Block entity tickers memory leak in Level

At least present in 1.18 as well from the tests I made.

Explanations

There is a list of BlockEntityTicker in net.minecraft.world.level.Level that doesn't update itself correctly when chunks that didn't tick once are unloaded. Since a Block entity ticker have to be ticked at least once in order to put itself to a "removed" mode, the Level that will iterate through all of its block entity tickers won't be able to confirm that nor remove them.

This situation can make a Level accumulate more and more block entity tickers that take more and more memory as it goes. We can especially observe this behavior when generating a large portion of chunks without ticking them (as in chunk pre-generation).

Fixing this issue can very easily be done, as simple as removing from the list of its corresponding Level all block entity tickers of a chunk about to be unloaded.

Recreating the memory leak

The simplest way to check that is to create a heapdump after generating loads of chunks using the Chunky and Spark mods:

1. Create a new world.
2. Launch a chunk generation task in the Overworld and in the Nether using Chunky. I would say more than 1000000 chunks, but really depends on how much RAM you assigned to Minecraft.
3. Pause the chunk generation before Minecraft crashes with OutOfMemoryError.
4. Create a heapdump using Spark, with the --run-gc-before flag preferably.
5. Inspect the heapdump and see where all the retained memory is.

 

You can also find a heapdump of my own here if you don't want to bother with the whole process.