaoxuanPKU

[LiuXiaoxuanPKU]

Hi @sighingnow, sorry for the very late reply.

After digging into #6052, we find there is correctness issue with cudagraph + flash_attn_varlen_func. We will look into the support. However, at the same time, we can move forward with this PR. Could you clean a bit and let us know when it's good to review. Thanks for the contribution and sorry for the delayed reply!

[cadedaniel]

cc @comaniac any thoughts on how this cuda graph approach works with model runner / prepare inputs ?

[comaniac]

Took a brief look. The approach is ok in general, but I could see that some more overheads are introduced due to more tensors/logic being processed. I could review the PR once it's rebased and ready to eliminate the overheads as possible.

[cadedaniel]

One alternative is to move this to a custom model runner, just for spec decode. Do you think that's better or worse than the current approach?

[comaniac]

One alternative is to move this to a custom model runner, just for spec decode. Do you think that's better or worse than the current approach?

In general I imagine that would have lots of code duplications, and this logic (without CUDA graph) should be very similar as prefix caching. Another way is maybe we quickly support a version without CUDA graph first and see how good/bad is?

[cadedaniel]

SGTM. @sighingnow we can merge this PR as is if we remove the cuda graph stuff, or we can add cuda graph stuff (using one of the two approaches @comaniac mentions above)

[sighingnow]

I will rebase and fixes the cudagraph issue, add testsing cases for this PR during this weekend, thanks all for your careful review.

[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had any activity within 90 days. It will be automatically closed if no further activity occurs within 30 days. Leave a comment if you feel this pull request should remain open. Thank you!

[mergify[bot]]

This pull request has merge conflicts that must be resolved before it can be merged. Please rebase the PR, @sighingnow.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork