JVM flags

JVM flags

Celeris works without any startup flags. The flags on this page turn on faster code paths that Java keeps closed by default. Add them if you want the extra speed. Skip them if you do not.

Which flags to use

Minecraft	Java	Flags	What they turn on
1.21.x	21	--enable-preview	The FFM code paths. In the current release this is also what the native physics kernel needs. See the note below.
1.21.x	21	--enable-preview --add-modules=jdk.incubator.vector	The same, plus faster SIMD math.
26.x	25	--add-modules=jdk.incubator.vector	Faster SIMD math. Java 25 already has the other paths open.

Note for version 0.3.1.1

In version 0.3.1.1, the native parts (the physics kernel and zstd) were built with function names that do not match the Java classes. Java cannot link them, so Celeris uses the FFM path or the Java code instead. On Java 21, --enable-preview lets Celeris use FFM, so the native physics kernel can run.

The fix is already in the source code and will ship in the next release. After that, the native parts load without flags, and these flags only add FFM memory and SIMD math.

Advanced switches

These are for testing and for solving problems. You do not need them normally.

Switch	What it does
-Dceleris.compatMode=true	Uses only plain Java code. Slower, but it works everywhere.
-Dceleris.physics.engine=java	Uses the Java physics kernel.
-Dceleris.physics.engine=off	Turns the physics engine off. Mods that use it fall back to their normal code.
-Dceleris.physics.isa=avx2	Picks the native instruction set. Choose one of generic, avx2, avx512 or neon.
-Dceleris.physics.threads=4	Sets how many worker threads the physics uses.

Where to add the flags

• Modrinth App: Settings, then Java and memory.
• CurseForge: Settings, then Minecraft, then Additional arguments.
• Prism Launcher: open the settings of your instance, then Java.
• Server: open the file user_jvm_args.txt in the server folder, and put the flags on one line.

Example for a Java 21 server:

--enable-preview --add-modules=jdk.incubator.vector

Two things to avoid

• Do not add --enable-native-access on NeoForge. On Java 21 it blocks the FFM path for NeoForge mods. Fabric is fine with --enable-native-access=ALL-UNNAMED, which also silences native access warnings.
• Do not add --enable-preview on Java 25. It is only for Java 21.

Check that it worked

Start the game or server, and search the log for these lines:

• Celeris memory backend: FFM means FFM memory is on.
• Celeris physics engine followed by a name that starts with native means the native physics kernel is on. A name of java means the Java kernel is used.