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// Use the below command to launch your server with 1500 megabytes of allocated RAM.
java -Xms1500M -Xmx1500M -jar server.jar --nogui
// You can also replace the M with a G for gigabytes of RAM instead:
java -Xms2G -Xmx2G -jar server.jar --nogui
// Use the cited source for more info.