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77 lines
3.6 KiB
Markdown
77 lines
3.6 KiB
Markdown
# nigig-traffic perf baseline (plan Phase 6.3)
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Machine: MacBook Pro, Apple Silicon (darwin x86_64 binary under Rosetta —
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`target/debug/nigig-traffic: Mach-O 64-bit executable x86_64`), debug
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profile. Two consecutive runs, `cargo test -p nigig-traffic --test perf
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-- --test-threads=1 --nocapture`. Catalog: **47 scenarios** (the old
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"36+" copy is a floor, still true).
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## build_world_for — median µs over 20 reps
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| Category | Run 1 | Note |
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|------------|-------|-----------------------------|
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| Lanes | 2 | |
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| Roundabout | 11 | 16 ring segs + island |
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| Parking | 5 | 4 bays |
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| UTurns | 4 | |
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| Signs | 3 | |
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| Priority | 2 | |
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| Hazards | 3 | |
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| Speed | 2 | |
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Single-digit microseconds everywhere. World build is not a cost center —
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no caching/baking work is justified.
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## tick — 600 ticks/scenario, idle input, per-tick average
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| | Run 1 | Run 2 |
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|------------|-------|-------|
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| median | 14 µs | 26 µs |
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| slowest #1 | haz_rain 501 µs | lane_bus_lane 450 µs |
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| slowest #2 | pri_yield_car 255 µs | pri_yield_car 386 µs |
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| slowest #3 | rb_multi_exits 248 µs| ut_single_turn 257 µs|
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Reading, per HPC ch.5.6: the slowest set **moves between runs**
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(`haz_rain`/`rb_multi_exits` appear once each) — that is noise (debug
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build, shared machine), not a scenario-intrinsic cost. `pri_yield_car`
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ranks top-3 twice; it spawns a moving AI car, so it is the prime suspect
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for the only genuine above-median cost — unconfirmed until a pinned,
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quiescent re-run. No optimization is authorized off this table alone.
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## Live run (NIGIG_TRAFFIC_STATS=1, same machine, windowed)
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```
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traffic stats: tick us p50=48 p95=270 p99=5761 (n=300)
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traffic stats: draw us p50=69 p95=563 p99=4182 (n=300)
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traffic stats: tick us p50=55 p95=332 p99=1839 (n=300)
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traffic stats: draw us p50=76 p95=663 p99=4272 (n=300)
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traffic stats: tick us p50=59 p95=561 p99=3681 (n=300)
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```
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Live tick p50 (~55µs) runs hotter than the harness median — it shares the
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machine with the window server and includes rig/camera work around the
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timed region. p99 spikes are millisecond-scale frame hitches (scheduler /
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allocator / display-link, not sim code): the next profiling step is an
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Instruments sample attributing p99, not optimizing p50.
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## Blocked: release-vs-size profile decision (Phase 9)
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`release` currently sets `opt-level = "z"` with no LTO. Whether the game
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binary wants `opt-level = 3` (throughput over size) must be measured on
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the Phase-6 bench — but the workspace-wide fork rev bump broke
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compilation of every crate (upstream `b9a083c` references a never-landed
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isolate refactor: `enter_isolate`/`leave_isolate`/`IsolateEntry` used in
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4 files, defined in 0; plus a 4-vs-5-arg `camera_boom_limit` drift), so
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no release numbers exist yet. Decision and both measurements go here
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once the workspace builds again. Do not choose on aesthetics.
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## Conclusions authorizing later phases
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1. Tick ≈ 15–25µs median (debug). At 60Hz the budget is 16,600µs —
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the sim consumes ~0.1%. There is no throughput emergency.
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2. Costs are latency-shaped (lookups, allocations, branches over dozens
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of entities), never compute-shaped. Phases 7–8 target exactly that;
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hand-written SIMD is off the table (documented anti-goal).
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3. Next measurement before any Phase 7 code: quiescent release-profile
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re-run + Instruments sample confirming `step_world` vs `evaluate`
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vs text-shaping split.
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