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Aikar's Flags

Recommended JVM startup flags​

Use these flags exactly, only changing Xmx and Xms. These flags work and scale accordingly to any size of memory.

For an automated script to generate these flags for you, see our Script Generator.

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=https://mcflags.emc.gs
-Daikars.new.flags=true -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 8GB of memory, do not use 8GB!

Minecraft (and Java) needs additional memory on top of that Xmx parameter. It is recommended to reduce your Xmx and Xms by about 1000-1500MB to avoid running out of memory or OOMKiller killing your server. This also leaves room for the operating system to use memory too.

Do you have 8GB of memory? Use 6500MB 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.

Recommended memory​

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.

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

-Xloggc:gc.log -verbose:gc -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintGCTimeStamps
-XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=5 -XX:GCLogFileSize=1M

Java 11+

-Xlog:gc*:logs/gc.log:time,uptime:filecount=5,filesize=1M

GC logging does not hurt your performance and can be left on at all times. The files will not take up much space (5MB)

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-Minecraft/OS level things. Therefore, you should never run Minecraft 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 memory allocation rate, ranging to at least 800MB/second on a 30 player server! And this is mostly short-lived objects (Block Position).

    Now, this means Minecraft really needs more focus on new gen 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 GCs. 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 GCs 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.

    Default is 65 to 85 depending on the Java version, we are setting that to 90 to ensure we reclaim garbage in old gen as fast as possible to retain as much free regions as we can.

  5. G1ReservePercent=20: Minecraft 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.

  6. MaxTenuringThreshold=1: Minecraft has a really high allocation rate of memory. Of that memory, most is reclaimed in the eden generation. However, transient data will overflow into survivor. Initially played with completely removing survivor and had decent results, but does result in transient data making its way to old which is not good. Max Tenuring 1 ensures that we do not promote transient data to old generation, but anything that survives 2 passes of GC is just going to be assumed as longer-lived.

    Doing this greatly reduces pause times in young collections as copying data up to 15 times in survivor space for a tenured object really takes a lot of time for actually old memory. Ideally the GC engine would track average age for objects instead and tenure out data faster, but that is not how it works.

    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 GCs.

  7. SurvivorRatio=32: Because we drastically reduced MaxTenuringThreshold, we will be reducing use of survivor space drastically. This frees up more regions to be used by eden instead.

  8. AlwaysPreTouch: AlwaysPreTouch gets the memory setup and reserved at process start ensuring it is contiguous, improving the efficiency of it more. This improves the operating systems memory access speed. Mandatory to use Transparent Huge Pages

  9. +DisableExplicitGC: Many plugins think they know how to control memory, and try to invoke garbage collection. Plugins that do this trigger a full garbage collection, triggering a massive lag spike. This flag disables plugins from trying to do this, protecting you from their bad code.

  10. MaxGCPauseMillis=200: This setting controls how much memory is used in between the minimum and maximum ranges specified for your new generation. This is a "goal" for how long you want your server to pause for collections. 200 is aiming for at most loss of 4 ticks. This will result in a short TPS drop, however the server can make up for this drop instantly, meaning it will have no meaningful impact on your TPS. 200ms is lower than players can recognize. In testing, having this value constrained to an even lower number results in G1 not recollecting memory fast enough and potentially running out of old gen triggering a full collection. Just because this number is 200 does not mean every collection will be 200. It means it can use up to 200 if it really needs it, and we need to let it do its job when there is memory to collect.

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

  12. G1RSetUpdatingPauseTimePercent=5: Default is 10% of time spent during pause updating RSets, reduce this to 5% to make more of it concurrent to reduce pause durations.

  13. G1MixedGCCountTarget=4: Default is 8. Because we are aiming to collect slower, with less old gen usage, try to reclaim old gen memory faster to avoid running out of old.

  14. G1HeapRegionSize=8M+: Default is auto calculated. Super important for Minecraft, especially 1.15, as with low memory situations, the default calculation will in most times be too low. Any memory allocation half of this size (4MB) will be treated as "Humongous" and promote straight to old generation and is harder to free. If you allow Java to use the default, you will be destroyed with a significant chunk of your memory getting treated as Humongous.

  15. +PerfDisableSharedMem: Causes GC to write to file system which can cause major latency if disk IO is high - see https://www.evanjones.ca/jvm-mmap-pause.html

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 Minecraft or its impact with AlwaysPreTouch, so this section is for the advanced users who want to experiment.