The late-wake detector suspended idle sleeping on stable hosts, which both
spammed a Warning and cost the shard several seconds of full-core spin each
time for no reason. The log level was the visible symptom; the detector was
the bug.
Three causes, all fixed:
Lateness was a bare count, not a rate. An idle loop performs ~400-500 sleeps
a second (2ms each, bounded by the 8ms wheel tick), and the trip condition
was more than one late wake per second across two consecutive samples. That
is a 0.4% tail-outlier rate -- reachable by a co-tenant burst, a page fault,
or another process changing the system timer resolution. A host that
genuinely cannot schedule the process returns *most* of its waits late, two
orders of magnitude away. Gate on the proportion (server.lateWakePercent,
default 10) and keep server.lateWakeThreshold as a floor for windows with
few sleeps, where a percentage means nothing.
GC pauses were charged to the host. The GC collects preferentially during
idle sleeps -- that is the natural pause point it looks for, as
dev-docs/debugging-event-loop.md already documents -- so its pauses landed
in the measurement by design. Sample GC.CollectionCount(1) either side of
the wait and skip the sample if a collection intervened. The second read
short-circuits behind the overshoot test, so the common path pays for one
counter read per sleep.
Every backoff logged at Warning. Tier it to the escalation that already
existed: Debug for the first two (recoverable, not actionable), Warning once
the host has survived several doublings, and the existing Error at the
ceiling. Adds an Information line when a clean streak clears an escalation.
Also moves the BackoffResetAfterCleanMs reset to run on every health sample
rather than only on the path to a new backoff, where it was unreachable for
a host that recovered for good -- such a host never cleared its escalation
or re-armed the ceiling Error.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
## Problem
`RunEventLoop` span through its body regardless of whether there was anything to do — ~10% of a desktop core for an empty shard, and ~70% of a core on a 3 vCPU VPS. A process that never idles is exactly what burstable vCPU plans throttle, which is how this surfaced: lag spikes that went away when the operator bought more cores. The spin also denied the GC its natural pause points, so memory climbed until a world save forced a collection — alarming in task manager, harmless in practice, and a recurring source of "is my server leaking?" reports.
## Result
Windows desktop, real world of **190,728 items / 33,158 mobiles**, no players, saves and prebake off, three consecutive runs:
| | Legacy spin | Idle sleeping |
|---|---|---|
| **CPU** | 10.42 – 10.50% of one core | **0.78 – 1.00%** |
| **Tick lag** (peak/15s) | 4–10 ms | 5–11 ms |
**~10× less CPU with tick lag unchanged** — the CPU came free rather than being traded for latency. Slower hosts gain proportionally more. Spin mode (`server.eventLoopIdleWaitMs=0`) independently gained **7× the iterations per core** (1.19M → 8.3M cycles/sec) from the ring's AcceptEx rework.
## How
The loop blocks in `NetState.WaitForCompletion` whenever every queue it drains is empty (all the drains are bounded, so leftovers keep it awake). Receive completions, new connections, and cross-thread `LoopContext.Post` (via the ring's sticky `Wake()`) are all in the wait set, so sleeping adds no latency to any of them. Only timer-driven logic sees wheel lag, bounded by the idle wait.
**Health is measured at the only place sleeping can cause harm.** A sleep is bounded by the time to the next wheel turn, so a correctly honoured sleep can never miss a deadline — the only failure mode is the host returning the wait late. That overshoot is measured on every sleep (one extra timestamp read; production's entire accounting cost), and an escalating backoff suspends sleeping when it persists. By construction, server work — saves, heavy staff commands, deep timer callbacks — cannot trip it, so the warning means exactly one thing: *the host is not scheduling the process promptly*, with two known remedies (dedicated CPU, or `=0`). Hosts with no high-resolution wait mechanism at all are detected once at startup and spin instead.
**CPS is removed.** `Core.CyclesPerSecond`/`AverageCPS` measured nothing actionable before and became actively misleading once the loop sleeps (the rate is set by the sleep, not by shard health). The admin gump's Performance page now shows the verdict instead: `Healthy` / `Sleep suspended (host)` / `Spinning (configured)`.
## Configuration
| Setting | Default | Meaning |
|---|---|---|
| `server.eventLoopIdleWaitMs` | `2` | Longest idle block. Measured across 1/2/4/8 ms, 2 is where the trade stops being free. `0` = never sleep: ~98% of a core, zero scheduling overhead — for large shards on dedicated CPU. |
| `server.lateWakeThreshold` | `1` | Idle waits the host may return a full tick late, per second, before sleeping backs off. Raise for jittery hosts; very high disables the backoff. |
## Diagnostics (compiled out by default)
`dotnet build -p:EventLoopProfiling=true` compiles in `EventLoopProfiler` — every hook is `[Conditional("EVENT_LOOP_PROFILING")]`, so normal builds contain zero profiling IL. The profiling build decomposes each second of wall time into **work (per loop phase) / sleep / GC pause / stolen residual**, keeps ~15 minutes of history in a ring buffer, and the `[LoopStats` command prints the last minute and dumps the full history to CSV. `dev-docs/debugging-event-loop.md` is the diagnosis guide (for humans and AI): what production already tells you, when to flip the profiling build, the signature table for host-steal vs deep-processing vs GC vs wake bugs, why dotnet-trace comes last, and the GC/RAM "leak" misconception.
## Verification
- 815 Server.Tests green; both build configurations compile.
- Docker echo harness green on epoll and io_uring (ping-pong mode); kqueue verified manually on an M1 Max.
- A/B measurements and per-change numbers: `measure/event-loop` branch.
## Notes
The full measurement harness and vendored ring sources used to develop this live on the [`measure/event-loop`](https://github.com/modernuo/ModernUO/tree/measure/event-loop) branch, kept for future loop work.
Kamron Batman