96 lines
5.2 KiB
Markdown
96 lines
5.2 KiB
Markdown
# Performance and resource observations
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ffmpeg.wasm runs native media code compiled to WebAssembly. It is normally much
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slower than desktop FFmpeg and may hold source, decoded frames, filters and
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output at once. No hardware encoder is used.
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## Fixed download footprint
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Uncompressed self-hosted core assets for
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`0.12.10-reviewed-stack5m.1`:
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| Mode | JavaScript/worker | WASM | Total |
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| ------------- | ------------------: | -----------: | -----------: |
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| single-thread | 111,804 B | 32,232,580 B | 32,344,384 B |
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| multithread | 128,953 B + 2,115 B | 32,718,455 B | 32,849,523 B |
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Only the selected mode is loaded. HTTP compression may reduce JavaScript
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transfer but not necessarily WASM to the same degree; report transfer and
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decoded sizes separately. Versioned core assets may be cached immutably.
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## Benchmark record
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One opt-in pinned-core baseline was recorded on 2026-07-24 with Chrome for
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Testing 149.0.7827.55 (Playwright 1.61.1), Linux 7.0.0-28 on Ubuntu 24.04,
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Intel Core i7-8700 (6 cores/12 threads) and 31 GiB RAM.
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`navigator.hardwareConcurrency` reported 12 and every sample was
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cross-origin-isolated. Each row used a fresh browser context; the first core
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fetch may populate the browser HTTP cache, so later rows are explicitly
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potentially warm-cache observations.
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Preset: MP4, H.264/libx264 CRF 23 medium, AAC. Times are milliseconds.
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“Export end-to-end” spans the Convert click through result construction,
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cleanup, verification re-probe and visible result. The three narrower engine
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columns come from the export validation report.
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| Source | Input bytes | Mode | Init | Probe | FFmpeg exec | Output read | Cleanup | Export end-to-end | Output bytes | Observed JS heap |
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| --------------------------------------- | ----------: | ----------------- | ----: | ----: | ----------: | ----------: | ------: | ----------------: | -----------: | ---------------: |
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| 2 s, 160×90 H.264/AAC synthetic pattern | 34,057 | ST | 294.4 | 131.4 | 315.6 | 1.0 | 1.2 | 422.4 | 68,700 | 10,000,000 B |
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| same | 34,057 | MT (`-threads 2`) | 914.5 | 126.4 | 294.3 | 1.3 | 1.9 | 416.8 | 68,753 | 10,000,000 B |
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| 6 s, 640×360 H.264/AAC synthetic loop | 526,632 | ST | 270.2 | 163.6 | 6,854.1 | 1.3 | 0.8 | 7,030.9 | 585,981 | 15,200,000 B |
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| same | 526,632 | MT (`-threads 2`) | 946.3 | 121.6 | 4,053.1 | 1.6 | 0.5 | 4,497.5 | 585,884 | 11,200,000 B |
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Command:
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```sh
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npm run benchmark:browser
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```
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Result: **1 passed (18.2 s)**; test body 17.6 s. Chromium exposed
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`performance.memory.usedJSHeapSize`, but this coarse post-operation value is
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not a peak-WASM-memory measurement. One run is not a statistical distribution;
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cold-cache repetitions, median/range, mobile and other-browser measurements
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remain release-quality evidence gaps. Never present this workstation as a
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universal promise.
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## Measurement boundaries
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- initialization: wrapper construction through successful capability query;
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- probe: input mount/write through parsed ffprobe JSON;
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- conversion: accepted command through exit code;
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- output read: exit through Blob/result construction;
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- cleanup: final result read through job directory unmount/delete;
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- cancellation: cancel request through worker termination and recoverable idle
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state (fresh-engine load is a separate measure).
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Progress events are experimental estimates and not a timer. Use monotonic
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browser timing around lifecycle boundaries.
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## Product limits
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Default soft input warning is 256 MiB; hard single/total input is 1.5 GiB.
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Estimated output warns at 512 MiB and is blocked at 1 GiB. FFmpeg jobs are
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serial; the application retains at most eight scheduled operations in its
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in-memory queue. Timeline clips are capped at 100, streams per input at 64,
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images at 8192×8192/67,108,864 pixels, generated output files at 255,
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contact-sheet cells at 500, scene-change results at 100, crossfades at 10
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seconds and runtime at four hours. Scene selection is a bounded two-step
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workflow but its analysis step can still decode the complete source when few
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frames meet the threshold. The documented ffmpeg.wasm input ceiling is 2 GiB,
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but real limits can be substantially lower on mobile or memory-constrained
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browsers.
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Import does not start FFmpeg. Recognized local audio/video can begin browser
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playback from its temporary object URL while a native media element reads
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lightweight duration/dimensions. Detailed FFprobe inspection starts only when
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requested or required for an export and has a bounded, size-aware watchdog:
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30 seconds for small files, a two-MiB-per-second budget for larger files, and a
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five-minute ceiling. A detailed-inspection timeout is nonfatal; browser
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playback and basic file information remain available. Completed probe metadata
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remains authoritative for stream mapping, remux safety, chapters, tags,
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subtitles/attachments and export validation.
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Lowering resolution, duration, frame rate, passes and output count reduces
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work. MT can be faster but consumes more memory/CPU and requires isolation; ST
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is not inherently a low-memory guarantee.
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