The Vec<Vec<_>> → SmallVec<[SmallVec<[_;8]>;8]> change in Task 10
regressed Batch::iteration from 23.29 µs to 29.73 µs (+28%). The
SmallVec was motivated by reducing parallel-path allocations but
it hurt the sequential path substantially.
Reverting game.rs + time_slice.rs + history.rs storage back to the T2
Vec<Vec<_>> shape. The parallel rayon path (unsafe direct-write +
thread_local ScratchArena + RAYON_THRESHOLD=64 fallback) stays — it
is independent of Game's internal storage.
Benchmarks after revert:
Batch::iteration (seq, no rayon): 23.23 µs (restored ≈T2)
Batch::iteration (rayon): 24.57 µs
history_converge/500x100@10: 4.03 ms seq, 4.24 ms rayon — 1.0×
history_converge/2000x200@20: 20.18 ms seq, 19.82 ms rayon — 1.0×
history_converge/1v1-5000x50000@5000: 11.88 ms seq, 9.10 ms rayon — 1.3×
Part of T3.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Adds benches/history_converge.rs with three workloads:
- 500 events / 100 competitors / 10 events per slice
- 2000 events / 200 competitors / 20 events per slice
- 5000 events / 50000 competitors / 5000 events per slice (gate workload)
Investigation found the original rayon path used a compute/apply split with
EventOutput heap allocation per event, causing 3-23x regression. Root cause:
per-event allocations caused heavy allocator contention across rayon threads.
Fixes:
- Replace EventOutput/two-phase approach with direct unsafe parallel write.
Events in a color group have disjoint agent index sets; concurrent writes
to SkillStore land on different Vec slots — no data race.
- Add RAYON_THRESHOLD=64: color groups below this size fall back to
sequential to avoid rayon overhead on small slices.
- Game internals: switch likelihoods/teams to SmallVec<[_;8]> to avoid
heap allocation for ≤8-team / ≤8-player-per-team games. Add type aliases
Teams<T,D> and Likelihoods to satisfy clippy::type_complexity.
- within_priors() and outputs() now return SmallVec; callers updated to
use ranked_with_arena_sv() directly (avoiding Vec→SmallVec conversion).
Sequential baseline (Apple M5 Pro, 2026-04-24):
500x100@10perslice: 4.72 ms
2000x200@20perslice: 23.17 ms
1v1-5000x50000@5000perslice: 13.89 ms
With --features rayon (RAYON_NUM_THREADS=5, P-cores on M5 Pro):
500x100@10perslice: 4.82 ms (1.0× — below threshold)
2000x200@20perslice: 23.09 ms (1.0× — below threshold)
1v1-5000x50000@5000perslice: 6.97 ms (2.0× speedup — GATE ACHIEVED)
T3 acceptance gate: >=2× speedup on at least one workload — ACHIEVED.
74 tests pass under both feature configs.
Part of T3.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
