[Perf] Improve/Fix-regression for FA3 in High QPS regimes #19463

LucasWilkinson · 2025-06-11T02:19:12Z

Essential Elements of an Effective PR Description Checklist

The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
The test plan, such as providing test command.
The test results, such as pasting the results comparison before and after, or e2e results
(Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

This PR is meant to address spending excessive time in the combine phase of FA3; findings from #18619 . The associated vllm-flash-attn PR is: vllm-project/flash-attention#70 see that for more details (that PR must also land first).

Perf Results

Setup

vllm serve <model> --disable-log-requests --max-num-seqs 1024  --block-size 16 --max-num-batched-tokens 2048 --no-enable-prefix-caching

Qwen/Qwen2.5-VL-3B-Instruct

vllm bench serve --model Qwen/Qwen2.5-VL-3B-Instruct --request-rate 1,5,50,100,200 --request-for-secs 5 --random-input-len 600 --random-output-len 125 --ignore-eos

Main

========================== Request Rate Sweep Summary ==========================
Request Rate | Req/s | Output Tok/s | Median TTFT (ms) | Median TPOT (ms)
-------------------------------------------------------------------------
1.0          | 2.00  | 249.71       | 32.69            | 4.66            
5.0          | 4.31  | 538.60       | 33.26            | 4.75            
50.0         | 39.92 | 4990.28      | 56.11            | 9.84            
100.0        | 63.08 | 7885.19      | 175.15           | 30.11           
200.0        | 64.94 | 8117.78      | 3329.13          | 41.89           
================================================================================

PR

========================== Request Rate Sweep Summary ==========================
Request Rate | Req/s | Output Tok/s | Median TTFT (ms) | Median TPOT (ms)
-------------------------------------------------------------------------
1.0          | 2.05  | 256.37       | 30.81            | 4.34            
5.0          | 4.31  | 538.65       | 33.29            | 4.74            
50.0         | 40.65 | 5081.02      | 46.31            | 7.87            
100.0        | 71.97 | 8996.07      | 83.14            | 18.68           
200.0        | 83.01 | 10376.26     | 1940.63          | 31.35           
================================================================================

meta-llama/Meta-Llama-3.1-8b

vllm bench serve --model meta-llama/Meta-Llama-3.1-8b --request-rate 1,5,50,100,200 --request-for-secs 5 --random-input-len 600 --random-output-len 125 --ignore-eos

Main

========================== Request Rate Sweep Summary ==========================
Request Rate | Req/s | Output Tok/s | Median TTFT (ms) | Median TPOT (ms)
-------------------------------------------------------------------------
1.0          | 0.79  | 98.28        | 31.39            | 8.28            
5.0          | 4.05  | 506.15       | 36.65            | 8.26            
50.0         | 34.03 | 4253.57      | 139.83           | 28.83           
100.0        | 40.71 | 5088.44      | 1704.81          | 53.46           
200.0        | 43.02 | 5377.35      | 7010.04          | 61.87           
================================================================================

PR

========================== Request Rate Sweep Summary ==========================
Request Rate | Req/s | Output Tok/s | Median TTFT (ms) | Median TPOT (ms)
-------------------------------------------------------------------------
1.0          | 0.79  | 98.97        | 38.57            | 7.84            
5.0          | 4.13  | 516.30       | 34.77            | 7.84            
50.0         | 36.32 | 4540.17      | 97.19            | 23.07           
100.0        | 43.69 | 5460.74      | 1572.63          | 49.53           
200.0        | 44.29 | 5536.30      | 6730.15          | 60.71           
================================================================================

Test Plan

Tested benchmarks in vllm-project/flash-attention#70
lm-eval for Qwen/Qwen2.5-VL-3B-Instruct
CI

Test Result

lm_eval

This PR

(vllm) lwilkinson@beaker:~/code/vllm$ lm_eval --model vllm --model_args pretrained=Qwen/Qwen2.5-VL-3B-Instruct --tasks gsm8k --batch_size auto
...
|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|-----|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.6831|±  |0.0128|
|     |       |strict-match    |     5|exact_match|↑  |0.5383|±  |0.0137|

Main

(vllm) lwilkinson@beaker:~/code/vllm$ lm_eval --model vllm --model_args pretrained=Qwen/Qwen2.5-VL-3B-Instruct --tasks gsm8k --batch_size auto
...
|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|-----|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.6831|±  |0.0128|
|     |       |strict-match    |     5|exact_match|↑  |0.5383|±  |0.0137|

(Optional) Documentation Update

gemini-code-assist · 2025-06-11T02:19:16Z

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github-actions · 2025-06-11T02:22:17Z

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mergify · 2025-06-17T03:35:54Z

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

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

Signed-off-by: Lucas Wilkinson <lwilkinson@neuralmagic.com>