Aikar's Flags

Aikar's Startup Flags are command-line options that can be used to improve and optimize the performance of a Java Minecraft server. These flags are particularly useful for servers who could benefit from a performance boost, such as servers with a large player base or servers with a lot of plugins.

All Minecraft server lineups from BerryByte come equipped with Aikar's flags by default.

Setting up the flags

Setting up Aikar's Flags for your server is a very simple process, simply update the parameters of the java command, that you use to start up your server with the flags listed below. Servers with less than 12GB of RAM:

java -Xms{SERVER_MEMORY}M -Xmx{SERVER_MEMORY}M -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

Servers with 12GB of RAM or more:

java -Xms{SERVER_MEMORY}M -Xmx{SERVER_MEMORY}M -XX:+UseG1GC -XX:+ParallelRefProcEnabled -XX:MaxGCPauseMillis=200 -XX:+UnlockExperimentalVMOptions -XX:+DisableExplicitGC -XX:+AlwaysPreTouch -XX:G1NewSizePercent=40 -XX:G1MaxNewSizePercent=50 -XX:G1HeapRegionSize=16M -XX:G1ReservePercent=15 -XX:G1HeapWastePercent=5 -XX:G1MixedGCCountTarget=4 -XX:InitiatingHeapOccupancyPercent=20 -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

Remember to replace {SERVER_MEMORY} with the amount of RAM you want to allocate to your server.

Flag breakdown

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If you're running on a shared hosting (one bulky machine runs multiple Minecraft servers) with strict resource allocation (e.g. Docker containers; this applies for BerryByte's Minecraft lineup), it is recommended to leave out roughly 1GB of RAM for the JVM's overhead (this is automatically done at BerryByte).

-Xmx: Sets the maximum amount of memory that the JVM can use. This can help prevent the server from crashing due to out-of-memory errors.

-Xms: Sets the initial amount of memory that the JVM uses when starting up. Aikar's flags set this to the same value as -Xmx, because unused memory is wasted memory.

-XX:+UseG1GC: Enables the Garbage-First (G1) Garbage Collector, which is a low-latency garbage collector that adaptively chooses how much memory to give to each region to optimize pause time

-XX:+ParallelRefProcEnabled: Enables parallel reference processing which helps improves performance and limit lag spikes.

-XX:MaxGCPauseMillis: Sets the maximum amount of time that the JVM can spend on garbage collection. This can help reduce lag spikes.

-XX:+UnlockExperimentalVMOptions: Unlocks experimental JVM options.

-XX:+DisableExplicitGC: Disables explicit garbage collection, which can help reduce lag spikes.

-XX:+AlwaysPreTouch: Enables pre-touching, which can help reduce lag spikes.

-XX:G1NewSizePercent: Sets the percentage of the heap that is used for new objects.

-XX:G1MaxNewSizePercent: Sets the maximum percentage of the heap that is used for new objects.

-XX:G1HeapRegionSize: Sets the size of the G1 heap region.

-XX:G1ReservePercent: Sets the percentage of the heap that is reserved for G1.

-XX:G1HeapWastePercent: Sets the percentage of the heap that is wasted.

-XX:G1MixedGCCountTarget: Sets the target number of mixed garbage collections.

-XX:InitiatingHeapOccupancyPercent: Sets the percentage of the heap that is used before a mixed garbage collection is triggered.

-XX:G1MixedGCLiveThresholdPercent: Sets the percentage of live objects that are required for a mixed garbage collection to be triggered.

-XX:G1RSetUpdatingPauseTimePercent: Sets the percentage of time that is spent on updating the remembered set.

-XX:SurvivorRatio: Sets the ratio of the survivor space to the eden space.

-XX:+PerfDisableSharedMem: Disables shared memory, which can help reduce lag spikes.

-XX:MaxTenuringThreshold: Sets the maximum tenuring threshold, which can help reduce lag spikes.

Final Notes

It is important to carefully consider the values you use for each startup flag, as setting them too high or too low can have negative impacts on the performance of the server. If you are having trouble using Aikar's Startup Flags, check the server.log file in the logs folder of your server for any error messages.

If you continue to experience issues with the flags, feel free to ask for help on the BerryByte's Discord Community (opens in a new tab) and refer to Aikar's blog (opens in a new tab) for more information. If these flags have helped your server signifcantly, consider donating to Aikar (opens in a new tab) to support his work.

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