Hermod

Performance and networking for Valheim dedicated servers. No gameplay changes.

It compresses most of the server's outgoing traffic away, makes two limits the game hardcodes adjustable, and writes a per-minute record of where the server's time actually goes.

Named for Hermod, the swift messenger of the gods, who rode nine nights down to Hel and back to carry word - which is a dedicated server's whole job.

What it does

Packet compression, measured at a 2.8x reduction. On a real server 8.6 MB of outgoing traffic became 3.0 MB, holding around 36% while a player was moving, with no change to frame time or CPU. It uses Zstandard with a dictionary trained on real Valheim traffic, which is what makes it work - the packets are small, and generic compression on a two kilobyte buffer has almost nothing to learn from.

Compression is negotiated per player and needs the mod on both ends. A player without it simply exchanges ordinary packets.

An adjustable ZDO send budget. The game stops sending a player world data once 10240 bytes are in flight to them, and that count includes bytes already on the wire awaiting acknowledgement - so it is really a throughput cap that tightens with distance, roughly 82 KB/s at 100 ms of latency and 33 KB/s at 250 ms. hermod_queue 32768 took the 95th-percentile packet from 16 KB to 64 KB on the test server.

Raise it for a reason, not by taste. Every message shares one ordered stream, so a bigger backlog delays chat and player positions too.

A zone generation budget. ZoneSystem.Update asks for a new zone once for the server plus once per player and discards every result, so one 100 ms tick can generate several zones. Zone generation is the largest single cost on a dedicated server - the worst frame measured was 1493 ms, of which 1481 ms was one location being built. The budget caps how much of a tick goes into it.

Nagle off, and the Steam send rate ceiling raised. The game pins Steam's minimum and maximum send rate to the same number, which switches Steam's own rate estimator off. Hermod leaves the minimum exactly where the game had it and raises only the maximum, so the result is never less than before.

The frame rate is deliberately not capped. The server sends world updates to one player per frame behind a 0.05 second gate, so each player's update rate is 1 / (0.05 + players/fps). Capping a six-player server to 60 fps cuts that from 17.9 a second to 6.7. Hermod leaves it alone and refuses a cap that would push delivery below ten a second for the number of people online.

A ZDO relay rewrite, off by default. The same one-player-per-frame loop means a server that slows down delivers updates more slowly. The relay drives it from a clock instead and serves every player on each pass. It ships off because the case it most improves is a player with real latency; turn it on with numbers from your own server. If frame times get materially worse it turns itself off.

A portal ghost detector. When a player goes through a portal, the server is supposed to tell everyone else to drop the copy of them standing at the old one. Hermod watches whether that actually happened: a player who moved far enough to have teleported, and any other player still holding their character record while they are outside that player's active area, is a client showing a ghost. It is reported in the log by name and counted in the ghosts column.

It does not fix them. Nothing in Hermod changes how the game behaves until there are numbers saying it should, and a fix for this would be a gameplay change. If ghosts stays at zero on your server, there is nothing here to fix.

Installing

Install it on the server. Install it on players too if you want compression, which needs both ends - everything else is server-side.

The commands also need it on the client you type them from: a Valheim dedicated server has no console input, so commands travel to the server over the wire and are checked for admin rights before they run.

What it writes

Three files in BepInEx/config/:

A short summary also goes to the BepInEx log every few minutes.

Percentile columns are histogram bucket edges, so ms_p99 = 16.7 means 99% of frames were at or below 16.7 ms. The _max columns are exact.

teleports counts players who moved more than 64 metres between samples, which is a portal hop rather than running. ghosts is the worst count seen in any one sample that minute, not a total: the same ghost persists across samples, so adding them up would measure how long the server ran rather than how bad the problem is. The log summary also states how many players it is watching and the furthest anyone moved, so a zero can be read as "nothing wrong" rather than "not looking".

extradata_ratio climbing while zdo_total stays flat is the leak fixZdoExtraDataLeak closes.

Commands

Settings

BepInEx/config/gbv.valheim.hermod.json. The two switches in the BepInEx config are Enabled and MeasureOnly.

A config file that will not parse leaves whatever was already in force alone, rather than reading as empty.

The flags column in the session log records what was live each minute: N Nagle, R send rate, F frame cap, Z relay, C compression, Q send budget raised, G zone budget.

Running alongside other mods

Run one networking mod, not two. If Hermod detects another it stands down from the send rate, the Nagle delay, the relay and compression and says so in the log; measurement carries on. Two mods framing the same packet would produce corrupt traffic rather than a clean failure, which is why the stand-down is automatic.

If a Valheim update moves one of the game internals Hermod hooks, it names it in the log and carries on with everything that does not need it. Reading the log for Hermod: once after an update is worth doing.

Credits

Built for Guys Being Vikings.

Packet compression is adapted from CW-Jesse's BetterNetworking, by way of LabodiDavid's Valheim 1.0 fork, under the MIT licence - including the Zstandard dictionary trained on real Valheim traffic, which is the part that makes compression worth doing. Full notice in THIRD-PARTY-NOTICES.md, licence in LICENSE-BetterNetworking.txt.

Zstandard itself comes from ZstdSharp.Port by Oleg Stepanischev.

Everything else was written from Valheim's own decompiled code.