JVM tuning

Server Startup Flags Generator

Generate an optimized JVM start command and script for Paper, Purpur, Velocity, and Waterfall.

Software

Environment

Memory

4GB

Flags

The community standard for Paper servers. Balanced G1GC tuning that suits almost everyone.

Jar file name

No GUIAdd --nogui so the server runs without the built-in window. Recommended for headless boxes.

Advanced options

Modern VectorsEnables SIMD to speed up map rendering on Pufferfish and its forks. Needs Java 17 or newer.Auto-restartWrap the script in a loop so the server restarts after it stops.Use variablesDefine memory and file name as variables at the top of the script for quick edits.Reserve memory overheadLower the heap below total RAM (11x/12 - 1200) to leave room for the JVM itself and avoid out-of-memory kills.

Start script

start.sh

#!/usr/bin/env bash

java -Xms4096M -Xmx4096M -XX:+UseG1GC -XX:+ParallelRefProcEnabled -XX:MaxGCPauseMillis=200 -XX:+UnlockExperimentalVMOptions -XX:+DisableExplicitGC -XX:+AlwaysPreTouch -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 -XX:G1NewSizePercent=30 -XX:G1MaxNewSizePercent=40 -XX:G1HeapRegionSize=8M -XX:G1ReservePercent=20 -jar server.jar --nogui

Flags only

Just the JVM flags, for pasting into a host panel's startup field.

jvm flags

-XX:+UseG1GC -XX:+ParallelRefProcEnabled -XX:MaxGCPauseMillis=200 -XX:+UnlockExperimentalVMOptions -XX:+DisableExplicitGC -XX:+AlwaysPreTouch -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 -XX:G1NewSizePercent=30 -XX:G1MaxNewSizePercent=40 -XX:G1HeapRegionSize=8M -XX:G1ReservePercent=20

Why these flags?

Xms is pinned to Xmx so the JVM grabs its full heap up front, avoiding mid-game resizes that cause lag spikes.
Aikar's Flags are the trusted default: G1GC tuned to keep garbage-collection pauses short and predictable.
At 12 GB or more, the G1GC presets switch to large-heap tuning (bigger regions, earlier collection) so big servers stay smooth.
Most servers should pick Aikar's Flags and a sensible RAM size first. The other presets are for owners who benchmark their own hardware.

These flags are a starting point, not a substitute for right-sizing RAM. Verify against each author's current reference before relying on them in production.