perf(windows): raise the capture helpers' timer resolution and use MMCSS

[EtienneLescot]

Fixes #921. Part of #920.

Change

• wgc-capture and cursor-sampler hold a 1 ms timer resolution for their lifetime (timeBeginPeriod).
• Both opt out of power throttling, which Windows 11 uses to ignore that request from a minimized or occluded process.
• The video writer, WASAPI capture, audio mixer and webcam threads register with MMCSS ("Capture", "Pro Audio").
• Every call failing is logged once and survived: the helper records at the old tick, as before.
• One new header, realtime_scheduling.h, in the style of dpi_awareness.h.

Measured

• A 5 ms sleep, median of 20: 15.45 ms before, 5.06 ms after (new check in audio_sample_utils_test, run by npm run build:native:win).
• No MMCSS registration warning in the mixer threads the suite starts.
• Build and all three native test executables pass.

Pending: a real take

• Webcam interval histogram: >95 % at 33 ms ±2, duplicates <2 % (today 25.9 unique fps for 30).
• Cursor telemetry at 30 Hz ±1 (today 22.7 Hz).
• [pacing] frames/elapsed ≥ 59.5 at 60 fps, also with the app minimized.

Coordination

The mixer's MMCSS scope sits in AudioMixer::start(), away from the mixLoop rewrite in #916, so the two PRs do not overlap.

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Summary by CodeRabbit

• Improvements
  • Audio, video, and cursor capture now use higher-priority scheduling and finer timer resolution. This may help keep capture timing more responsive and consistent, particularly during demanding workloads. Audio and video capture tasks receive scheduling profiles suited to their workloads, while capture processes use more precise timing. These changes apply in the background and do not require any changes to capture settings.

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• electron/native/wgc-capture/src/audio_sample_utils_test.cpp

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📝 Walkthrough

Walkthrough

The Windows recording and sampling code now requests 1 ms timer resolution and registers selected worker threads with MMCSS. A test compares sleep durations before and after timer-resolution setup.

Changes

Windows real-time scheduling

Layer / File(s)	Summary
Scheduling helpers and build linkage electron/native/wgc-capture/src/realtime_scheduling.h, electron/native/wgc-capture/CMakeLists.txt	Adds HighResolutionTiming and MmcssThread. Links wgc-capture, cursor-sampler, and audio_sample_utils_test with avrt and winmm.
Process timer-resolution setup and test electron/native/wgc-capture/src/main.cpp, electron/native/wgc-capture/src/cursor-sampler.cpp, electron/native/wgc-capture/src/audio_sample_utils_test.cpp	Constructs HighResolutionTiming in wgc-capture and cursor-sampler. The test compares median sleep durations before and after constructing the helper and expects the later median to be under 8 ms.
MMCSS registration for recording threads electron/native/wgc-capture/src/audio_sample_utils.cpp, electron/native/wgc-capture/src/dshow_webcam_capture.cpp, electron/native/wgc-capture/src/wasapi_loopback_capture.cpp, electron/native/wgc-capture/src/webcam_capture.cpp, electron/native/wgc-capture/src/main.cpp	Registers the audio mixer and WASAPI thread with Pro Audio. Registers the DirectShow, webcam, and video-writer threads with Capture.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: 🔵 Low · up to 6ac0c

The timing check can fail native Windows builds and CI jobs after retries when scheduling is delayed. This is a bounded reliability risk, but its frequency on current runners is unknown.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 6ac0c

The changes adjust scheduling for existing recording helpers rather than adding access to recordings or granting new permissions. Scheduling requests have fallback behavior and scoped cleanup. Remaining risk is bounded to local runtime behavior, including increased resource use and behavior not yet validated during a real recording.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The supported exposure is local scheduling and resource contention associated with existing capture helpers. The inspected controls change execution policy, not the identity or data-access authority of the recording process.

Trust Boundaries and Controls

• observed — Inspected production workers supply fixed MMCSS task names, Capture or Pro Audio. Caller-supplied capture configuration is not passed through to the new scheduling-profile selection.

Resilience and Maintainability Implications

• inferred — Failed scheduling acquisition does not strand a required security setup step: workers continue under ordinary scheduling, and cleanup is conditional on successful acquisition. The production process-lifetime ownership bounds the non-restored power policy, although runtime cleanup failures and forced interruption remain unvalidated.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 41.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 12 functions across 8 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main changes: increasing Windows capture-helper timer resolution and enabling MMCSS scheduling.
Description check	✅ Passed	The description is detailed and relevant. It covers the change, linked issues, Windows impact, measured results, testing, and pending validation. It does not use every template heading and does not ex…
Linked Issues check	✅ Passed	The PR implements the coding requirements in [#921]. HighResolutionTiming requests a 1 ms timer period, disables process power throttling, logs failures, and restores the timer period after successf…
Out of Scope Changes check	✅ Passed	The changed build links, scheduling helper, thread registrations, and timer-resolution test directly support [#921]. No unrelated change is established by the whole-PR summary.

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Actionable comments posted: 1

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Inline comments:
Review comments at @electron/native/wgc-capture/src/audio_sample_utils_test.cpp:
- Line 242: Update the latency assertion in the timer-resolution test so the
measured threshold does not affect the default test result. Keep TIMER_RAW
diagnostic output, and apply the threshold only in an opt-in performance check
intended for a controlled host.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: 504ecfc4-71cd-43a8-b660-44a6d79cf32d

📥 Commits

Reviewing files that changed from the base of the PR and between 46c304e and 74e6c31.

📒 Files selected for processing (9)

• electron/native/wgc-capture/CMakeLists.txt
• electron/native/wgc-capture/src/audio_sample_utils.cpp
• electron/native/wgc-capture/src/audio_sample_utils_test.cpp
• electron/native/wgc-capture/src/cursor-sampler.cpp
• electron/native/wgc-capture/src/dshow_webcam_capture.cpp
• electron/native/wgc-capture/src/main.cpp
• electron/native/wgc-capture/src/realtime_scheduling.h
• electron/native/wgc-capture/src/wasapi_loopback_capture.cpp
• electron/native/wgc-capture/src/webcam_capture.cpp

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