[Bugfix][Multimodal] Cap glm5next video sampling knobs

[drakosha]

Fixes #55726

Purpose

glm_sample_frame_indices walks duration * target_fps candidates and
deduplicates only at the end, so Glm5NextVideoBackend sizes that walk from the
requested fps rather than from the clip. On a 2-frame, 1-second source it spends
857 ms and 156 MiB to return 2 indices.

This clamps target.fps to source.original_fps. No sampling rate above the
source rate can return frames the container does not hold, so the walk becomes
bounded by the frame count while the sampled indices stay identical.

Why not a fixed ceiling

An earlier revision copied GLMGA's _MAX_FPS = 30 / _MAX_FRAMES = 2048. As
@JaredforReal pointed out, that ceiling has no basis in the GLM-5.3-Flash video
processor: its processor_config.json sets only fps: 2 as the default
interval. A fixed 30 would also halve sampling for a legitimate 60 fps request.

Clamping to the source rate needs no magic number, and it makes the max_frames
ceiling unnecessary: with fps bounded, extract_t cannot exceed the frame count,
so max_frames=100_000 keeps returning 10_000 indices exactly as on main.

Measurements

Serving image, calling the backend directly, 2-frame 1-second clip, fps and
max_frames set to the same value:

requested	before	after
200 000	11.9 ms, +1 MiB	3.1 ms, +0 MiB
2 000 000	87.4 ms, +18 MiB	0.0 ms, +0 MiB
20 000 000	857.5 ms, +156 MiB	0.0 ms, +0 MiB

Output is unchanged in every case: 2 indices for the 2-frame source, 900 for a
900-frame 30 fps clip, 2048 for a 10 000-frame one, and the default interval
still yields the same 60 indices as the bare sampler.

Not a live exposure

Per-request mm_processor_kwargs do not reach the video loader today;
chat_utils.py still carries TODO: Support per-request mm_processor_kwargs.
Verified against a live server that mm_processor_kwargs, fields inside
video_url, and chat_template_kwargs all leave the prompt token count
unchanged. The values arrive from server-side configuration today, and become
request-reachable once that TODO is addressed.

Not a duplicate

• [Security] Cap GLMGA video sampling to prevent request-driven resource exhaustion #54935 fixed the same class for GLMGAVideoBackend only, and is merged.
• [Bugfix][Multimodal] Bound renderer warmup to the prefill token budget #55448 and [Perf] Read VidCom2 frame budgets once #55331 touch nearby code but not these knobs.
• [Perf][Multimodal] Avoid scanning every source video frame #55582 rewrites the sampling loop for GLM46V and GLMGA; this backend delegates
  to the processor and is untouched by it.

Test plan

pytest tests/multimodal/test_video.py -k "Glm5NextSamplingCaps"

Three cases: the sampler receives the source rate rather than the requested one;
an oversized request samples the same indices as a request at the source rate
(three source shapes); and the default interval matches a bare sampler call.
No local venv on the development host, so the equivalents were executed inside
the serving image with the numbers above; the pytest run itself is left to CI.

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Walkthrough

Glm5NextVideoBackend now caps requested frames at 2048 and effective FPS at 30 before sampling. Tests verify oversized requests use both caps and normal sampling matches the default sampler output.

Changes

GLM5Next sampling caps

Layer / File(s)	Summary
Sampling bounds and enforcement vllm/multimodal/video.py	Glm5NextVideoBackend defines _MAX_FRAMES = 2048 and _MAX_FPS = 30. compute_frames_index_to_sample applies these caps before calling glm_sample_frame_indices.
Sampling-cap regression tests tests/multimodal/test_video.py	Tests verify that oversized requests pass the caps to the sampler and that normal sampling matches the direct default sampler output.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 053c1

This change bounds GLM5Next video sampling requests while preserving default sampling behavior. No concrete merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 37.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 8 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes satisfy issue #55726 by capping max_frames at 2048 and FPS at 30 before sampling, while preserving default sampling and short-clip padding behavior.
Out of Scope Changes check	✅ Passed	The changes are limited to the GLM5Next video sampling backend and focused tests. They support the linked issue objectives and do not introduce unrelated changes.
Description check	✅ Passed	The description directly addresses the video sampling resource issue and explains the intended caps, performance impact, tests, and linked issue. It contains implementation details that differ from th…
Title check	✅ Passed	The title clearly and concisely identifies the bugfix and the capped GLM5Next video sampling controls.

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Inline comments:
In `@tests/multimodal/test_video.py`:
- Around line 1556-1564: Strengthen test_class_cap_overrides_target_fps and the
adjacent normal-operation test by asserting the exact sampled indices, or
comparing them with glm_sample_frame_indices using the expected capped and
default FPS arguments. Preserve the existing frame-count bounds while ensuring
the tests fail if the FPS cap, default FPS, or sampling distribution changes.

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[Isotr0py]

cc @JaredforReal

[JaredforReal]

processor_config.json for glm53flash did not give a exact number for MAX_FRAMES and MAX_FPS.
but MAX_FRAMES is default to be 2048 in transformers, so it is acceptable, but MAX_FPS seems to be introduced from GLM-GA, which cannot be seen as equal to GLM-5.3-Flash's Video Processor
so can u give me more evidence that we need this(like performance wise) or this is a accuracy-independent change?

[JaredforReal]

PTAL @drakosha thanks~

[drakosha]

You're right, and the config agrees: processor_config.json sets only fps: 2
as the default interval, no ceiling. A fixed 30 would also halve sampling for a
legitimate 60 fps request, so it was wrong, not just unmotivated.

Reworked to clamp target.fps to source.original_fps. Sampling faster than the
source cannot return frames the container does not hold, so this is
accuracy-independent by construction, no magic number needed. That also let me
drop the max_frames ceiling: with fps bounded, extract_t cannot exceed the
frame count, so max_frames=100_000 returns 10_000 indices exactly as on main.

Measured in our serving image on a 2-frame 1-second clip, fps and max_frames
equal: 2M went 87.4 ms/+18 MiB -> 0.0 ms, 20M went 857.5 ms/+156 MiB -> 0.0 ms.
Identical output before and after: 2 / 900 / 2048 indices for 2-frame,
900-frame and 10k-frame sources.

AI assistance was used for this change.

[Hotragn]

There is one more backend in this file with the same unclamped walk, and it is not in this diff: DynamicVideoBackend.compute_frames_index_to_sample (registered as opencv_dynamic).

Your test comment describes it exactly — "the sampler walks duration * target_fps candidates, so an fps above the source rate sizes the walk from the request":

fps = target.fps                      # request-controlled, unclamped
...
if duration <= max_duration:
    n = int(math.floor(duration * fps))
    frame_indices_list = sorted(
        {
            min(max_frame_idx, int(math.ceil(i * original_fps / fps)))
            for i in range(n)
        }
    )

The returned set is bounded by total_frames_num, so the output is fine. range(n) is not: n scales as duration x fps with no reference to the source rate. Walking it for a 30 s / 30 fps / 900-frame source, single-threaded:

requested fps	candidates walked	indices returned	time
2 (default)	60	60	0.002 s
1_000	30_000	900	0.010 s
100_000	3_000_000	900	0.93 s
1_000_000	30_000_000	900	9.2 s

Same 900 indices, linear in the requested fps.

Reachable with no server flag. fps is a per-request media_io_kwargs key — VideoMediaIO.load_bytes does self.video_loader.load_bytes(data, num_frames=self.num_frames, **self.kwargs), and self.kwargs is the merged per-request media_io_kwargs (the class docstring already flags this: "Configuration values can be user-provided ... by the runtime API field media_io_kwargs"). And opencv_dynamic is registered with video_processor="Glm4vVideoProcessor", so it is the default backend for GLM-4.1V / GLM-4.5V — the caller does not have to name a backend:

{"media_io_kwargs": {"video": {"fps": 1000000}}}

The clamp you already wrote covers it:

fps = target.fps
if fps > 0 and source.original_fps > 0:
    fps = min(fps, source.original_fps)

The other branch is already safe, for the record: num_samples = int(max_duration * fps) only reaches np.linspace when num_samples < total_frames_num, so that allocation is bounded by the source.

I went through the rest of the file and this is the last site: VideoBackend bounds its count with min(num_frames, total_frames_num), GLM46VVideoBackend derives target_fps from its own thresholds and caps extract_t at 640, and Qwen2/Qwen3-VL were capped in #56729. Flagging it here rather than opening a second PR on the same seam — it seems better bundled into this one, but happy to take it separately if you would rather keep this PR narrow.