Server Setup Tutorial
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Download the Minecraft Server: Java Edition
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Server Start Parameters I've found to work really well.
java -Xmx1G -Xms1G -XX:+UseG1GC -XX:+UnlockExperimentalVMOptions -XX:+ParallelRefProcEnabled -XX:MaxGCPauseMillis=200 -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:G1HeapRegionSize=32M -jar server.1.21.4.jar nogui