[Perf] Optimize Qwen3-VL multi-video prompt processing

[Sirius29]

Replace text-level prompt expansion (decode + string replace + re-tokenize) with token-level replacement in _call_hf_processor.

Co-authored-by: DeepSeek

Purpose

Optimize _call_hf_processor in Qwen3-VL for multi-video inputs.

Problem: Each video's expanded token IDs were decoded to text and
inserted into the prompt via prompt.replace(), causing the prompt to
grow with every video. The final super()._call_hf_processor() call
then re-tokenized this massively inflated prompt string — work that
scales poorly and is entirely redundant since the token IDs were
already computed.

Bug fix (EVS): The original code computed the EVS-adjusted
(video_pruning_rate) token sequence via get_video_repl, but then
immediately overwrote it with the raw, unpruned HF processor
input_ids (batch_decode(input_ids)). The EVS result was dead code.
This silently discarded the pruning and crashed whenever EVS was
enabled: _validate_mm_placeholders expects the per-frame placeholder
structure that get_video_repl produces with
select_token_id=False, but the overwritten flat HF output has no
timestamps and no per-frame structure — so zero placeholders are found
and a RuntimeError is raised.

Solution: Replace the text-level expansion with token-level
replacement. Per-video input_ids are collected directly and
substituted into the final output via a lightweight token scan, keeping
the HF processor call operating on the small original prompt.

Key design decisions:

• Uses get_video_repl (not raw HF input_ids) to generate per-video
  token sequences, ensuring EVS pruning is correctly applied and the
  resulting tokens match what _find_mm_placeholders expects downstream
• Discards the HF processor's input_ids (video_outputs.pop(
  "input_ids", None)) so the stale, unpruned, flat sequence can no
  longer overwrite the EVS-adjusted result — this fixes the EVS crash
• Moves config lookups (hf_config, tokenizer, merge_size,
  video_pruning_rate) outside the per-video loop to avoid redundant
  queries
• Eliminates the decode to text to prompt.replace to re-tokenize
  round-trip entirely

No duplicate PRs found for this optimization.

Test Plan

• Run existing Qwen3-VL processor regression test to verify no
  regression in single-video timestamp handling
• Run new test_processor_multi_video (2/4 videos) to verify
  token-level replacement produces correct placeholder count,
  consistent lengths, and non-overlapping ranges
• Manually verify EVS-enabled path produces fewer tokens than non-EVS
• Run ruff check and ruff format on changed files

.venv/bin/python -m pytest \
  tests/models/multimodal/processing/test_qwen3_vl.py -v

Test Result

All Passed

---

This PR was developed with AI assistance.

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[DarkLight1337]

You seem to have ignored the logic related to video_pruning_rate

[Sirius29]

You seem to have ignored the logic related to video_pruning_rate

Fixed as suggested. Thanks!

[DarkLight1337]

Please update the PR description accordingly

[Sirius29]

Please update the PR description accordingly

PR description updated to reflect the new changes. Thanks!

[DarkLight1337]

Could you post benchmarks showing how much improvement is achieved by this optimization?

[Sirius29]

Could you post benchmarks showing how much improvement is achieved by this optimization?

Model：Qwen3-VL-2B
Test Environment：
H20-96G
vllm 0.23.0
transformers 4.57.6
torch 2.11.0

It seems that I can't upload images. In my work setup, I use 7 video inputs, with a total of 2366 video tokens. From the torch profiler, the original call to _call_hf_processor took 3.981ms, and after optimization it took 0.496ms. In my initial test environment(around v0.19.0), the unoptimized time seemed to exceed 10ms, so the optimization effect would be more obvious, but after updating to v0.23.0, it seems a lot faster already. Still, this optimization has its advantages.

I also create a benchmark test, here’s the test script and results.
bench_qwen3_vl_processor.py

Config	Videos	Frames	Tokens	Original (ms)	Optimized (ms)	Speedup
1v_16f_256	1	16	576	5.42	4.59	1.18x
2v_16f_256	2	16	1152	10.04	8.49	1.18x
4v_8f_128	4	8	384	8.64	7.28	1.19x
4v_16f_256	4	16	2304	19.87	14.56	1.37x
8v_8f_128	8	8	768	16.60	14.15	1.17x
8v_16f_256	8	16	4608	48.28	35.34	1.37x

At the same time, in the original implementation, the placeholder on line 1315 would override the placeholder on line 1311, making the evs trimming ineffective. This change also fixes that issue, and I added this part to the PR description as well.

[DarkLight1337]

Ok, thanks for showing the results!