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swactor/CLAUDE/notes/baseline_benchmarks.md
Developer ef87f7e1b9 feat: per-actor message budget for tick fairness 
Research across ractor, tokio, Erlang/OTP BEAM, Linux CFS, and libuv
revealed that tick_all drained the entire mailbox per actor per tick,
allowing one hot actor to starve all others on the same worker.

- Add `actor_message_budget` to RuntimeConfig (default: 64 msgs/actor/tick)
- Modify tick_all to break after budget messages, yielding to next actor
- budget=0 restores unlimited (backward compatible) behavior
- 3 new fairness tests validating hot-cold actor scenarios
- New fairness benchmark group (cold_latency_under_pressure, throughput_by_budget)
- Fix RuntimeConfig struct literals across workspace crates

Inspired by BEAM's 4000-reduction budget and tokio's 128-op cooperative budget.
All 45 tests pass (42 original + 3 new).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
2026-02-12 11:11:30 +00:00

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Baseline Benchmarks (pre-improvements)

Latency (single-threaded)

Benchmark Time
spawn 1.28 µs
message_roundtrip 2.24 µs
send_fire_and_forget 1.50 µs
inbox_creation 1.63 µs

Throughput (single-threaded)

Benchmark Time Throughput
single_actor/100 15.0 µs 6.65 Melem/s
single_actor/1000 57.5 µs 17.4 Melem/s
single_actor/10000 474.6 µs 21.1 Melem/s
multi_actor/10x100 80.6 µs 12.4 Melem/s
multi_actor/100x100 610.6 µs 16.4 Melem/s
multi_actor/100x1000 6.10 ms 16.4 Melem/s
ring/10 6.3 µs 1.75 Melem/s
ring/100 99.9 µs 1.01 Melem/s
ring/500 919.0 µs 545 Kelem/s
spawn/100 33.5 µs 2.99 Melem/s
spawn/1000 326.2 µs 3.07 Melem/s
spawn/5000 1.69 ms 2.96 Melem/s

Key Observations

  • Single-actor throughput scales well: 6.65M → 21.1M msgs/s as batch size grows (amortized overhead)
  • Multi-actor throughput lower due to iteration overhead across actors
  • Ring throughput degrades with ring size (expected: each message traverses more actors)
  • Spawn throughput steady at ~3M/s regardless of batch size
  • Message roundtrip latency: 2.24µs (spawn + deliver + process + reply + deliver)