[1.20.1] Server freeze / timeout when loading specific chunk region (caused by dungeon\_difficulty structure checks)

[RockZe]

Hello, I have a reproducible server freeze related to dungeon_difficulty.
Environment: Fabric 1.20.1, ~450 mods, multiplayer server (30–40 players).

Problem

• When any player approaches chunk region (-180,64) to (-162,68) the server completely freezes and disconnects clients with a timeout.
• Deleting entities and tile entities in these chunks with MCA Selector does not solve the issue.
• Even deleting and regenerating the chunks still reproduces the problem.

Diagnostics

• Running /spark profiler --timeout 60 during the freeze shows the main thread blocked in dungeon_difficulty logic.

• The stacktrace points to structure pattern matching: calls into net.minecraft.server.level.ServerChunkCache / ChunkMap and dungeon_difficulty structure scanning.

• Example excerpt from the captured stack:

  java.lang.Thread.run
   net.minecraft.server.MinecraftServer.tickChildren
   net.minecraft.server.level.ServerChunkCache.getChunk
   net.minecraft.server.level.ChunkMap.getChunk
   net.minecraft.world.level.chunk.ChunkStatus.generate
   modernfix.structure.CachingStructureManager.readStructureTag
   dungeon_difficulty.structure.StructureDifficultyHandler.matchStructures
   dungeon_difficulty.structure.StructureDifficultyHandler.updateChunkDifficulty
  

• Disabling the mod dungeon_difficulty immediately resolves the freeze; the region becomes playable again.

• Other structure mods present (BetterDungeons, BetterMineshafts, BetterStrongholds, RespawningStructures, etc.), but only dungeon_difficulty consistently triggers the lockup.

Config

Even with a stock config (dungeon_difficulty_v2.json), the issue persists. Customizing values (perPlayerDifficulty, entity matchers, zones) does not change behavior — the server still freezes as soon as dungeon_difficulty is active.

Expected

Dungeon Difficulty should not hard-freeze the server during chunk load. At worst, it should skip invalid or unexpected structure data.

Request

Could you please investigate:

• Why structure scanning in dungeon_difficulty hangs indefinitely in certain regions?
• Is there a way to disable structure-based difficulty logic entirely, and only use per-dimension or per-player scaling?
• Could the structure matcher run asynchronously or with a timeout, so it cannot lock up the main server thread?

Thanks for your work — this mod is very useful for RPG balancing, but this bug makes some areas of the world completely unplayable.

[RockZe]

From personal observation, downloading regions through chunky with Dungeon Difficulty is almost 2 times slower than without it.

On average:

With Dungeon Difficulty - Rate: 2.3 cps

Without Dungeon Difficulty - Rate: 5.2 cps

Maybe it's because of my configuration, I'll attach it below. I tried to make the most hardcore enhancements, but maybe I went a little overboard :)

dungeon_difficulty_v2.json

[ZsoltMolnarrr]

Hello,

The 1.20.1 version is archived, players can use it as is.

I recommend updating to 1.21.1 where this mod (and the whole ecosystem) works much better. :)