DioCrafts
fd5808c157
Decode JPEGs at the smallest DCT scale (1/8·1/4·1/2·1/1) whose long axis is still ≥ the largest needed thumbnail (800px), via jpeg-decoder, instead of a full-resolution decode through the image crate. The full-res bitmap — the dominant time and RAM cost — is never materialised. PNG/GIF/WebP and unusual JPEG colour spaces (CMYK / 16-bit grey) fall back to a full decode. Extracts the shared decode + EXIF-orientation logic into decode_oriented(), removing the duplication that existed between render_thumbnail_from_data and render_all_thumbnails_from_data. Measured on 14 cores (see benches/BASELINE.md): - render_all 1.8-2.0× faster (12MP 111->61ms, 48MP 398->203ms) - peak heap 5.5-14.8× lower, now decoupled from source MP (~18-25MB regardless) - saturated throughput 3-3.6× (parallel efficiency 4.9×->8.5×) - quality SSIM 0.987-0.999 (>=0.98 gate), PSNR 47-55dB Also adds the Phase 0 benchmark harness (gated behind the `bench` feature, zero prod impact): deterministic image corpus (src/bench_support.rs), criterion latency bench (benches/thumbnails.rs), and a peak-RAM/throughput/SSIM harness (examples/bench_thumbnails_mem.rs). Baseline + before/after in benches/BASELINE.md. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
155 lines
7.5 KiB
Markdown
155 lines
7.5 KiB
Markdown
# Thumbnail performance — Phase 0 baseline
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> **Phase 1.1 (shrink-on-load) is now merged — see "Phase 1.1 results" at the
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> bottom for the before/after.** The tables below remain the Phase 0 baseline
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> (the "before").
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The "before" numbers every later phase must beat. Captured on **14 cores** with
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the current `image` 0.25 pipeline (`render_thumbnail_from_data` /
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`render_all_thumbnails_from_data` in
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`src/infrastructure/services/thumbnail_service.rs`).
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> Heap = logical allocation high-water mark (counting allocator), not RSS.
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> The synthetic corpus is high-entropy (gradient + noise), so JPEG sizes and
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> decode work are realistic-to-slightly-pessimistic. Drop real photos into
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> `benches/corpus/` (same filenames) to re-baseline on real data.
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## Reproduce
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```bash
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# Peak RAM + saturated throughput (Task 0.3) → target/bench-baseline-fase0.json
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cargo run --release --features bench --example bench_thumbnails_mem
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# Per-size latency + output bytes (Task 0.2) → target/criterion/report/index.html
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cargo bench --features bench # do NOT pipe through `tail` — it truncates the log;
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# results are saved under target/criterion/ regardless
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```
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## A. Per-image — peak heap, single-thread latency, output size
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| case | fmt | source | MP | render_all ms | peak heap MB | out KB (3 sizes) |
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|------------------|------|-----------|-----:|--------------:|-------------:|-----------------:|
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| jpeg_12mp | jpeg | 4000×3000 | 12.0 | 111.30 | 96.1 | 57 |
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| jpeg_24mp | jpeg | 6000×4000 | 24.0 | 207.23 | 151.0 | 41 |
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| jpeg_48mp | jpeg | 8000×6000 | 48.0 | 397.67 | 260.9 | 38 |
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| jpeg_exif_orient | jpeg | 4000×3000 | 12.0 | 121.82 | 107.6 | 59 |
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| png_large | png | 3000×2000 | 6.0 | 34.36 | 58.4 | 63 |
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| webp_large | webp | 1280×853 | 1.1 | 21.05 | 20.7 | 113 |
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| gif_large | gif | 600×600 | 0.4 | 12.86 | 15.4 | 232 |
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| small_300 | jpeg | 300×300 | 0.1 | 9.94 | 8.0 | 184 |
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## B. Saturated throughput (14 threads, 3 s window)
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| case | source | MP | photos/sec | eff ms/photo |
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|-----------|-----------|-----:|-----------:|-------------:|
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| jpeg_12mp | 4000×3000 | 12.0 | 44.4 | 22.52 |
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| jpeg_24mp | 6000×4000 | 24.0 | 25.3 | 39.51 |
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| jpeg_48mp | 8000×6000 | 48.0 | 12.4 | 80.49 |
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Scaling is sub-linear (14 threads ≈ 4.9× single-thread): memory-bandwidth bound
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(moving 96–261 MB per decode) + rayon oversubscription (each caller thread fans
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3 sizes onto the shared rayon pool).
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## C. Per-size latency — criterion median ms (one size in isolation vs all-three)
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| case | Icon ms | Preview ms | Large ms | all-3 ms | Large/all |
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|------------------|--------:|-----------:|---------:|---------:|----------:|
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| jpeg_12mp | 75.42 | 97.17 | 106.30 | 107.11 | 99.3% |
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| jpeg_24mp | 148.65 | 190.05 | 200.70 | 203.44 | 98.7% |
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| jpeg_48mp | 303.31 | 375.33 | 386.04 | 391.24 | 98.7% |
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| jpeg_exif_orient | 90.68 | 122.57 | 119.27 | 119.33 | 100.0% |
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| png_large | 13.32 | 25.87 | 32.26 | 32.59 | 99.0% |
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| webp_large | 15.83 | 19.51 | 24.47 | 24.45 | 100.1% |
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| gif_large | 2.28 | 5.15 | 12.94 | 12.92 | 100.2% |
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| small_300 | 0.85 | 3.22 | 9.94 | 9.92 | 100.2% |
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## Key findings (these steer Phase 1)
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1. **Decode dominates: 70–99 % of total time.** For jpeg_12mp, rendering all
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three sizes (107 ms) costs barely more than rendering Icon alone (75 ms) —
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the full-resolution decode is the shared cost; per-size resize+encode is
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cheap on top. ⇒ **Shrink-on-load (Task 1.1) is the single biggest lever**,
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bigger than first estimated.
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2. **Peak heap scales linearly with megapixels** (~2× the RGBA bitmap):
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12 MP→96 MB, 48 MP→261 MB. With the real `cpus/2` semaphore that is up to
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7×261 MB ≈ 1.8 GB on a 48 MP burst — the OOM ceiling that caps concurrency.
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Shrink-on-load collapses this ~16× and unlocks Task 1.5 (raise the semaphore).
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3. **Task 2.1 "defer Large" is now DROPPED — the benchmark refutes it.**
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Because all three sizes share one decode (Large/all ≈ 99 %), deferring Large
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saves ~9 ms eager but forces a *second full decode* (~106 ms) when the
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lightbox opens — it roughly **doubles** total decode work. Keep generating
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all sizes in one pass.
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4. **No-upscale (Task 1.4) confirmed minor:** small_300's Large (9.9 ms)
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upscales 300→800; clamping recovers a few ms and avoids artefacts.
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5. **PNG/GIF/WebP get no DCT shrink-on-load** — only `fast_image_resize`
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(Task 1.2) speeds their resize portion.
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---
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# Phase 1.1 results — shrink-on-load (DCT scale-on-decode for JPEG)
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Implemented via `jpeg-decoder` in `decode_oriented` / `decode_jpeg_scaled`
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(`src/infrastructure/services/thumbnail_service.rs`). The JPEG decoder now emits
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the image at the smallest DCT scale (1/8·1/4·1/2·1/1) whose long axis is still ≥
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the largest needed thumbnail (800 px), so the full-resolution bitmap is never
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materialised. Non-JPEG and unusual JPEG colour spaces fall back to a full decode.
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Same machine (14 cores), same corpus.
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### Latency — `render_all`, single thread (ms)
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| case | before | after | speedup |
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|-----------|-------:|-------:|--------:|
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| jpeg_12mp | 111.30 | 60.64 | 1.84× |
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| jpeg_24mp | 207.23 | 113.71 | 1.82× |
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| jpeg_48mp | 397.67 | 202.88 | 1.96× |
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| jpeg_exif | 121.82 | 60.12 | 2.03× |
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| png_large | 34.36 | 33.67 | ~1× (no DCT, expected) |
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### Peak heap per decode (MB) — the headline win
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| case | before | after | reduction |
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|-----------|-------:|------:|----------:|
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| jpeg_12mp | 96.1 | 17.6 | 5.5× |
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| jpeg_24mp | 151.0 | 24.9 | 6.1× |
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| jpeg_48mp | 260.9 | 17.6 | 14.8× |
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| jpeg_exif | 107.6 | 18.9 | 5.7× |
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Peak heap is now **decoupled from source resolution** (~18–25 MB regardless of
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MP — bounded by the 800 px decode, not the original). 48 MP now uses *less* than
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24 MP because it hits the 1/8 scale (1000×750) vs 24 MP's 1/4 (1500×1000).
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### Saturated throughput (14 threads, photos/sec)
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| case | before | after | speedup |
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|-----------|-------:|------:|--------:|
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| jpeg_12mp | 44.4 | 140.8 | 3.17× |
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| jpeg_24mp | 25.3 | 74.7 | 2.95× |
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| jpeg_48mp | 12.4 | 45.3 | 3.65× |
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Throughput improved **more** than single-thread latency (3.2× vs 1.8× at 12 MP):
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parallel efficiency rose from ~4.9× to ~8.5× across 14 threads because the 16×
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smaller decode buffers relieve the memory-bandwidth ceiling.
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### Quality gate — shrink-on-load vs full decode (Preview 400 px)
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| case | SSIM | PSNR dB |
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|-----------|-------:|--------:|
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| jpeg_12mp | 0.9875 | 47.42 |
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| jpeg_24mp | 0.9927 | 48.91 |
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| jpeg_48mp | 0.9939 | 49.37 |
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| small_300 | 0.9995 | 55.17 |
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All **SSIM ≥ 0.98** (acceptance criterion met) and PSNR 47–55 dB (>40 dB =
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visually indistinguishable). Output bytes unchanged (e.g. 12 MP: 57→58 KB).
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### Follow-ups this unlocked
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- **Task 1.5** (raise `cpus/2` → `cpus`): peak heap no longer scales with MP, so
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the OOM ceiling that justified halving concurrency is largely gone.
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- The `MAX_DECODE_PIXELS` 50 MP reject could be relaxed — huge JPEGs now decode
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cheaply at 1/8 — but that is a behaviour change, deferred.
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