Excessive lag due to FastCraft handling hopper activity #222
Description
Server and plugin version:
Paper version 1.21.11-112-main@d10c312 (2026-02-10T12:05:47Z) (Implementing API version 1.21.11-R0.1-SNAPSHOT)
The plugin version we were using was 3.2.13.
The issue:
Earlier today our server experienced an extreme lag with a TPS of 3-4. A spark profile showed it was caused by FastCraft taking an excessively long time getting a snapshot of the hopper's inventory during inventory change event and item pickup event.
https://spark.lucko.me/rcQCdKwGO7
Steps to reproduce (not quite):
This occurred while I was asleep so I couldn't provide details on how or why this happened, only some guesses I have. It is possible that an item or multiple items in a hopper had an excessive amount of data, which led to a long deserialization time seen in the profile. However this is unusual since we have not experienced anything like this for a long time.
In my attempts at reproducing the issue I used the typical method of creating an area ban as that was an easy way to make an item with large amount of data. I have tried putting the item into two hoppers facing each other, into a chest in a hopper chain, rapidly moving the item between the hopper inventory and my inventory. All only succeeded in causing a short lag spike and produced a different profile result compared to the one above. A couple of profiles from my attempts if you wish to see them: https://spark.lucko.me/WkInr21KAt and https://spark.lucko.me/Um3fZNMsox
I have looked at the server log during the lag and checked last known locations of players who were online at the time and were unable to find an obvious cause such as a player attempting an area ban.
I'm sorry I couldn't explain much more about this issue. I will be happy to do more attempts at reproduce the issue if you have any more ideas on what may trigger the poor performance.