Highly Performant & Scalable Ultima Online Server Emulator
https://www.modernuo.com
c-sharpcsharpcsharp-coredotnetdotnet-coreemulatorgplgplv3mmommo-enginemmorpgmmorpg-serveronlinerpgrunuoserverservuoultimaultima-onlineuo
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## Problem The late-wake detector added in #2559 suspends idle sleeping on perfectly healthy hosts. The visible symptom is this Warning firing periodically on stable machines: > This host returned a 2ms idle wait at least 8ms late 2 time(s) in the last second; idle sleeping suspended for 5000ms Demoting it to Debug would hide the symptom but not the cost: every one of those lines means the shard dropped idle sleeping for 5s and burned a full core for no reason. The detector is what was mis-tuned. ## Cause 1 — lateness was a count, not a rate An idle loop performs **~400–500 sleeps per second** (2ms each, bounded by the 8ms wheel tick). The trip condition was `late > 1` across two consecutive one-second samples — a **0.4% tail-outlier rate**. A co-tenant burst, a page fault, or another process changing the system timer resolution clears that bar on a healthy host. A host that genuinely cannot schedule the process — throttled burstable vCPU — returns *most* of its waits late. Signal and noise were two orders of magnitude apart, and the check sat in the noise. Now gated on the proportion, with the absolute count kept as a floor: ```csharp if (late <= _lateWakeThreshold) { _consecutiveBadSamples = 0; return; } // floor if (late * 100 < sleeps * _lateWakePercent) { _consecutiveBadSamples = 0; return; } // rate ``` New `server.lateWakePercent` (default `10`). The floor is what keeps a window with only a handful of sleeps from tripping on a meaningless percentage; `server.lateWakeThreshold` keeps its existing meaning. ## Cause 2 — GC pauses were charged to the host `dev-docs/debugging-event-loop.md` already documents that the GC collects preferentially **during idle sleeps** — that is the natural pause point it looks for. So the detector was systematically measuring the GC's chosen pause point and billing it to the host's scheduler. Not an occasional coincidence; a designed-in one. ```csharp var collections = GC.CollectionCount(1); NetState.WaitForCompletion(requested); ... if (elapsed - requested >= Timer.TickRate && GC.CollectionCount(1) == collections) ``` Gen1 (which counts gen2 with it) rather than gen0 — gen0 pauses don't approach the 8ms `TickRate` bar anyway, and gating on them would discard useful samples. The second read short-circuits behind the overshoot test, so the common path costs **one** `GC.CollectionCount` per sleep: an internal counter read, single-digit nanoseconds, ~500/sec. ## Cause 3 — every backoff logged at Warning Tiered to the escalation that already existed, since a single suspension is recoverable and not something an operator can act on: | Backoff | Level | |---|---| | 1–2 | `Debug` | | 3–5 | `Warning` (now includes the sleep count and "for the Nth time running") | | ceiling | `Error`, unchanged | | recovery | `Information` (new) | Each backoff doubles the suspension, so every line is already a distinct escalation step — no further rate limiting needed. ## Drive-by The `BackoffResetAfterCleanMs` reset only ran on the path to a *new* backoff, making it unreachable for a host that recovered for good — such a host never cleared its escalation or re-armed `_loggedBackoffCeiling`. It now runs on every health sample, which is also what makes the new recovery line reachable. ## Testing Full solution builds clean, 0 warnings. No tests added: the state is private static in `Core` coupled to `_tickCount` with no injection point, and nothing covered it before — adding a seam purely to test it seemed worse than the gap. Happy to add one if reviewers disagree. |
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| LICENSE | ||
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| README.md | ||
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ModernUO 
Ultima Online Server Emulator for the modern era!
Requirements
Supported Operating Systems
Required Frameworks
All Operating Systems
Windows
Hardware
| 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 |
Game logic is single-threaded, so single-core clock speed matters more than core count, and dedicated vCPU matters more than either — burstable or shared plans throttle once credits run out, which is the most common cause of unexplained lag spikes. Save size drives RAM more than player count does.
See dev-docs/server-requirements.md for the reasoning and tuning options.
Development
Supported IDEs
Getting Started
- Install prerequisite requirements
- Clone this repository (or download the latest):
git clone https://github.com/modernuo/ModernUO.git
- Open
ModernUO.slnto start developing
Building/Publishing
Interactive Mode (Recommended for new users)
Run ./publish.cmd (Windows) or ./publish.sh (Linux/macOS) with no arguments to launch the guided build tool. It will:
- Check prerequisites (.NET SDK, native libraries)
- Walk you through configuration and platform selection
- Build and publish the server to the
Distributiondirectory - Show deployment instructions for cross-compiled builds
Command Line
./publish.cmd [release|debug] [os] [arch]
os- Supported operating systemsarchx64- Intel/AMD 64-bitarm64- ARM 64-bit
Linux Prerequisites
Fedora, CentOS, RHEL, AlmaLinux, Rocky, etc
dnf upgrade --refresh -y
# RHEL-family distributions (CentOS Stream, AlmaLinux, Rocky, RHEL) need CRB and EPEL enabled.
# Fedora skips this block.
dnf install -y dnf-plugins-core
dnf config-manager --set-enabled crb
dnf install -y epel-release
# Prerequisites
dnf install -y findutils libicu libdeflate libargon2 tzdata
Ubuntu, Debian, etc
apt-get update -y
# The ICU runtime package carries the ABI version in its name (libicu74, libicu76, …) and has no
# stable alias, so match it by pattern rather than pinning a release-specific name.
apt-get install -y '^libicu[0-9]+$' libdeflate0 libargon2-1 tzdata
Only the runtime libraries are needed — the -dev/-devel packages are not. Run
./build-tool --check-prereqs to check the current machine and print the exact packages your
release needs.
zstd is not listed because ZstdNet bundles libzstd for every platform, and liburing is not
listed because IORingGroup issues io_uring syscalls directly.
If the shard's configured time zone is a legacy alias such as US/Eastern, Debian 12 and Ubuntu
24.04 also need tzdata-legacy. See Platform Prerequisites
for what each dependency is for and what breaks without it.
OSX Requirements
brew install icu4c libdeflate argon2
Running the Server
- Follow the publish instructions
- The
Distributiondirectory is portable — copy it to your production server for deployment - Run
ModernUO.exeordotnet ModernUO.dllfrom theDistributiondirectory - On first run, the server will prompt you to configure game data file locations
Troubleshooting / FAQ
- See FAQ
Want to sponsor?
Thank you for supporting us! You can find out how by visiting the sponsors page.
Collaborators
Thanks
- RunUO Team & Community
- Voxpire, the ServUO Team & Community
- Karasho, Jaedan and the ClassicUO Community
Development Tools & Plugins provided with ♥ by
Code Signing Policy
Free code signing provided by SignPath.io, certificate by SignPath Foundation.
This program will not transfer any information to other networked systems unless specifically requested by the user or the person installing or operating it
Teams & Roles
Approvers & Committers: Development Team