ModernUO/dev-docs/server-requirements.md

Server Requirements

Hardware guidance for running a ModernUO shard.

Tiers

Use	vCPU	RAM	Storage
Development / test	2 dedicated	2 GB	SSD
Small live shard (< 50 concurrent)	4 dedicated	4 GB	NVMe
Medium (50–200)	4–8	8 GB	NVMe
Large (200+)	8+, high clock	16 GB+	NVMe

These are starting points. Save size drives RAM more than player count does, and single-thread clock speed drives tick latency more than core count does. Both are explained below.

Dedicated vCPU, not burstable

This matters more than any other line on this page.

Budget VPS plans sold as "2 vCPU" are frequently shared or burstable: you get a CPU credit balance or a cgroup quota, and once it is exhausted the hypervisor throttles you. Throttling shows up in game as periodic freezes that correlate with nothing in your logs, and it is the single most common cause of "ModernUO is laggy on my $3/month VPS".

Symptoms worth checking before blaming the server:

• Steal time above ~1% (top, the %st column on Linux)
• Lag that disappears when you move to a larger plan with the same core count
• Tick lag spikes with no matching CPU spike in the process itself

Cores

Game logic is single-threaded. Every mobile, item, timer, and packet handler runs on one thread, so a shard's headroom is bounded by how fast one core is. Two fast cores beat four slow ones.

Cores beyond the first are used by:

• World saves. world.useMultithreadedSaves (default on) spins up ProcessorCount - 1 serialization workers plus one inline on the main thread. On a 2-core box that is one worker; on a 2-core box with a large world, consider setting it to false so saves do not contend with the loop.
• The .NET runtime. Tiered JIT compilation (heaviest in the first minutes after boot) and background GC.
• Everything else on the machine, including your OS and, on Windows, antivirus.

Since ModernUO 2026 the loop sleeps when idle, so an empty shard costs roughly 1% of a core rather than spinning. That change disproportionately helps small hosts.

Memory

Three things dominate, and only one of them scales with players.

World size. A world of ~190,000 items and ~33,000 mobiles loads in about a second and is not itself large. Items and mobiles are the cheap part.

Saves. Each serialization worker pre-allocates a heap sized to its share of the last save, at roughly 1.25× total save size, and those buffers are retained afterwards. A 400 MB save therefore implies about 500 MB of resident serialization heap on top of the live world. This is the reason 1 GB hosts are not viable for a real shard, even though an empty one boots fine.

Map residency. TileMatrix reads map blocks from disk on demand and caches them permanently — there is no eviction. Memory climbs toward full-facet residency as players explore. Felucca's land tiles alone are around 117 MB, and statics are larger.

Optional systems can add substantially more. The pathfinding prebake (pathfinding.prebakeMaps) peaks above 1 GB of heap while baking. Budget for it or leave it off on small hosts.

Network buffers come from four pools that grow and shrink with the population rather than being sized for a full shard. A connection that has not yet presented valid credentials holds a 4 KB receive and a 4 KB send buffer (the platform's page size). On Windows Server 2012 R2 / 2016 the transport's legacy mapping path floors at 64 KB: the pre-auth receive pool is off there (its base is 64 KB), while the pre-auth send buffer starts at 64 KB under the 256 KB base. Everything the server sends before that must fit in that ring — a connection that overruns it is dropped; the stock login sequence uses under 2 KB. Otherwise, when the game server verifies the account the connection is promoted to a 64 KB receive buffer and a network.sendBufferSize send buffer from the base pools, and nothing ever moves back. A flood of unauthenticated connections tops out at about 32 MB across the full 4096-connection cap where the platform minimum is 4 KB (the transport's retained slabs and the base pools used by logged-in players are separate), and never allocates a base-pool slab. At boot the network holds network.initialBufferSlabs slab(s) of each pool — at the defaults one 2 MB receive slab, one 8 MB send slab and two 128 KB pre-auth slabs, about 10 MB — and allocates another slab only when the population needs one. Each slab covers 32 connections at the 4096-connection maximum. After 15 quiet minutes idle slabs are trimmed back towards current usage, never past the last 15 minutes' peak, at one slab per pool per minute and never below network.initialBufferSlabs. Only the newest slab is trimmed, and buffers are handed out from the oldest slab first, so ordinary churn empties the newest slabs; a shard that drops from 4096 players to a handful takes about two hours to shrink fully, longer if a long-lived connection still holds a buffer in a newer slab.

Send memory per authenticated connection is network.sendBufferSize at rest and can grow to network.sendBufferMaxSize under load. Shared send-buffer tier memory is capped by network.sendBufferGrowthBudget, and growth is refused when process memory exceeds network.memoryCeilingPercent of available memory. The worst case is the receive and base send-buffer sizes times the number of logged-in connections, plus the shared growth budget: a full 4096 logged-in connections is roughly 1.25 GB of base buffers, and the growth budget can add up to another 256 MB.

ModernUO runs Workstation GC, which is the right default for small hosts. Do not switch to Server GC on a 2-core box.

Storage

Saves are write-heavy bursts. Cheap network-attached storage with throttled IOPS will stall the save path, and World.WaitForWriteCompletion blocks the loop at shutdown. Use local NVMe or SSD.

Budget disk for: the world save, plus archives and backups if autoArchive is enabled (retention defaults keep 24 hourly, 30 daily, and 12 monthly copies), plus the pathfinding cache if enabled.

Operating systems

See the README for the full supported list. Two things are worth calling out:

• Windows Server 2012 R2 and 2016 sleep via a raised timer resolution. Sleeping for a couple of milliseconds prefers a high-resolution waitable timer, which requires Windows 10 1803 / Server 2019. On older versions the ring falls back to timeBeginPeriod(1), which raises the system timer resolution to 1 ms so the plain wait timeout is accurate enough. The trade-off is a higher interrupt rate (system-wide on those versions) — an acceptable price on a dedicated game server, and the reason the high-resolution timer is preferred where it exists.

  Only if both mechanisms fail does the server detect it at startup, log it, and spin instead — the same behaviour as setting server.eventLoopIdleWaitMs to 0: a full core at idle, and zero missed deadlines. A host that claims short waits but cannot deliver them is caught at runtime by the adaptive backoff.

• Linux kernel 6.1 or newer (Debian 12 and equivalents). io_uring is used where available, with automatic epoll fallback.

Tuning for a small host

Setting	Default	Why change it
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.
server.lateWakeThreshold	1	Floor for the backoff: idle waits the host may return a full tick late, per second, before the rate test below applies at all. Raise on a jittery host; set very high to disable the backoff.
server.lateWakePercent	10	Share of a second's idle waits that must come back late before idle sleeping backs off. An idle loop sleeps hundreds of times a second, so a bare count cannot tell a few tail outliers from a host that never schedules the process — a genuinely bad host misses most of its waits. 0 leaves lateWakeThreshold in sole charge.
world.useMultithreadedSaves	true	Set false on 2-core hosts so saves do not contend with the game loop.
pathfinding.prebakeMaps	varies	Leave off on memory-constrained hosts; it peaks above 1 GB while baking.
network.sendBufferSize	256 KB	Lower it if you are memory-bound with many connections.
network.sendBufferMaxSize	2 MB (2097152)	Ceiling a single connection's send buffer can grow to under load. Lower it on memory-constrained hosts; raise it if slow clients are disconnected with "send buffer exhausted".
network.sendBufferGrowthBudget	256 MB (268435456)	Cap on the shared memory the larger send-buffer tiers may use. Lower it on memory-constrained hosts.
network.memoryCeilingPercent	80%	Refuse send-buffer growth once the process is above this share of available memory; 0 turns the check off.
network.initialBufferSlabs	1	Slabs of each base pool held from boot, and the floor the trim never goes below. Raise it on a large shard to pre-warm the pools instead of paying for a slab as the population climbs.
network.maxBufferSlabs	128	Divides the connection maximum into base-pool slabs: a slab holds MaxConnections / maxBufferSlabs connections, 32 at the default. Raise it for finer slabs on a small host (the slab floor is 16 buffers); lowering it makes each slab, and the boot allocation, larger. It is not a connection or memory cap — both pools still reach the connection maximum.
autoArchive.* retention	24h/30d/12m	Reduce if disk is tight.

Am I undersized?

Watch the log. The server warns when the host returns idle waits late and suspends idle sleeping, and says so at startup if the host cannot honour short waits at all. Those warnings mean the host is not scheduling the process promptly — typical of burstable or shared vCPU plans — and no server-side change fixes that. For anything deeper, see debugging-event-loop.md.