[Perf][DSV4] Optimize sparse top-k metadata kernels for higher prefill throughput

[chaunceyjiang]

Purpose

Optimize sparse top-k metadata kernels for higher prefill throughput

Test

vllm bench serve \
    --backend vllm \
    --base-url http://localhost:8000 \
    --model deepseek-ai/DeepSeek-V4-Flash-0731 \
    --dataset-name random \
    --random-input-len 1024 \
    --random-output-len 64 \
    --num-prompts 128 \
    --num-warmups 8 \
    --request-rate inf \
    --ignore-eos \
    --temperature 0 \
    --seed "$seed" \
    --save-result 

vllm serve deepseek-ai/DeepSeek-V4-Flash-0731 \
  --trust-remote-code \
  --kv-cache-dtype fp8 \
  --block-size 256 \
  --enable-expert-parallel \
  --tensor-parallel-size 8 \
  --tokenizer-mode deepseek_v4 \
  --tool-call-parser deepseek_v4 \
  --enable-auto-tool-choice \
  --reasoning-parser deepseek_v4 \
  --no-enable-prefix-caching \
  --max-num-batched-tokens 16384

Test Plan

Tokens	128 Workers	256 Workers	Relative Change
1,024	9.75 us	8.27 us	15.2% faster
4,096	22.40 us	15.52 us	30.7% faster
16,384	74.76 us	47.14 us	36.9% faster

Paired A/B/A Serving Results

Run	Workers	Mean Output Throughput	Mean TPOT
A1	256	638.67 tokens/s	102.26 ms
B	128	629.61 tokens/s	104.80 ms
A2	256	639.44 tokens/s	101.88 ms

The second optimized run improved output throughput by 1.56% and reduced
mean TPOT by 2.79% relative to the paired 128-worker baseline.

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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.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[zyongye]

/ci run

[github-actions]

✅ Triggered Buildkite CI #84067 for commit 45ea33e48648.