ModernUO/dev-docs/debugging-event-loop.md

Debugging Event Loop Performance

How to diagnose "the server feels slow" — written for both humans and AI assistants. Follow the funnel in order; most incidents resolve before the last step. Do not start with dotnet-trace.

The model

Every second of the main thread's wall time goes to exactly one of four places:

1. Work — the loop's phases: mobile deltas, item deltas, timer callbacks (Timer.Slice), network processing (NetState.Slice), posted tasks (LoopContext), world snapshots (WorldSnapshot — the on-loop portion of a save).
2. Sleep — idle blocking in NetState.WaitForCompletion, bounded by the next timer tick and server.eventLoopIdleWaitMs.
3. GC pauses — land inside whichever phase (or sleep) was running.
4. Stolen — the host ran something else: hypervisor scheduling, noisy neighbors, CPU credit throttling.

A sleep is bounded by the time to the next wheel turn, so a correctly honoured sleep can never cost a deadline. The only way sleeping harms the game is the wait returning late — that is stolen time, and the server measures it directly on every sleep.

Step 0 — Read what production already tells you

No build changes needed. Three signals exist, all actionable:

Signal	Meaning	Action
Startup error: host cannot honour short waits	No high-resolution timer and timeBeginPeriod failed. Very old or unusual Windows.	Nothing is wrong with the server; it spins and uses a full core. Upgrade the OS or accept the core.
Warning: host returned a Nms idle wait late … for the Nth time running	The OS did not reschedule the process promptly after a 1–2ms wait, through several escalating backoffs. Shared/burstable vCPU signature.	Move to dedicated CPU, or set server.eventLoopIdleWaitMs=0 to spin permanently. This is a host problem — no amount of server-side change fixes it.
Error: keeps returning idle waits late and sleeping has backed off N times	The escalation hit its 120s ceiling. The host is not going to recover.	As above, but stop waiting for it to settle. Logged once per degradation, re-armed after a clean minute.
Admin gump → Performance → Event Loop	Healthy / Sleep suspended (host) / Spinning (configured) / Spinning - host cannot honor short waits	Same as above; the last verdict is the startup error's state, not a config choice.

The first two backoffs of any episode log at Debug, not Warning: a single suspension is recoverable and not something an operator can act on. Raise the log level if you are chasing a marginal host and want to see them. Late wakes that coincide with a gen1-or-higher GC are not counted at all — the GC deliberately collects during idle sleeps, so its pauses land there by design and are not the host's fault.

If none of these fired and the shard still feels laggy, the cause is work, GC, or something a boot-time signal cannot see. Continue.

Step 1 — Flip the profiling build

dotnet build -p:EventLoopProfiling=true

This compiles in EventLoopProfiler (Server) and the [LoopStats command (UOContent). Without the flag every hook call site is removed by the compiler ([Conditional]), so there is nothing to "turn off" in normal builds and no cost to leave the hooks in the code. The profiling build's own overhead is a handful of timestamp reads per iteration — small enough to run for days while hunting an intermittent problem.

Capture a baseline first. Run [LoopStats while the shard feels fine and keep the CSV. The profiler also keeps ~15 minutes of history in memory, so if the problem is episodic you can wait for an episode and the good minutes on either side are already recorded. Numbers without a baseline are how RunUO's profiler became useless — always compare bad minutes to good minutes on the same box, build, and world.

Step 2 — Read the decomposition

[LoopStats prints the last minute and writes the full history CSV (one row per second). Match the shape against these signatures:

Signature	Diagnosis	Next step
One phase consistently hot (e.g. TimerSlice 40%/s)	Deep processing in that subsystem	Step 3 — find the culprit in that phase
All phases near zero, stolen high, lateWakes > 0	Host is stealing CPU	Host problem; see step 0 actions
gcPauseMs high, gen2 counts rising	GC pressure — something is allocating heavily	Step 3 on the allocating phase, or dotnet-counters for alloc rate
Iterations ≫ sleeps while shard is idle	The loop is not sleeping: a queue never drains or a wake storm	Check IsIdle inputs; a stuck signal in the ring is the historical example
Sleeps ≈ iterations, each sleep ~0ms	Spurious wake storm	Ring backend issue; count wakesIssued vs actual cross-thread posts
Everything normal, complaint persists	Not the event loop	Look at the network path, client, or DB/save timing

Wheel lag vs player lag: wheelLagMaxMs is how late timer callbacks fired. Receives are handled the moment they arrive (they wake the loop), so player-felt lag with a clean wheel points away from the loop entirely.

Step 3 — Find the culprit inside a hot phase

Add a temporary culprit hook rather than reaching for a tracer. The pattern: same [Conditional("EVENT_LOOP_PROFILING")] attribute, own file or the profiler file, record only the worst offender per second (identity + duration), never a per-event log. Examples:

• TimerSlice hot → time each timer callback, keep the max and its timer.ToString().
• NetworkSlice hot → time packet handlers by packet id, keep the max.
• GC pressure → dotnet-counters monitor --counters System.Runtime for alloc rate first; it is cheap and often names the culprit generation without a trace.

Keep the hook after the hunt if it earns its cost in the profiling build; delete it otherwise.

Step 4 — dotnet-trace, last and targeted

Only when a hot phase resists the culprit hook. Know the costs: EventPipe visibly slows the process (worst exactly when things are already bad) and adds artifacts to the trace — on small vCPU hosts the tracer's own threads appear as hotspots and Rider/PerfView hotspot views can mislead. Mitigate by being narrow:

• Trace the specific minutes the decomposition flagged, not "a while".
• dotnet-trace collect --profile cpu-sampling --duration 00:00:30 is usually enough.
• Compare against a trace of a good minute (same rule as step 1: no baseline, no conclusions).

The RAM / GC misconception (read before declaring a leak)

ModernUO allocates very little, and the GC collects opportunistically — mostly during idle sleeps and world saves. Under a spinning loop (eventLoopIdleWaitMs=0, or the pre-2026 default) the GC may find no natural pause point: memory climbs to a large fraction of physical RAM, a forced collection eventually drops part of it, and fragmentation keeps the baseline permanently above where it started. Task manager shows alarming numbers; the in-game numbers do not. Performance is unaffected — this is lazy collection working as designed, not a leak. Idle sleeping largely removes the effect because every sleep is a natural GC opportunity. Before investigating "a leak": check gen0/1/2 and gcPauseMs in the decomposition, and compare working set after a world save, which forces the collection the spin loop never allowed.

Rules of thumb

• Never trade always-on profiling for the numbers. Production carries one timestamp per sleep and nothing else; everything heavier lives behind the build flag or on the measure/event-loop branch (full harness, A/B scripts, vendored ring experiments).
• One decomposition chart beats a thousand log lines. Resist adding warnings the reader cannot act on; the three production signals are deliberate.
• When filing or reporting: attach the baseline CSV and the episode CSV. Relative statements ("TimerSlice went from 4% to 61% during the episode") are the useful form.