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The loop span through its body regardless of whether there was anything to do -- ~10% of a desktop core for an empty shard, ~70% of a small VPS core, and a process that never idles is exactly what burstable vCPU plans throttle. The loop now blocks in NetState.WaitForCompletion whenever every queue it drains is empty, waking on the next timer tick or the moment work arrives. Receive completions, new connections, and cross-thread LoopContext.Post (via IORingGroup 1.0.10'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 server.eventLoopIdleWaitMs (default 2ms, 0 = never sleep). Measured on a real world of 190k items / 33k mobiles: 10.4% of a core to 0.8-1.0%, with peak tick lag unchanged. Spin mode independently gained 7x the iterations per core from the ring's AcceptEx rework. Sleeping also gives the GC natural pause points, which the old spin loop denied it -- memory no longer climbs until a save forces a collection. A sleep is bounded by the next wheel turn, so a correctly honoured sleep can never miss a deadline; the only way sleeping harms the wheel is the host returning the wait late. That overshoot is measured on every sleep, and an escalating backoff (server.lateWakeThreshold) suspends sleeping when it persists -- server work like saves or heavy commands cannot trip it by construction. Hosts without high-resolution waits are detected once at startup and spin instead. The admin gump shows the verdict instead of the now-meaningless CPS figure, which is removed. Time accounting for diagnosis is compiled out of normal builds: build with -p:EventLoopProfiling=true to enable EventLoopProfiler (per-phase wall time, sleep overshoot, GC pauses, stolen-time residual, ~15min ring buffer) and the [LoopStats command with CSV dump. See dev-docs/debugging-event-loop.md for the diagnosis funnel. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
120 lines
6.2 KiB
Markdown
120 lines
6.2 KiB
Markdown
# Server Requirements
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Hardware guidance for running a ModernUO shard.
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## Tiers
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| Use | vCPU | RAM | Storage |
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|---|---|---|---|
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| Development / test | 2 **dedicated** | 2 GB | SSD |
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| Small live shard (< 50 concurrent) | 4 dedicated | 4 GB | NVMe |
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| Medium (50–200) | 4–8 | 8 GB | NVMe |
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| Large (200+) | 8+, high clock | 16 GB+ | NVMe |
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These are starting points. Save size drives RAM more than player count does, and single-thread
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clock speed drives tick latency more than core count does. Both are explained below.
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## Dedicated vCPU, not burstable
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This matters more than any other line on this page.
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Budget VPS plans sold as "2 vCPU" are frequently shared or burstable: you get a CPU credit balance
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or a cgroup quota, and once it is exhausted the hypervisor throttles you. Throttling shows up in
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game as periodic freezes that correlate with nothing in your logs, and it is the single most common
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cause of "ModernUO is laggy on my $3/month VPS".
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Symptoms worth checking before blaming the server:
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- Steal time above ~1% (`top`, the `%st` column on Linux)
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- Lag that disappears when you move to a larger plan with the same core count
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- Tick lag spikes with no matching CPU spike in the process itself
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## Cores
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Game logic is **single-threaded**. Every mobile, item, timer, and packet handler runs on one
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thread, so a shard's headroom is bounded by how fast one core is. Two fast cores beat four slow
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ones.
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Cores beyond the first are used by:
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- **World saves.** `world.useMultithreadedSaves` (default on) spins up `ProcessorCount - 1`
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serialization workers plus one inline on the main thread. On a 2-core box that is one worker; on
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a 2-core box with a large world, consider setting it to `false` so saves do not contend with the
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loop.
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- **The .NET runtime.** Tiered JIT compilation (heaviest in the first minutes after boot) and
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background GC.
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- **Everything else on the machine**, including your OS and, on Windows, antivirus.
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Since ModernUO 2026 the loop sleeps when idle, so an empty shard costs roughly 1% of a core rather
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than spinning. That change disproportionately helps small hosts.
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## Memory
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Three things dominate, and only one of them scales with players.
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**World size.** A world of ~190,000 items and ~33,000 mobiles loads in about a second and is not
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itself large. Items and mobiles are the cheap part.
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**Saves.** Each serialization worker pre-allocates a heap sized to its share of the last save, at
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roughly 1.25× total save size, and those buffers are retained afterwards. A 400 MB save therefore
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implies about 500 MB of resident serialization heap on top of the live world. **This is the reason
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1 GB hosts are not viable for a real shard**, even though an empty one boots fine.
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**Map residency.** `TileMatrix` reads map blocks from disk on demand and caches them permanently —
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there is no eviction. Memory climbs toward full-facet residency as players explore. Felucca's land
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tiles alone are around 117 MB, and statics are larger.
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Optional systems can add substantially more. The pathfinding prebake
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(`pathfinding.prebakeMaps`) peaks above 1 GB of heap while baking. Budget for it or leave it off on
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small hosts.
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Network buffers are minor by comparison: 64 KB receive plus a configurable 256 KB send
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(`network.sendBufferSize`) per connection, so 100 players is roughly 32 MB.
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ModernUO runs **Workstation GC**, which is the right default for small hosts. Do not switch to
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Server GC on a 2-core box.
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## Storage
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Saves are write-heavy bursts. Cheap network-attached storage with throttled IOPS will stall the
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save path, and `World.WaitForWriteCompletion` blocks the loop at shutdown. Use local NVMe or SSD.
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Budget disk for: the world save, plus archives and backups if `autoArchive` is enabled (retention
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defaults keep 24 hourly, 30 daily, and 12 monthly copies), plus the pathfinding cache if enabled.
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## Operating systems
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See the README for the full supported list. Two things are worth calling out:
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- **Windows Server 2012 R2 and 2016 sleep via a raised timer resolution.** Sleeping for a couple
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of milliseconds prefers a high-resolution waitable timer, which requires Windows 10 1803 /
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Server 2019. On older versions the ring falls back to `timeBeginPeriod(1)`, which raises the
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system timer resolution to 1 ms so the plain wait timeout is accurate enough. The trade-off is a
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higher interrupt rate (system-wide on those versions) — an acceptable price on a dedicated game
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server, and the reason the high-resolution timer is preferred where it exists.
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Only if *both* mechanisms fail does the server detect it at startup, log it, and spin instead —
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the same behaviour as setting `server.eventLoopIdleWaitMs` to 0: a full core at idle, and zero
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missed deadlines. A host that claims short waits but cannot deliver them is caught at runtime by
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the adaptive backoff.
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- **Linux kernel 6.1** or newer (Debian 12 and equivalents). io_uring is used where available, with
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automatic epoll fallback.
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## Tuning for a small host
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| Setting | Default | Why change it |
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|---|---|---|
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| `server.eventLoopIdleWaitMs` | `2` | `0` never sleeps: ~98% of one core, but zero skipped timer slots and zero lag. The choice for a large shard on dedicated CPU that would rather spend a core than risk a late wake. Above `2` the wheel starts losing slots. |
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| `server.lateWakeThreshold` | `1` | Idle waits the host may return a full tick late, per second, before idle sleeping backs off. Raise on a jittery host; set very high to disable the backoff. |
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| `world.useMultithreadedSaves` | `true` | Set `false` on 2-core hosts so saves do not contend with the game loop. |
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| `pathfinding.prebakeMaps` | varies | Leave off on memory-constrained hosts; it peaks above 1 GB while baking. |
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| `network.sendBufferSize` | 256 KB | Lower it if you are memory-bound with many connections. |
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| `autoArchive.*` retention | 24h/30d/12m | Reduce if disk is tight. |
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## Am I undersized?
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Watch the log. The server warns when the host returns idle waits late and suspends idle sleeping,
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and says so at startup if the host cannot honour short waits at all. Those warnings mean the host
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is not scheduling the process promptly — typical of burstable or shared vCPU plans — and no
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server-side change fixes that. For anything deeper, see
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[debugging-event-loop.md](debugging-event-loop.md).
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