137 lines
9 KiB
Markdown
137 lines
9 KiB
Markdown
# Performance / Memory Cost Audit
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## Purpose
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Audit the integrated Mandelbrot viewer for processing that is low-value, redundant, or harmful from a speed / memory perspective. This audit intentionally **does not remove or alter any production behavior** in `index.html`.
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Production source audited:
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- `index.html`
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- SHA-256: `544820c5024cf6d56e012db6d1bec3ddfcef5d833691b890baa64b26c5c2ee5a`
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- Regression: PASS, 19 WGSL kernels
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Measurements are CPU/static surrogates unless explicitly stated. Real WebGPU frame timing is not available in this container because the browser WebGPU backend cannot initialize successfully here.
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## Conclusions
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### A — High-confidence waste / harmful cost
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| ID | Finding | Cost | Benefit observed | Confidence |
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|---|---|---|---|---|
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| A1 | Numeric history is recopied after every accepted recolor | 8 B/px copy per recolor | Numeric data did not change | Very high |
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| A2 | `cpuFrameRgba` retains the assembled CPU fallback frame but is never read | 4 B/px JS heap retained | None | Very high |
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| A3 | `certifyInterior()` runs a full-frame unknown-statistics pass and waits for GPU completion before primary compute | Full meta scan + queue synchronization | Partial proof/history-seed caller does not consume these stats | Very high |
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| A4 | Strict direct rendering still updates Brent-cycle state even though periodicity classification is disabled | Per-iteration ALU/register state | None in strict mode | Very high |
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| A5 | Export workspace survives export completion | Up to ~24 MiB GPU memory with 512 tiles / 3 slots | None while not exporting | Very high |
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| A6 | Deep active/correction workspaces survive the hard frame that required them | active workspace up to ~44.1 MiB standard / ~176.5 MiB high; dense correction queue 4/16 MiB | None on later easy frames until reuse/resize | High |
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### B — Strong optimization candidates, but keep until GPU A/B validation
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| ID | Finding | Evidence | Assessment |
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|---|---|---|---|
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| B1 | Numeric history is always resident although exact-grid pan reuse is restrictive | 8 B/px resident. Under common budget-scaled resolutions, integer CSS pans 1–100 px produced 0/100 exact-grid reuse hits in the static alignment test | Likely over-allocated; make lazy/telemetry-driven rather than delete immediately |
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| B2 | CPU exact cardioid/bulb tile proof runs for every numerical frame | 1024×768: ~4.8–6.1 ms median; 2048²: ~25–28 ms median. Four of six fixed corpus views certified 0 pixels | Add cheap spatial gate; direct backend especially needs A/B comparison |
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| B3 | `fast-coverage-repair` runs a full-screen FAST pass after tiled coverage | Tile loops cover the full compute domain; mode 2 only repairs reason=0 untouched metadata | Likely redundant defensive pass; instrument reason=0 count before removal |
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| B4 | `captureColorSource()` snapshots `meta+smooth` during rendering | 8 B/px copy per snapshot; provisional snapshots can occur every 125 ms. Snapshot allocation is 12 B/px | Useful only for live recoloring while render is in flight; gate more aggressively |
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| B5 | Color-auto idle mode retains a color snapshot | standard ~12 MiB, high ~48 MiB at preset caps | Idle color-auto recolor path uses current field, not `recolorPublished()` | Snapshot should be render-only/on-demand |
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| B6 | Adaptive initial-iteration probe uses high-precision scoring before every adaptive frame | Fixed corpus: ~1.3–12.5 ms and all six chose 350. Exploratory views can choose 512, so feature is not useless | Cache/gate/replace with cheaper trigger; do not delete blindly |
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| B7 | Active continuation can replay already-performed iterations | initial OP_LIMIT session starts state at n=0; reference change resets progress to 0; recovered pixels can rebuild whole active session | Potentially large on the hardest views; add replay counters before redesign |
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### C — Expensive but currently justified / not a deletion target
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- General periodicity detection: CPU surrogate showed ~98% iteration reduction in period-3 interiors. It can add overhead on boundary-only views, but removal is not justified.
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- Sparse numerical correction and reason bucketing: it adds full scans/queue construction, but isolates expensive DS/BigInt work. Needs GPU A/B before structural change.
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- Stable texture reprojection: consumes one history texture but directly improves interaction latency.
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- Reference cache / guided multi-reference: memory cost is bounded and prior evaluation showed reduced perturbation mismatch.
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- Series approximation: small reference metadata cost with large deep-zoom iteration skip in prior evaluation.
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## Key measurements
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### Recolor numeric-history copy bandwidth
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`commitHistory()` always calls `commitNumericHistory()`. Therefore a color-only update copies unchanged `meta` and `smooth` buffers (8 B/px). Color auto can request a pass every 50 ms while idle.
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| preset cap | unchanged numeric copy / recolor | at 20 Hz |
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|---|---:|---:|
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| fast, 524,288 px | 4 MiB | 80 MiB/s |
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| standard, 1,048,576 px | 8 MiB | 160 MiB/s |
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| high, 4,194,304 px | 32 MiB | 640 MiB/s |
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This is in addition to the actual color pass bandwidth.
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### Color snapshot
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`captureColorSource()` allocates/copies:
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- meta: 4 B/px
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- smooth: 4 B/px
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- texture: 4 B/px
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Resident cost = 12 B/px: ~6 MiB fast, 12 MiB standard, 48 MiB high at preset caps. A provisional snapshot copies 8 B/px and can be refreshed at up to 8 Hz, equivalent to ~32 / 64 / 256 MiB/s at the preset caps.
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### Exact interior tile proof
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Seven-run median on this host:
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| view | 1024×768 | certified | 2048×2048 | certified |
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|---|---:|---:|---:|---:|
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| overview | ~6.1 ms | 7.3% | ~25.5 ms | 8.6% |
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| period3 bulb | ~5.1 ms | 0% | ~26.5 ms | 0% |
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| period3 core | ~4.8 ms | 0% | ~28.1 ms | 0% |
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| seahorse boundary | ~6.0 ms | 0% | ~25.1 ms | 0% |
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| seahorse deep | ~5.6 ms | 0% | ~26.1 ms | 0% |
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The exact proof is still useful in views that overlap the main cardioid / period-2 bulb, especially on the perturbation path. The issue is unconditional invocation, not the proof mechanism itself.
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### Exact-grid numeric-history reuse
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The history seed requires identical span and an approximately integer render-pixel shift. When the resolution is pixel-budget limited, render pixels often do not align with CSS pixels.
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Examples from the current resize rule, testing integer CSS horizontal pans of 1–100 px:
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| screen / quality | render size | reuse hits |
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|---|---:|---:|
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| 1920×1080 / fast | 965×543 | 0/100 |
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| 1920×1080 / standard | 1365×768 | 0/100 |
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| 1920×1080 / high | 2731×1536 | 0/100 |
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| 1024×768@1× / standard | 1024×768 | 100/100 |
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This is a static alignment test, not real interaction telemetry. It supports making numeric history conditional rather than proving that it has no value.
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### Persistent workspaces
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- Sparse active workspace, worst full-active capacity: ~44.1 MiB standard, ~176.5 MiB high.
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- At 10% active: ~5.64 MiB standard, ~22.51 MiB high.
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- Dense deep correction queue: 4 MiB standard, 16 MiB high.
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- Export workspace at 512 tile, depth 3: ~24 MiB retained after export.
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- Non-AA export currently allocates four 512² sample textures, ~4 MiB per slot, although 1× rendering does not need the four AA sample textures.
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## Static code locations
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- `index.html:1299-1300`: pre-primary unknown stats + synchronization in `certifyInterior()`.
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- `index.html:1383-1384`: full numeric-history copy.
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- `index.html:1416`: `commitHistory()` always commits numeric history.
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- `index.html:1822`: color-only recolor promotes history and therefore triggers numeric copy.
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- `index.html:1718`: `state.cpuFrameRgba=rgba`; the state value has no reader.
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- `index.html:162-163`: cycle state still updated in strict direct mode.
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- `index.html:1349-1350`: tiled FAST coverage followed by full-screen coverage-repair pass.
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- `index.html:1751-1756`: provisional color snapshots at 125 ms throttle.
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- `index.html:1774`: old completed field snapshot at every numerical render start.
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- `index.html:1866`: idle color-auto snapshot allocation.
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- `index.html:1218`: active workspace persists unless another active allocation request causes a >4× shrink.
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- `index.html:1217`: full-pixel dense correction queue persists once allocated.
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- `index.html:1222-1226, 1898-1944`: export workspace lifetime extends beyond export job.
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- `index.html:1306-1307, 1648, 1654`: active continuation reinitialization/rebuild can reset progress.
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## Recommended instrumentation before deleting anything
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1. Add counters for numeric-history `attempt / hit / reused pixels / bytes copied`.
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2. Count reason-0 pixels immediately before `fast-coverage-repair`; if consistently zero, remove/disable that pass in an A/B branch.
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3. Record `certifiedInteriorTiles` CPU ms and certified ratio per frame; gate when hit rate stays zero.
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4. Record active-continuation `reinitCount` and `replayedPixelIterations`.
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5. Record peak/resident bytes for active, correction, export, color snapshot and release-on-idle experiments.
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6. Separate color-history promotion from numeric-history promotion and A/B color-auto frame time.
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## Production integrity
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No production deletion or behavioral modification was made by this audit. Regression was rerun after inspection and passed.
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