swactor/CLAUDE/notes/progress.md

# Progress Log

## Current Stage: Phase 1 — Research + First Improvement Cycle

### Status: Cycle 3 COMPLETE

## Plan Overview
1. **Phase 0**: Codebase audit — understand current swactor architecture, existing tests, benchmarks ✅
2. **Phase 1**: Broad survey + interleaved improvements
3. **Phase 2**: Deeper improvements based on findings
4. **Phase 3**: Testing methodology improvements
5. **Phase 4**: Final evaluation & documentation

## Completed This Session

### Cycle 1: Fairness (Message Budget)
- **Research**: Studied ractor, tokio, Erlang/OTP BEAM, Linux CFS/EEVDF, libuv
- **Finding**: `tick_all` drained ENTIRE mailbox per actor per tick — critical fairness bug
  - BEAM uses 4000 reduction budget, tokio uses 128-op cooperative budget
  - Swactor had zero budget — one hot actor could starve all others on same worker
- **Implementation**: Added `actor_message_budget` to `RuntimeConfig` (default: 64)
  - Modified `tick_all` to break after `budget` messages per actor
  - `budget=0` means unlimited (backward compatible)
- **Tests**: 3 new fairness tests (hot_actor_does_not_starve_cold_actor, unlimited_budget_drains_all, budget_messages_drain_across_multiple_ticks)
- **Benchmarks**: Added fairness benchmark group (cold_latency_under_pressure, throughput_by_budget)
- **Fixes**: Updated RuntimeConfig struct literals across crates (python, runtime-dashboard, mt_benchmarks)
- **Result**: 45 tests pass (42 original + 3 new), all workspace crates compile

### Cycle 2: Stress Tests, Benchmarks, Research Expansion
- **Research**: Added Kameo and Actix analysis to synthesis
  - Actix uses custom Vyukov lock-free MPSC queue (why it's fastest)
  - Kameo has dual bounded/unbounded mailbox, default capacity 64
  - Both use vtable dispatch (not Box<dyn Any> downcast)
  - Actix has 256-message assertion guard (validates our budget approach)
- **Stress tests**: 6 new tests
  - `message_ordering_preserved_under_budget` — FIFO order with budget=8
  - `mt_stress_many_senders_one_receiver` — 50 senders × 100 msgs, 4 threads
  - `mt_stress_concurrent_spawn_and_send` — 200 concurrent spawn+send, 4 threads
  - `mt_chain_spawning_under_load` — 50-level chain across 2 workers
  - `mt_panic_isolation_under_load` — 10 panicking + 10 healthy actors, 4 threads
  - `sustained_throughput_does_not_drop_messages` — 10 batches × 100 msgs
- **Benchmarks**: 2 new benchmark groups
  - `msg_size`: throughput and send_latency by message size (8B-4KB)
  - `contention`: fanin (1-100 senders to 1 sink), cross_worker (1-4 threads)
- **Result**: 51 tests pass (42 original + 3 fairness + 6 stress), all workspace compiles

### Cycle 3: Thread Parking (Adaptive Backoff)
- **Implementation**: Replaced `thread::sleep` with `thread::park_timeout` in worker run loop
  - Workers register `thread::current()` via `OnceLock<Thread>` on startup
  - `send_to` and `spawn` call `Thread::unpark()` on target worker
  - Cross-worker sends from `WorkerContext` also unpark target
  - Zero new dependencies (uses `std::sync::OnceLock` + `std::thread::park_timeout`)
- **Design source**: Tokio's parker state machine, Linux NO_HZ adaptive ticks
- **Benefits**: Parked workers wake instantly when work arrives (vs waiting for sleep timer)
  - Reduces idle-to-active latency from up to 1ms to near-zero
  - No overhead on hot path — `unpark()` is no-op if thread isn't parked
- **Tests**: 1 new test (`mt_parked_worker_wakes_on_send`)
- **Result**: 52 tests pass (51 + 1 new), all workspace compiles

### Research Notes
- Full analysis in `CLAUDE/notes/research_synthesis.md`
- Baseline benchmarks in `CLAUDE/notes/baseline_benchmarks.md`
- Constraints in `CLAUDE/notes/constraints.md`

## Next Steps
- [ ] **Cycle 2: Stress testing + property-based tests**
  - Concurrent spawn+send stress tests
  - Multi-threaded fairness validation
  - Property: message ordering preserved under budget
  - Property: all messages eventually delivered with budget > 0
- [x] **Cycle 3: Adaptive backoff with thread parking** ✅
- [ ] **Cycle 4: Enhanced benchmarks**
  - Message size sensitivity (8B, 64B, 256B, 1KB)
  - Latency percentiles (p50, p99, p999)
  - Many-to-one fanin contention
  - Cross-worker vs same-worker delivery comparison
- [ ] **Cycle 5: Work stealing exploration**
  - Evaluate feasibility of actor migration between workers
  - BEAM two-tier approach: reactive steal + periodic migration

## Open Questions
- Should budget be configurable per-actor (not just per-runtime)?
- Is 64 the right default budget? Benchmarks show budget=32 slightly faster for throughput
- Thread parking: how to handle the notification mechanism without adding deps?

## Blockers
- (none)