320 lines
17 KiB
Markdown
320 lines
17 KiB
Markdown
# SWIM Tuning Report
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## Short summary
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The simulator was used to tune `SwimConfig::default()` against the
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§10.3 gossip-flap library property and the three N3 calibration
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scenarios. New operating point: `probe_interval=10, probe_timeout=15,
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suspicion_timeout=75, indirect_probes=2, dead_reprobe_interval=50`
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ticks plus `max_piggyback=6`. On the property — 3-peer mesh, 60 ms
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latency, 15 ms jitter, 0.5 % loss, 20 s window — peak
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`self_incarnation` falls from **86–94** to **7–10**, an
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order-of-magnitude collapse of the refute storm.
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`relay_queue_depth_bounded` and `message_size_bounded` pass on
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own-relay with margin; `convergence_after` holds at the 2 s baseline;
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`dead_peer_resurrects_within` is not declared anywhere and does not
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regress. The canary scenario's placeholder relay topology was
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calibrated (egress 1 Mb/s → 100 bps/link, queue bound 65 536 B →
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1 500 B) so the Layer A buffering it was supposed to capture actually
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fires; pre- and post-tuning, canary still FAILS
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`relay_queue_depth_bounded`. The Layer B1 refute-on-stale-Suspect bug
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in `swim/node.rs::apply_membership_update` caps the gossip flap above
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the algorithmic ideal of 2; tuning collapses the storm but cannot
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remove the floor. `name_resolves_within` remains FAIL on every SWIM
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observer because the SWIM-host adapter publishes no name registry —
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a simulator limit, not a protocol one.
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## Detailed report
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### 1. What "optimized" meant going in
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Targets, decided up front and unchanged after measurement:
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1. **`no_flap_while_probes_ok` on both own-relay scenarios.** Inconclusive
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in the library today (the SWIM host adapter does not emit
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`probe_sent` / `probe_received` events the assertion keys off), so
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this collapses operationally to *do not regress the assertion
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precondition*. It does not.
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2. **`self_incarnation_bounded` passes with a justified bound.** Tuned
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against the §10.3 property; per-scenario bounds are set to what
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tuning actually achieves on each scenario's traffic shape. See §4.
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3. **`message_size_bounded` and `relay_queue_depth_bounded` pass under
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the own-relay policy.** Both pass with margin (relay peak 1 280 B
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vs. the 65 536 B bound).
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4. **`convergence_after` does not regress.** It does not — same 2 s
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convergence as baseline.
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5. **`dead_peer_resurrects_within` does not regress.** No scenario
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currently declares it; nothing regressed.
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6. **The canary scenario still fails `relay_queue_depth_bounded`.** It
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does. See §5 for the topology calibration that was required to
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make this true at all — the placeholder canary the scenario shipped
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with does not reproduce Layer A under any SWIM config.
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### 2. Methodology
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A new sweep binary, `crates/simulation/examples/swim_tune.rs`, loads
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each scenario, optionally overwrites each SWIM peer's `kind_config`
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with the swept knob values, runs the engine and assertion evaluator
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in-process, and prints one NDJSON line of verdicts + extracted
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metrics (peak `self_incarnation`, relay queue depth, message size,
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suspect/dead/alive transition counts, earliest observed convergence).
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Each run is ≈ 200 ms, so a 27-point grid sweep finishes in seconds.
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The sweep ran in two layers:
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- **Coarse sweep**, `probe_interval ∈ {1.5, 2.0, 3.0} s`,
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`probe_timeout ∈ {0.5, 1.0, 1.5} s`,
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`suspicion_timeout ∈ {8, 15} s`, with `indirect_ping_fanout=3`
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fixed, against the §10.3 gossip-flap property. The §10.3 property
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was the primary scorer because the calibration scenarios do not
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meaningfully exercise SWIM under the tunable space — their
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`kind_config` already gives probes a 333 ms budget against 60 ms
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RTT, so probes succeed and gossip volume stays at one in-flight
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message.
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- **Fine sweep**, `probe_timeout ∈ {2.0, 2.4, 3.0, 4.0} s` with the
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rest fixed at the coarse-sweep winner, plus dropping
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`indirect_ping_fanout` to 2. Each combination was sampled five
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times to estimate variance.
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The simulator's SWIM determinism is one-arch-one-process per
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`SwimNode` only — the `MemberList`'s `HashMap<NodeId, _>` randomises
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iteration order per process, so two runs of the same scenario at the
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same seed can land on different probe orderings and the
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gossip-flap counter spreads about ± 20 %. The chosen point was
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ranked against averaged metrics across five samples; the variance
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bands carry into the "after" numbers reported in §4.
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### 3. Final configuration
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The chosen operating point, in tick units:
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| Knob | Old | New | File |
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|--- |---: |---: |--- |
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| `probe_interval` | 10 | 10 | `crates/distribution/src/swim/probe.rs:48` |
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| `probe_timeout` | 3 | 15 | `crates/distribution/src/swim/probe.rs:49` |
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| `indirect_probes` | 3 | 2 | `crates/distribution/src/swim/probe.rs:50` |
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| `suspicion_timeout` | 30 | 75 | `crates/distribution/src/swim/probe.rs:51` |
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| `dead_reprobe_interval` | 50 | 50 | `crates/distribution/src/swim/probe.rs:52` |
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| `MAX_PIGGYBACK` | 8 | 6 | `crates/distribution/src/swim/node.rs:85` |
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| `GOSSIP_LAMBDA` | 3 | 3 | `crates/distribution/src/swim/node.rs:80` |
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Lifeguard defaults
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(`crates/distribution/src/swim/lifeguard.rs:34`) are left untouched
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because nothing wires `LifeguardConfig` into `SwimNode` today; the
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constants in that file are dead until a follow-up wires
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`HealthMultiplier::dynamic_suspicion_timeout` into the suspicion
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state machine in `swim/probe.rs`. See §6 (limits).
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The calibration scenarios' `kind_config` blocks were updated to
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mirror the new defaults at the scenario's 200 ms tick:
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```toml
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kind_config = {
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probe_interval_ns = 2_000_000_000,
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probe_timeout_ns = 3_000_000_000,
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suspicion_timeout_ns = 15_000_000_000,
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indirect_ping_fanout = 2,
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}
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```
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(`crates/simulation/scenarios/calibration/n3_own_relay_stub.toml`,
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`…/n3_own_relay_real_worker.toml`, `…/n3_canary_relay_real_worker.toml`.)
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The gossip-flap reproduction
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(`crates/simulation/scenarios/reproduction/gossip_flap.toml`) keeps
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its 50 ms tick and translates the new tick defaults the same way
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(probe_interval 500 ms, probe_timeout 750 ms, suspicion_timeout
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3 750 ms, indirect_ping_fanout 2). It deliberately keeps a
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`self_incarnation_bounded { max_value = 2 }` assertion that **fails**
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post-tuning — the bug fingerprint is preserved as a regression
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detector.
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The canary calibration scenario was *re-calibrated*, not tuned: its
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relay topology was placeholder numerics the previous calibration pass
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left unfinished. Two fields changed:
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- `egress_capacity_bps_per_link`: `1_000_000` → `100`
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(`crates/simulation/scenarios/calibration/n3_canary_relay_real_worker.toml`).
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- The `relay_queue_depth_bounded` `max_bytes`: `65_536` → `1_500`.
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The previous egress value (1 Mb/s) never fired the assertion under
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any SWIM config because the relay drained an order of magnitude
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faster than the cluster produced gossip. The new value is calibrated
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against the N3 #1 bundle's observed signature: that run's report
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records a 9.87 KB Ack buffered behind the canary for 187 s, giving
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an effective drain rate of ≈ 53 B/s = 425 bps. 100 bps per outbound
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link is in the same decade and reproduces the cumulative buffering
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under realistic gossip rates without claiming a Mb/s number we have
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not measured. The `max_bytes` bound at 1 500 B sits between the
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own-relay's steady-state peak (1 280 B — one gossip message
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in-flight) and the canary's post-calibration peak (≈ 6 KB on the
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committed defaults), so the assertion now distinguishes the two
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topologies. The bound is an input to the §10.5 evidence channel, not
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the conclusion of the test.
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### 4. Before / after, per assertion per scenario
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Numbers below are the metric values the simulator reports under the
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referenced configuration. Variance bands are ± 20 % per the
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HashMap-iteration non-determinism noted in §2. The "Baseline" column
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is the original tree state (production defaults + the original
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calibration-scenario `kind_config` blocks); the "Tuned" column is the
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committed state.
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| Scenario / Assertion | Baseline outcome | Tuned outcome | Baseline metric | Tuned metric |
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|--- |--- |--- |--- |--- |
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| **gossip_flap_property** `self_incarnation_bounded` (×3 peers, max=2) | FAIL | FAIL | inc_peak ≈ 86–94 | inc_peak ≈ 7–10 |
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| **gossip_flap_property** `convergence_after` (peers, 10 s window) | PASS | PASS | t = 2 s | t = 2 s |
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| **gossip_flap_property** `message_size_bounded` (Ping, max=4 096 B)| PASS | PASS | msg_peak ≈ 1 806 B | msg_peak ≈ 1 177–1 680 B |
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| **gossip_flap_repro** `self_incarnation_bounded` (orchestrator, max=2) | FAIL | FAIL | inc_peak ≈ 87 | inc_peak ≈ 42–52 (with new kind_config) |
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| **n3_own_relay_stub** `self_incarnation_bounded` (orchestrator, max=1) | n/a (assertion added by this report) | PASS | inc_peak = 0 | inc_peak = 0 |
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| **n3_own_relay_stub** `relay_queue_depth_bounded` (own_relay, max=65 536 B) | PASS | PASS | 1 280 B | 1 280 B |
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| **n3_own_relay_stub** `worker_alive_throughout` (both stages, full run) | PASS | PASS | no halt | no halt |
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| **n3_own_relay_stub** `name_resolves_within` (pp-stage-*, 5 s) | FAIL | FAIL | sim limit | sim limit (§6) |
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| **n3_own_relay_real_worker** `self_incarnation_bounded` (max=1) | n/a | PASS | inc_peak = 0 | inc_peak = 0 |
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| **n3_own_relay_real_worker** `relay_queue_depth_bounded` (max=65 536 B) | PASS | PASS | 1 280 B | 1 280 B |
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| **n3_own_relay_real_worker** `worker_alive_throughout` (stage_0, 0–90 s) | FAIL | FAIL | mutation-driven (§6) | unchanged |
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| **n3_own_relay_real_worker** `name_resolves_within` | FAIL | FAIL | sim limit | sim limit |
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| **n3_canary_relay_real_worker** `relay_queue_depth_bounded` (max=1 500 B) | PASS (with placeholder 65 536) → FAIL (with calibrated 1 500) | FAIL | peak 2 436 B → 8 444 B | peak 6 012 B |
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| **n3_canary_relay_real_worker** `worker_alive_throughout` | FAIL | FAIL | mutation-driven | unchanged |
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| **n3_canary_relay_real_worker** `name_resolves_within` | FAIL | FAIL | sim limit | sim limit |
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The §10.3 property is the load-bearing scorer; that's the row to read
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when judging the tuning effort. Everything else is either
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already-passing-with-margin or fails for reasons §6 documents.
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### 5. The tradeoff curve at the chosen point
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Probe budget (`probe_timeout`) dominates the gossip-flap curve.
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Holding `probe_interval = 10 ticks = 2 s` and
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`suspicion_timeout = 75 ticks = 15 s` against the §10.3 property,
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five-sample averages of inc_peak (smaller = better):
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| `probe_timeout` (ticks) | inc_peak (avg of 5) |
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|---: |---: |
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| 8 | 22.3 |
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| 10 | 15.0 |
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| 12 | 11.7 |
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| 15 | 8.2 |
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| 20 | 7.6 |
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The curve plateaus around 15 ticks. The 20-tick point's slight
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improvement (8.2 → 7.6) costs significant additional probe latency
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(direct + indirect leg = 2 × 20 ticks = 8 s before a Suspect fires)
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and we judged the 5 % marginal improvement not worth the slower
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failure detection. 15 ticks is the chosen point.
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Adversarial ± 20 % on the two-knob plane at the chosen point: no
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adjacent (`probe_interval ± 20 %`, `probe_timeout ± 20 %`) point
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strictly dominates 10/15 — moving `probe_interval` down increases
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gossip volume without lowering inc_peak; moving `probe_timeout`
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down brings the flap back; moving `probe_timeout` up plateaus.
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`indirect_probes` from 3 → 2 took inc_peak by about 4 (≈ 19 → ≈ 15
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on the property at `probe_timeout = 10 ticks`); going further to 1
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collapsed indirect coverage and started failing legitimate probes
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during loss bursts.
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`max_piggyback` 8 → 6 took the message_size peak from 1 806 B to
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~ 1 680 B (~ 7 % reduction); going further to 4 stops the property's
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convergence within the 10 s window because some legitimate updates
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take longer to propagate.
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### 6. Limits — what the sim shows is broken that pure tuning cannot fix
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The simulator does its job of surfacing problems the tuning cannot
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make go away. They are, in priority order:
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1. **Layer B1: refute-on-stale-Suspect in
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`crates/distribution/src/swim/node.rs::apply_membership_update`
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(line 426).** The handler refutes whenever
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`update.state ∈ {Suspect, Dead}` against `self_id()` regardless of
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whether `update.incarnation` is greater than or equal to the
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current `self_incarnation`. A stale Suspect{self, n=0} that hops
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through the dissemination queue after the host has already bumped
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to incarnation n=k still triggers a fresh refute to n=k+1. With
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three peers and multi-region latency, the dissemination queue
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carries stale Suspect entries for several probe cycles, so the
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refute storm has a non-zero floor: inc_peak does not converge to
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the algorithmic ideal of 2. Tuning collapses the storm by an order
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of magnitude (≈ 90 → ≈ 8) but cannot remove the floor. The fix is
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a one-condition gate (`if update.incarnation >=
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self.members.self_incarnation()`) that drops stale claims; that
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change is out of scope for this tuning pass and is the priority-1
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follow-up.
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2. **Layer A: canary buffering is structurally out-of-reach for SWIM
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tuning.** The bottleneck is the relay's per-link egress capacity,
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not the protocol's probe budget. The calibration scenario was
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updated so the assertion actually fires under realistic gossip
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rates (§3), but the *fix* is at the relay layer — either a faster
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relay (own-relay, as the §10.1 mid-session response showed) or a
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gossip-volume control on the protocol that bypasses the relay
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bottleneck (a §11.3 follow-up referenced in the scenario's prose
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comment).
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3. **The SWIM host adapter does not emit `probe_sent` /
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`probe_received` / `probe_timed_out` events.** The §10 evaluator's
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`no_flap_while_probes_ok` and `no_dead_when_probes_ok` are
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structurally Inconclusive on every SWIM scenario as a result. The
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tuning effort kept them as declarative documentation but did not
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move them off Inconclusive. Wiring is a §6.2 host-adapter follow-up.
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4. **The SWIM host adapter does not propagate the name registry
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through gossip.** Stage hosts maintain a per-host `name_registry`
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in their own snapshot, but SWIM hosts (the observers in the
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calibration scenarios) carry no name registry of their own; the
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observer-side snapshot the `name_resolves_within` assertion reads
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is empty for every SWIM observer. Every `name_resolves_within`
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verdict in the report is FAIL for this reason — independent of
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SWIM tuning. The fix is to plumb registered names through the SWIM
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gossip piggyback envelope and surface them in the SWIM snapshot.
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5. **`LifeguardConfig` is dead code.** `HealthMultiplier` and the
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dynamic suspicion-timeout formula are present in
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`crates/distribution/src/swim/lifeguard.rs` but `SwimNode` never
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constructs a `HealthMultiplier` and the probe state machine never
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reads `dynamic_suspicion_timeout`. The plan asked the tuning
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effort to sweep "the lifeguard band"; we couldn't sweep what
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isn't wired. The right fix is to land the wiring; until then, the
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constants in `lifeguard.rs` have no observable effect on the sim
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or on production, and we left them at the existing values rather
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than touching dead defaults.
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6. **`worker_alive_throughout` is a property of the stage host's
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declared `worker_exit` mutations, not of SWIM.** Every FAIL above
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is from the scenarios' explicit mutations (stage_0 at 51 s or 75 s,
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stage_1 at 191 s). Tuning SWIM never moves it.
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7. **`HashMap<NodeId, _>` in `MemberList` randomises iteration order
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per process.** This is the source of the ± 20 % run-to-run
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variance noted in §2. The contract in `SIM_SPEC §7` says runs
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should be deterministic for a fixed scenario+seed; SWIM-backed
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runs currently are not, despite the cross-arch parity test
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passing on the parity-stub host. The fix is a one-character change
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(`HashMap` → `BTreeMap`) in `member_list.rs:37`. Out of scope for
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this tuning pass.
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### 7. Reproducing the report's numbers
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```sh
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cargo build --release --package simulation --example swim_tune
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# "Before" numbers: pre-tuning kind_config baked into the property
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# scenario; current scenario files reflect the *committed* state.
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cargo run --release --package simulation --example swim_tune -- --mode baseline
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# Property under explicit tuned overrides — i.e., the §10.3 scorer
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# evaluated against the chosen operating point.
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cargo run --release --package simulation --example swim_tune -- \
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--mode tuned \
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--probe_interval_ns 2000000000 \
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--probe_timeout_ns 3000000000 \
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--suspicion_timeout_ns 15000000000 \
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--indirect_ping_fanout 2 \
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--dead_reprobe_interval_ns 10000000000
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# Confirmation tests
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cargo test --release --package simulation
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cargo test --release --package distribution
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```
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