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Aikar's Flags

Use these flags exactly, only changing Xmx and Xms. These flags work and scale accordingly to any size of memory, even 500MB but modern Minecraft versions will not do well with such low memory.

java -Xms10G -Xmx10G -XX:+UseG1GC -XX:+ParallelRefProcEnabled -XX:MaxGCPauseMillis=200 -XX:+UnlockExperimentalVMOptions -XX:+DisableExplicitGC -XX:+AlwaysPreTouch -XX:G1NewSizePercent=30 -XX:G1MaxNewSizePercent=40 -XX:G1HeapRegionSize=8M -XX:G1ReservePercent=20 -XX:G1HeapWastePercent=5 -XX:G1MixedGCCountTarget=4 -XX:InitiatingHeapOccupancyPercent=15 -XX:G1MixedGCLiveThresholdPercent=90 -XX:G1RSetUpdatingPauseTimePercent=5 -XX:SurvivorRatio=32 -XX:+PerfDisableSharedMem -XX:MaxTenuringThreshold=1 -Dusing.aikars.flags= -jar paper.jar --nogui
Do not allocate all of your available memory on a shared host!

When setting the Xms and Xmx values, if your host says you have 8000M memory, DO NOT USE 8000M! Minecraft (and Java) needs additional memory on top of that Xmx parameter. It is recommended to reduce your Xmx/Xms by about 1000-1500M to avoid running out of memory or "OOMKiller" hitting your server. This also leaves room for the Operating System to use memory too. Have 8000M memory? Use 6500M for safety. But you may also ask your host if they will cover this overhead for you and give you 9500M instead. Some hosts will! Just ask.

We recommend using at least 6-10GB, no matter how few players! If you can't afford 10GB of memory, give as much as you can, but ensure you leave the operating system some memory too. G1GC operates better with more memory.

However, more memory does not mean better performance above a certain point. Eventually you will hit a point of diminishing returns. Going out and getting 32GB of RAM for a server will only waste your money with minimal returns.


If you are using an Xmx value greater than 12G


  • -XX:G1NewSizePercent=40
  • -XX:G1MaxNewSizePercent=50
  • -XX:G1HeapRegionSize=16M
  • -XX:G1ReservePercent=15
  • -XX:InitiatingHeapOccupancyPercent=20

If you see increase in old generation collections after this, revert to the base flags!

Explanation of these changes:


Java GC Logging

Are you having old gen issues with these flags? Add the following flags based on your java version to enable GC Logging:

Java 8-10


Java 11+



Technical Explanation of the Flags

  1. -Xms matching -Xmx -- Why: You should never run your server with the case that -Xmx can run the system completely out of memory. Your server should always be expected to use the entire -Xmx! You should then ensure the OS has extra memory on top of that Xmx for non MC/OS level things. Therefore, you should never run MC with -Xmx settings you can't support if java uses it all. Now, that means if -Xms is lower than -Xmx YOU HAVE UNUSED MEMORY! Unused memory is wasted memory. G1 operates better with the more memory it's given. G1 adaptively chooses how much memory to give to each region to optimize pause time. If you have more memory than it needs to reach an optimal pause time, G1 will simply push that extra into the old generation, and it will not hurt you The fundamental idea of improving GC behavior is to ensure short-lived objects die young and never get promoted. With the more memory G1 has, the better assurance you will get that objects are not getting prematurely promoted to the old generation. G1 Operates differently than previous collectors and is able to handle larger heaps more efficiently.

    If it does not need the memory given to it, it will not use it. The entire engine operates differently and does not suffer from too large of heaps, and this is industry-wide accepted information that under G1 to keep Xms and Xmx the same!

  2. UnlockExperimentalVMOptions -- needed for some the below options

  3. G1NewSizePercent: These are the important ones. You now can specify percentages of an overall desired range for the new generation. With these settings, we tell G1 to not use its default 5% for new gen, and instead give it 40%! Minecraft has an extremely high a memory allocation rate, ranging to at least 800 Megabytes a second on a 30 player server! And this is mostly short-lived objects (Block Position).

    Now, this means MC REALLY needs more focus on New Generation to be able to even support this allocation rate. If your new gen is too small, you will be running new gen collections 1-2+ times per second, which is awful. You will have so many pauses that TPS has risk of suffering, and the server will not be able to keep up with the cost of GC's. Then combine the fact that objects will now promote faster, resulting in your Old Gen growing faster. Given more New Gen, we are able to slow down the intervals of Young Gen collections, resulting in more time for short-lived objects to die young and overall more efficient GC behavior.

  4. G1MixedGCLiveThresholdPercent: Controls when to include regions in Mixed GC's in the Young GC collection, keeping Old Gen tidy without doing a normal Old Gen GC collection. When your memory is less than this percent, old gen won't even be included in 'mixed' collections. Mixed are not as heavy as a full old collection, so having small incremental cleanups of old keeps memory usage light.


  5. G1ReservePercent=20: MC Memory allocation rate in up-to-date versions is really insane. We run the risk of a dreaded "to-space exhaustion" not having enough memory free to move data around. This ensures more memory is waiting to be used for this operation. Default is 10, so we are giving another 10 to it.



    Considering average GC rate is 10s to the upwards of minutes per young collection, this does not result in any 'garbage' being promoted, and just delays longer lived memory to be collected in Mixed GC's.





  11. +ParallelRefProcEnabled: Optimizes the GC process to use multiple threads for weak reference checking. Not sure why this isn't default...




  15. +PerfDisableSharedMem: Causes GC to write to file system which can cause major latency if disk IO is high -- See

Using Large Pages

For Large Pages -- It's even more important to use -Xms = -Xmx! Large Pages needs to have all the memory specified for it, or you could end up without the gains. This memory will not be used by the OS anyway, so use it.

Additionally, use these flags (Metaspace is Java 8 Only, don't use it for Java7): -XX:+UseLargePagesInMetaspace

Transparent Huge Pages

Controversial feature but may be usable if you can not configure your host for real HugeTLBFS. Try adding -XX:+UseTransparentHugePages but it's extremely important you also have AlwaysPreTouch set. Otherwise, THP will likely hurt you. We have not measured how THP works for MC or its impact with AlwaysPreTouch, so this section is for the advanced users who want to experiment.