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Aikar's Flags

Aikar's Flags

"Aikar's Flags" are command-line options for common JVM (opens in a new tab) implementations like OpenJDK and OpenJ9 designed to improve performance of the JVM's G1GC garbage collector for the Minecraft Java Edition server.

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 command that you use to start up your server with the flags listed below.

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

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

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

Breakdown

⚠️

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.

This is only a quick recap, you should give Aikar's blog (opens in a new tab) a read if you're interested on how these really work.

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 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 our Discord server (opens in a new tab). If these flags have helped your server significantly, consider donating to Aikar (opens in a new tab) to support his work.

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