Server JVM Flags Generator

One-line explanations for every flag in the AIKAR preset. References: PaperMC — Aikar's flags · Oracle HotSpot GC Tuning.

• -Xms8G

  Initial heap size. Setting it equal to -Xmx avoids the JVM resizing the heap at runtime, which causes brief pauses.

• -Xmx8G

  Maximum heap size. The hard ceiling on Java heap memory. For Minecraft, set this to your total RAM minus 1-2 GB for the OS and overhead.

• -XX:+UseG1GC

  Selects the G1 (Garbage-First) collector. G1 splits the heap into regions and prioritizes those with the most garbage. Default since Java 9.

• -XX:+ParallelRefProcEnabled

  Processes weak/soft/phantom references in parallel during GC. Reduces pause times on heavy reference workloads (entities, chunks).

• -XX:MaxGCPauseMillis=200

  Soft target for max GC pause in ms. G1 tunes itself toward this. 200 ms is a safe ceiling for tick-bound servers.

• -XX:+UnlockExperimentalVMOptions

  Required to use experimental flags like G1NewSizePercent and G1MixedGCLiveThresholdPercent.

• -XX:+DisableExplicitGC

  Ignores System.gc() calls from plugins and mods. Prevents poorly-written code from triggering full-heap GC pauses.

• -XX:+AlwaysPreTouch

  Touches every page of the heap at startup so memory is committed up front. Slightly slower boot, far smoother first hour of runtime.

• -XX:G1NewSizePercent=30

  Minimum percentage of the heap reserved for the young generation. Raising it (Aikar uses 30-40%) gives short-lived chunks/entities room to die before promotion.

• -XX:G1MaxNewSizePercent=40

  Maximum young-generation size as a percentage of the heap. Pairs with G1NewSizePercent to size the eden+survivor regions for Minecraft's allocation rate.

• -XX:G1HeapRegionSize=8M

  Size of each G1 region. 8 MB suits typical heaps; 16 MB is recommended for ≥12 GB heaps to reduce region count.

• -XX:G1ReservePercent=20

  Reserve held back to avoid heap exhaustion during evacuation. Higher values trade headroom for fewer to-space failures.

• -XX:G1HeapWastePercent=5

  Tolerated waste percentage in old regions before mixed GC kicks in. Lower = more aggressive cleanup.

• -XX:G1MixedGCCountTarget=4

  Number of mixed GC cycles to amortize old-gen cleanup over. Smaller numbers = shorter individual pauses.

• -XX:InitiatingHeapOccupancyPercent=15

  Heap occupancy that triggers a concurrent marking cycle. Aikar lowers this aggressively to start GC before the heap fills.

• -XX:G1MixedGCLiveThresholdPercent=90

  Old-gen regions below this liveness ratio are eligible for mixed GC. Raising it lets G1 collect more regions per cycle.

• -XX:G1RSetUpdatingPauseTimePercent=5

  Cap on the percentage of GC pause spent updating the remembered set. Caps a known G1 pause-spike source.

• -XX:SurvivorRatio=32

  Ratio of eden to a single survivor space. 32 means each survivor is 1/32 of eden — small survivors push short-lived objects out of young gen faster.

• -XX:+PerfDisableSharedMem

  Disables JVM perf counter shared memory file. Avoids /tmp I/O stalls that can spike pauses on busy hosts.

• -XX:MaxTenuringThreshold=1

  Promotes objects from young to old gen after surviving 1 GC. Aikar's setting — Minecraft objects are mostly chunk-lifetime, so promote fast.