Editing Tutorials/Setting up a server (section)

=== Java options ===
Java options should be added between the  {{code|java}} and the {{code|-jar}}  on the command line, or in your startup script.

; Memory limits
: The most important thing for a ''Minecraft'' server is memory to run with. <code>-Xmx</code> defines how much memory it is allowed to use. {{code|-Xmx2G}} (2 gigabytes) is more than enough for a home server with 5 players on default [[simulation distance]] (5&times;21<sup>2</sup> = 2205 chunks simulated), but do scale it up by your actual settings. Setting the value too low causes frequent GC stops. Making it too high can make GC too long on Java 8 &ndash; a version you should not be using in the first place.
: {{code|-Xms}} (the initial memory size) is the next biggest knob, but ''only'' affects startup performance. If your server also runs things other than ''Minecraft'', set it to 1/4 of the maximum size (so for our case, {{code|-Xms512M}}) for a good balance: the JVM will be allowed to return some memory to the OS when it's not needed. (The return algorithm is ''very'' conservative, so it has very minimal effect on long-term performance.) If your server is fully dedicated to ''Minecraft'', feel free to go higher: making it equal to the maximum size disables heap scaling and gives maximum startup speed.
: A "soft max heap size" ({{code|1=-XX:SoftMaxHeapSize=1G}}) is available for some versions of JRE. The JRE will try to only use that much memory, but will go over to a maximum of <code>-Xmx</code> if necessary. If you are running many things on your server, this may be useful.
; GC
: The default GC on Java 9+ used by modern ''Minecraft'' is G1GC (Java 8 also has it, but it is off by default). The flags used by the official launcher {{code|1=-XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M}} works reasonably well, though of course [https://github.com/brucethemoose/Minecraft-Performance-Flags-Benchmarks#server-g1gc better flags (brucethemoose)] exist. ([https://aikar.co/2018/07/02/tuning-the-jvm-g1gc-garbage-collector-flags-for-minecraft/ Aikar's famous flags] are good, but not optimal.)
: On Java 14+, ZGC is available. This GC is almost lag-free with millisecond-level pauses. Delete all GC-related flags and simply use {{code|-XX:+UnlockExperimentalVMOptions -XX:+UseZGC}}. Tuning flags other than {{code|-Xmx}} are [https://docs.oracle.com/en/java/javase/17/gctuning/z-garbage-collector.html#GUID-9957D441-A99A-4CF5-9522-393E6DE7D898 a lot less important] compared to in GCs (the default is quite good), although {{code|1=-XX:-ZProactive}} is worth a try if you want to trade memory for lag.
; VM
: On [https://stackoverflow.com/questions/31078803/usage-of-d32-and-d64-while-launching-java Solaris], use <code>-d64</code> to tell the JVM to use your 64-bit processor properly. Since nobody uses Solaris, this is irrelevant.
: Calculations may be made faster with GraalVM, benefiting operations such as chunk generation. A basic version is bundled in the common "HotSpot" Java 9&ndash;15, but it was later removed due to maintenance burden. Now you need to download it separately: use [https://github.com/brucethemoose/Minecraft-Performance-Flags-Benchmarks#graalvm-enterprise-edition instructions from brucethemoose].

To sum up, a reasonable flag combination can be as simple as {{code|1=-Xmx4G -Xms1G -XX:SoftMaxHeapSize=3G -XX:+UnlockExperimentalVMOptions -XX:+UseZGC}}. That is it. There are bits that you can squeeze from complicated flags, but you would get much more from [[Mods|modified server software]]. In fact, all serious discussions (e.g. Aikar and brucethemoose) of server performance assume some degree of modding.