runtime: wake_slot default ON (RFC 005 accepted, finding 18); supervisor stops survivors sequentially so reverse-order teardown is guaranteed; deflake gen_server/channel tests; baseline.json regenerated slot-on; ROADMAP notes

ROADMAP.md

@@ -28,6 +28,15 @@ and **excised** (not worth the code cost; preserved on branch
 `rfc-004-spinning`). Also a false-sharing fix (`align(64)` on `SchedulerStats`)
 and a termination wake for idle siblings.
 Commits `2708042`, `37d9319`, `eddf3fe`.
+**Default flipped ON 2026-08-18** (history.md findings 17/18): the slot had
+shipped default-off "until the shootout accepts it" and the flip was never
+made, so every general.rs number since was slot-off. Acceptance sweep
+(rq_runtime, 1/2/4/8/20 schedulers, rq-mpmc, 7 runs): ping-pong-pairs
+−7% at 1T, 3.6×/5.3×/15.6×/10× faster at 2/4/8/20T, 100% slot hits, 0
+displaced; yield-storm and spawn-storm within ±10% noise both ways.
+general.rs on the flip: ping_pong_steady 20T 18485→1549 µs, 1T −15%,
+mpsc_contention 1T −51%, ping_pong_oneshot 20T −18%; no smarm regressions.
+`benches/baseline.json` regenerated slot-on.
 
 ---
 
@@ -146,7 +155,11 @@ Needs an RFC.
 #### Per-switch cost (context shims, epoch protocol)
 The shootout's residual: per-wake latency is 0.16–0.18 µs at N=1 and
 0.8–1.2 µs at N=8+, dominated by the context-switch shims and the epoch
-protocol, not the queue. On current evidence this is the larger constant —
+protocol, not the queue. **Premise corrected 2026-08-18 (finding 17): the
+N=8+ figure was the slot-off futex path (one futex_wake per landed wake,
+woken worker steals the pair); slot-on it is ~1.3× N=1. The shims are ~5
+cycles (finding 3) and the epoch CASes ~117 cycles/roundtrip (finding 15).
+Re-measure slot-on before spending anything here.** On current evidence this is the larger constant —
 "the whole game" alongside the v0.9 work — but there is no spec yet. Needs a
 profiling spike (where do the cycles actually go per park/unpark round-trip)
 and then an RFC before it can be scheduled.

benches/baseline.json

@@ -1,366 +1,366 @@
 {
+  "catch_unwind_panics": {
+    "smarm 1-thread": {
+      "result": 10000,
+      "median": 112187,
+      "min": 111086,
+      "max": 121834
+    },
+    "smarm 20-thread": {
+      "result": 10000,
+      "median": 20374,
+      "min": 19939,
+      "max": 21381
+    },
+    "tokio current_thread": {
+      "result": 10000,
+      "median": 10425,
+      "min": 10338,
+      "max": 10795
+    },
+    "tokio multi-thread": {
+      "result": 10000,
+      "median": 2420,
+      "min": 2309,
+      "max": 5657
+    }
+  },
   "chained_spawn": {
     "smarm 1-thread": {
       "result": 1000,
-      "median": 449,
+      "median": 454,
-      "min": 440,
+      "min": 443,
-      "max": 452
+      "max": 473
     },
     "smarm 20-thread": {
       "result": 1000,
-      "median": 2102,
+      "median": 1884,
-      "min": 1936,
+      "min": 1866,
-      "max": 2226
+      "max": 2124
     },
     "tokio current_thread": {
       "result": 1000,
-      "median": 109,
+      "median": 110,
-      "min": 108,
+      "min": 109,
-      "max": 125
+      "max": 124
     },
     "tokio multi-thread": {
       "result": 1000,
-      "median": 186,
+      "median": 172,
-      "min": 156,
+      "min": 160,
-      "max": 211
+      "max": 177
-    }
-  },
-  "yield_many": {
-    "smarm 1-thread": {
-      "result": 200000,
-      "median": 10563,
-      "min": 10181,
-      "max": 10589
-    },
-    "smarm 20-thread": {
-      "result": 200000,
-      "median": 43218,
-      "min": 42523,
-      "max": 43864
-    },
-    "tokio current_thread": {
-      "result": 200000,
-      "median": 5258,
-      "min": 5251,
-      "max": 5864
-    },
-    "tokio multi-thread": {
-      "result": 200000,
-      "median": 5583,
-      "min": 5512,
-      "max": 6563
-    }
-  },
-  "fan_out_compute": {
-    "smarm 1-thread": {
-      "result": 33860,
-      "median": 15030,
-      "min": 14584,
-      "max": 15039
-    },
-    "smarm 20-thread": {
-      "result": 33860,
-      "median": 2362,
-      "min": 2274,
-      "max": 2426
-    },
-    "tokio current_thread": {
-      "result": 33860,
-      "median": 14040,
-      "min": 14037,
-      "max": 14042
-    },
-    "tokio multi-thread": {
-      "result": 33860,
-      "median": 2386,
-      "min": 2342,
-      "max": 2418
-    }
-  },
-  "ping_pong_oneshot": {
-    "smarm 1-thread": {
-      "result": 1000,
-      "median": 805,
-      "min": 789,
-      "max": 829
-    },
-    "smarm 20-thread": {
-      "result": 1000,
-      "median": 7503,
-      "min": 7005,
-      "max": 7677
-    },
-    "tokio current_thread": {
-      "result": 1000,
-      "median": 410,
-      "min": 396,
-      "max": 440
-    },
-    "tokio multi-thread": {
-      "result": 1000,
-      "median": 8911,
-      "min": 8844,
-      "max": 9198
-    }
-  },
-  "spawn_storm_busy": {
-    "smarm 1-thread": {
-      "result": 10000,
-      "median": 103743,
-      "min": 103453,
-      "max": 104373
-    },
-    "smarm 20-thread": {
-      "result": 10000,
-      "median": 15364,
-      "min": 14982,
-      "max": 15642
-    },
-    "tokio current_thread": {
-      "result": 10000,
-      "median": 1071,
-      "min": 1063,
-      "max": 1079
-    },
-    "tokio multi-thread": {
-      "result": 10000,
-      "median": 12183,
-      "min": 11551,
-      "max": 12852
-    }
-  },
-  "mpsc_contention": {
-    "smarm 1-thread": {
-      "result": 320000,
-      "median": 6711,
-      "min": 6703,
-      "max": 6722
-    },
-    "smarm 20-thread": {
-      "result": 320000,
-      "median": 35331,
-      "min": 34287,
-      "max": 36335
-    },
-    "tokio current_thread": {
-      "result": 320000,
-      "median": 6207,
-      "min": 5495,
-      "max": 6278
-    },
-    "tokio multi-thread": {
-      "result": 320000,
-      "median": 72132,
-      "min": 66649,
-      "max": 75892
-    }
-  },
-  "many_timers": {
-    "smarm 1-thread": {
-      "result": 10000,
-      "median": 117002,
-      "min": 103516,
-      "max": 118009
-    },
-    "smarm 20-thread": {
-      "result": 10000,
-      "median": 53418,
-      "min": 52807,
-      "max": 53785
-    },
-    "tokio current_thread": {
-      "result": 10000,
-      "median": 12444,
-      "min": 12389,
-      "max": 12483
-    },
-    "tokio multi-thread": {
-      "result": 10000,
-      "median": 13948,
-      "min": 13791,
-      "max": 14268
-    }
-  },
-  "multi_thread_scaling": {
-    "smarm 1-thread": {
-      "result": 33860,
-      "median": 15036,
-      "min": 15029,
-      "max": 15060
-    },
-    "smarm 2-thread": {
-      "result": 33860,
-      "median": 7987,
-      "min": 7965,
-      "max": 8027
-    },
-    "smarm 4-thread": {
-      "result": 33860,
-      "median": 4299,
-      "min": 4269,
-      "max": 4309
-    },
-    "smarm 20-thread": {
-      "result": 33860,
-      "median": 2381,
-      "min": 2321,
-      "max": 2409
-    },
-    "tokio multi 1-thread": {
-      "result": 33860,
-      "median": 14300,
-      "min": 14213,
-      "max": 14386
-    },
-    "tokio multi 2-thread": {
-      "result": 33860,
-      "median": 7356,
-      "min": 7308,
-      "max": 7408
-    },
-    "tokio multi 4-thread": {
-      "result": 33860,
-      "median": 3725,
-      "min": 3714,
-      "max": 3728
-    },
-    "tokio multi 20-thread": {
-      "result": 33860,
-      "median": 2402,
-      "min": 2383,
-      "max": 2421
     }
   },
   "deep_recursion": {
     "smarm 1-thread": {
       "result": 1,
-      "median": 291,
+      "median": 289,
-      "min": 288,
+      "min": 283,
-      "max": 323
+      "max": 290
     },
     "smarm 20-thread": {
       "result": 1,
-      "median": 673,
+      "median": 683,
-      "min": 627,
+      "min": 632,
-      "max": 740
+      "max": 725
     },
     "tokio current_thread": {
       "result": 1,
-      "median": 14,
+      "median": 12,
-      "min": 13,
+      "min": 11,
-      "max": 17
+      "max": 12
     },
     "tokio multi-thread": {
       "result": 1,
-      "median": 53,
+      "median": 51,
-      "min": 50,
+      "min": 47,
-      "max": 54
+      "max": 56
     }
   },
-  "yield_in_hot_loop": {
+  "fan_out_compute": {
     "smarm 1-thread": {
-      "result": 1000000,
+      "result": 33860,
-      "median": 30361,
+      "median": 15040,
-      "min": 29592,
+      "min": 15026,
-      "max": 30509
+      "max": 15050
+    },
+    "smarm 20-thread": {
+      "result": 33860,
+      "median": 2365,
+      "min": 2324,
+      "max": 2393
     },
     "tokio current_thread": {
-      "result": 1000000,
+      "result": 33860,
-      "median": 69408,
+      "median": 14097,
-      "min": 67891,
+      "min": 14087,
-      "max": 70101
+      "max": 14102
-    }
-  },
-  "uncontended_channel": {
-    "smarm 1-thread": {
-      "result": 1000000,
-      "median": 11962,
-      "min": 11943,
-      "max": 12052
     },
-    "tokio current_thread": {
+    "tokio multi-thread": {
-      "result": 1000000,
+      "result": 33860,
-      "median": 14664,
+      "median": 2390,
-      "min": 14584,
+      "min": 2142,
-      "max": 14702
+      "max": 2827
     }
   },
-  "catch_unwind_panics": {
+  "many_timers": {
     "smarm 1-thread": {
       "result": 10000,
-      "median": 121767,
+      "median": 113255,
-      "min": 120583,
+      "min": 105504,
-      "max": 122123
+      "max": 118516
     },
     "smarm 20-thread": {
       "result": 10000,
-      "median": 19950,
+      "median": 53400,
-      "min": 19260,
+      "min": 53293,
-      "max": 20565
+      "max": 54901
     },
     "tokio current_thread": {
       "result": 10000,
-      "median": 10184,
+      "median": 12443,
-      "min": 10037,
+      "min": 12402,
-      "max": 10660
+      "max": 13170
     },
     "tokio multi-thread": {
       "result": 10000,
-      "median": 2430,
+      "median": 13701,
-      "min": 2339,
+      "min": 13374,
-      "max": 2628
+      "max": 13913
+    }
+  },
+  "mpsc_contention": {
+    "smarm 1-thread": {
+      "result": 320000,
+      "median": 3303,
+      "min": 3271,
+      "max": 3391
+    },
+    "smarm 20-thread": {
+      "result": 320000,
+      "median": 35379,
+      "min": 34372,
+      "max": 36334
+    },
+    "tokio current_thread": {
+      "result": 320000,
+      "median": 6034,
+      "min": 5428,
+      "max": 6037
+    },
+    "tokio multi-thread": {
+      "result": 320000,
+      "median": 63753,
+      "min": 59875,
+      "max": 77970
+    }
+  },
+  "multi_thread_scaling": {
+    "smarm 1-thread": {
+      "result": 33860,
+      "median": 15115,
+      "min": 15102,
+      "max": 15209
+    },
+    "smarm 2-thread": {
+      "result": 33860,
+      "median": 8002,
+      "min": 7972,
+      "max": 8061
+    },
+    "smarm 4-thread": {
+      "result": 33860,
+      "median": 4284,
+      "min": 4255,
+      "max": 4327
+    },
+    "smarm 20-thread": {
+      "result": 33860,
+      "median": 2402,
+      "min": 2313,
+      "max": 2414
+    },
+    "tokio multi 1-thread": {
+      "result": 33860,
+      "median": 14310,
+      "min": 14254,
+      "max": 14602
+    },
+    "tokio multi 2-thread": {
+      "result": 33860,
+      "median": 7374,
+      "min": 7248,
+      "max": 7567
+    },
+    "tokio multi 4-thread": {
+      "result": 33860,
+      "median": 3734,
+      "min": 3724,
+      "max": 3749
+    },
+    "tokio multi 20-thread": {
+      "result": 33860,
+      "median": 2390,
+      "min": 2195,
+      "max": 2411
+    }
+  },
+  "ping_pong_oneshot": {
+    "smarm 1-thread": {
+      "result": 1000,
+      "median": 796,
+      "min": 789,
+      "max": 814
+    },
+    "smarm 20-thread": {
+      "result": 1000,
+      "median": 6118,
+      "min": 5849,
+      "max": 6493
+    },
+    "tokio current_thread": {
+      "result": 1000,
+      "median": 422,
+      "min": 405,
+      "max": 426
+    },
+    "tokio multi-thread": {
+      "result": 1000,
+      "median": 8983,
+      "min": 8906,
+      "max": 9037
     }
   },
   "ping_pong_steady": {
     "smarm 1-thread": {
       "result": 10000,
-      "median": 1427,
+      "median": 1209,
-      "min": 1419,
+      "min": 1191,
-      "max": 1437
+      "max": 1222
     },
     "smarm 20-thread": {
       "result": 10000,
-      "median": 18485,
+      "median": 1549,
-      "min": 17162,
+      "min": 1523,
-      "max": 18897
+      "max": 1584
     },
     "tokio current_thread": {
       "result": 10000,
-      "median": 1285,
+      "median": 1283,
-      "min": 1281,
+      "min": 1279,
-      "max": 1287
+      "max": 1303
     },
     "tokio multi-thread": {
       "result": 10000,
-      "median": 79958,
+      "median": 82303,
-      "min": 77547,
+      "min": 81303,
-      "max": 82155
+      "max": 83249
     }
   },
   "spawn_pair_control": {
     "smarm 1-thread": {
       "result": 1000,
-      "median": 632,
+      "median": 558,
-      "min": 624,
+      "min": 546,
-      "max": 634
+      "max": 584
     },
     "smarm 20-thread": {
       "result": 1000,
-      "median": 5017,
+      "median": 4918,
-      "min": 4707,
+      "min": 4417,
-      "max": 5149
+      "max": 5022
     },
     "tokio current_thread": {
       "result": 1000,
-      "median": 287,
+      "median": 292,
-      "min": 277,
+      "min": 280,
-      "max": 335
+      "max": 323
     },
     "tokio multi-thread": {
       "result": 1000,
-      "median": 8621,
+      "median": 8561,
-      "min": 8415,
+      "min": 8546,
-      "max": 8828
+      "max": 9121
+    }
+  },
+  "spawn_storm_busy": {
+    "smarm 1-thread": {
+      "result": 10000,
+      "median": 105394,
+      "min": 97447,
+      "max": 106921
+    },
+    "smarm 20-thread": {
+      "result": 10000,
+      "median": 15479,
+      "min": 15041,
+      "max": 16061
+    },
+    "tokio current_thread": {
+      "result": 10000,
+      "median": 1237,
+      "min": 1114,
+      "max": 1248
+    },
+    "tokio multi-thread": {
+      "result": 10000,
+      "median": 12354,
+      "min": 11581,
+      "max": 15660
+    }
+  },
+  "uncontended_channel": {
+    "smarm 1-thread": {
+      "result": 1000000,
+      "median": 12085,
+      "min": 12020,
+      "max": 12162
+    },
+    "tokio current_thread": {
+      "result": 1000000,
+      "median": 14840,
+      "min": 14763,
+      "max": 15051
+    }
+  },
+  "yield_in_hot_loop": {
+    "smarm 1-thread": {
+      "result": 1000000,
+      "median": 30730,
+      "min": 30542,
+      "max": 30859
+    },
+    "tokio current_thread": {
+      "result": 1000000,
+      "median": 70423,
+      "min": 65167,
+      "max": 70699
+    }
+  },
+  "yield_many": {
+    "smarm 1-thread": {
+      "result": 200000,
+      "median": 10872,
+      "min": 10697,
+      "max": 10936
+    },
+    "smarm 20-thread": {
+      "result": 200000,
+      "median": 44358,
+      "min": 43982,
+      "max": 44790
+    },
+    "tokio current_thread": {
+      "result": 200000,
+      "median": 5312,
+      "min": 4751,
+      "max": 5478
+    },
+    "tokio multi-thread": {
+      "result": 200000,
+      "median": 5832,
+      "min": 5533,
+      "max": 6951
     }
   }
 }

src/runtime.rs

@@ -182,7 +182,7 @@ impl Config {
             stack_reserve: DEFAULT_STACK_RESERVE,
             stack_guard: DEFAULT_STACK_GUARD,
             max_actors: DEFAULT_MAX_ACTORS,
-            wake_slot: false,
+            wake_slot: true,
             node_id: crate::pg::DEFAULT_NODE_ID,
             incarnation: crate::pg::DEFAULT_INCARNATION,
         }
@@ -205,7 +205,7 @@ impl Config {
             stack_reserve: DEFAULT_STACK_RESERVE,
             stack_guard: DEFAULT_STACK_GUARD,
             max_actors: DEFAULT_MAX_ACTORS,
-            wake_slot: false,
+            wake_slot: true,
             node_id: crate::pg::DEFAULT_NODE_ID,
             incarnation: crate::pg::DEFAULT_INCARNATION,
         }
@@ -283,7 +283,7 @@ impl Config {
     /// thread's slot; it is resumed next on that core and inherits the
     /// remainder of the waker's timeslice. Scheduler-context wakes
     /// (timer/IO drain) and spawns always go to the shared queue.
-    /// Default: `false` (off until the slot shootout accepts it).
+    /// Default: `true` (accepted 2026-08-18, history.md finding 17/18).
     pub fn wake_slot(mut self, on: bool) -> Self {
         self.wake_slot = on;
         self
@@ -333,7 +333,7 @@ impl Default for Config {
             stack_reserve: DEFAULT_STACK_RESERVE,
             stack_guard: DEFAULT_STACK_GUARD,
             max_actors: DEFAULT_MAX_ACTORS,
-            wake_slot: false,
+            wake_slot: true,
             node_id: crate::pg::DEFAULT_NODE_ID,
             incarnation: crate::pg::DEFAULT_INCARNATION,
         }

tests/channel.rs

@@ -137,11 +137,19 @@ fn channel_ops_interleaved_with_monitor_churn_multi_thread() {
         for i in 0..32i64 {
             let tx = tx.clone();
             handles.push(spawn(move || {
-                // Short-lived target whose death fires the monitor below.
+                // Short-lived target whose death fires the monitor below. It
+                // waits for `go` so the monitor is registered before it can
+                // die — otherwise a fast target yields an immediate NoProc
+                // Down instead of the finalize-sent Exit this test is about
+                // (pre-existing ~8% flake at 4 threads, independent of the
+                // wake slot).
+                let (go_tx, go_rx) = channel::<()>();
                 let t = spawn(move || {
+                    let _ = go_rx.recv();
                     tx.send(i).unwrap();
                 });
                 let m = smarm::monitor(t.pid());
+                go_tx.send(()).unwrap();
                 t.join().unwrap();
                 // Down delivery exercises send-from-finalize.
                 let d = m.rx.recv().unwrap();

tests/gen_server.rs

@@ -396,8 +396,11 @@ impl GenServer for Pool {
 }
 
 // A worker spawned and watched from inside a handler delivers its Down to
-// handle_down. Down arms outrank the inbox, so the death is in the log by
+// handle_down. Nothing orders the worker's death before the follow-up
-// the time the follow-up call is answered.
+// call (the worker sits in the shared queue while call/reply wakes ride the
+// wake slot), so poll: the log must become exactly [Panic] within a bounded
+// number of yields. Down-outranks-inbox ordering is covered by
+// `watch_dead_pid_is_noproc_down`.
 #[test]
 fn worker_pool_down_reaches_handle_down() {
     let got = Arc::new(Mutex::new(Vec::new()));
@@ -409,7 +412,15 @@ fn worker_pool_down_reaches_handle_down() {
         });
         server.cast(PoolCast::SpawnDoomedWorker).unwrap();
         let _ = server.call(()).unwrap(); // sync point: cast handled, worker live
-        *got2.lock().unwrap() = server.call(()).unwrap();
+        let mut log = Vec::new();
+        for _ in 0..10_000 {
+            log = server.call(()).unwrap();
+            if !log.is_empty() {
+                break;
+            }
+            smarm::yield_now();
+        }
+        *got2.lock().unwrap() = log;
     });
     assert_eq!(*got.lock().unwrap(), vec![DownReason::Panic]);
 }

tests/wake_slot.rs

@@ -80,10 +80,11 @@ fn slot_off_means_zero_slot_traffic() {
     assert_eq!(rt.stats().slot_hits(), 0);
     assert_eq!(rt.stats().slot_displacements(), 0);
 
-    // …and off by default (RFC 005: default off until the shootout accepts).
+    // …and ON by default (RFC 005 accepted after the shootout — history.md
+    // finding 17/18: ping-pong 12.5× at 20T, ~9% at 1T, 100% slot hits).
     let rt = init(Config::exact(1));
     rt.run(ping_pong(2, 100));
-    assert_eq!(rt.stats().slot_hits(), 0, "wake_slot must default to OFF");
+    assert!(rt.stats().slot_hits() > 0, "wake_slot must default to ON");
 }
 
 // ---------------------------------------------------------------------------