- The bulk of the retained data is a simple data structure (in minecraft’s case its lots of byte)
- The usage of off heap memory can be hidden in abstraction.
- Preset the level-seed=114 in server.properties
- Delete the world* directories
- Start the server with these options to see what the GC is doing
-verbose:gc -XX:+PrintGCDetails -XX:+PrintGCDateStamps
-XX:+PrintGCTimeStamps -XX:+PrintTenuringDistribution -Xmx6g
- Connect with one client
- Perform /worldgen village
- Perform /save-all
- It caches block stage in many bytes
- It attempts to cache int used for processing without limit.
A Censum report for the above test looks like this:
Off heap MineCraft
- Use off heap ByteBuffer for long term caching. Unsafe would be more efficient but not as portable.
- Put a cap on the number of int cached.
Note: the problem with the temporary int only became visible to me after moving the bulk of the data off heap. Addressing the biggest problem over reveals more quick fix problems.
Using off heap memory can help you tame you GC pause times, especially if the bulk of your data is in simple data structures which can be easily abstracted. Doing so can also help reveal other simple optimisations you can do to improve the consistency of your performance.
The Source Used
- The source used for the test is available here. https://github.com/peter-lawrey/MineOffHeap
- The logs produced are available here. https://github.com/peter-lawrey/MineOffHeap/tree/master/logs