Java & memory
GPCL picks Java automatically based on the game version and allocates memory based on the machine's RAM. In most cases you do not need to change anything.
Java version mapping
| Minecraft version | Java required |
|---|---|
| 1.16.5 and older | Java 8 |
| 1.17 - 1.20.4 | Java 17 |
| 1.20.5 and newer | Java 21 |
If the matching Java is missing at launch, GPCL downloads and installs it automatically. If the download fails, go to Settings → Downloads → Java download and try another mirror.
Java is installed inside GPCL's own directory. It does not affect other Java installations on the system and does not change the system's JAVA_HOME.
Memory: automatic
The default is "Automatic". Allocation works in two steps:
- Start from 30% of physical memory, round down to whole GB, and clamp to 2 - 8 GB;
- Then look at currently available memory: leave the system at least 1 GB, use 2 GB as the floor, round the result down to 512 MB, and take the smaller of the two.
A few real results:
| Physical memory | Currently available | Automatic allocation |
|---|---|---|
| 16 GB | 8 GB | 4 GB |
| 16 GB | 3.9 GB | 2.5 GB |
| 8 GB | 4 GB | 2 GB |
The settings page shows system memory used / total and the current available value. When available memory is tight, automatic allocation steps down and suggests closing some programs first.
-Xmx is an upper bound, not something consumed the moment the game starts. Allocating against the memory actually available at the time is therefore more stable than forcing a large value.
Memory: custom
When a modpack or a large mod set is not enough, switch to "Custom" and drag the slider. Leave room for the system and other programs; do not give all your physical memory to the game.
OutOfMemoryError means memory ran out:
- Raise the allocation one step and try again;
- If that is not enough, remove some mods;
- Also close memory-hungry programs such as browsers.
G1GC tuning
Settings → Advanced → Memory management has G1GC tuning, off by default. Enable it only when the frame rate fluctuates or GC stutters are noticeable; enabling it appends a set of common G1GC flags.
Custom JVM arguments are in the same section. A wrong argument can stop the game from starting at all; if you are unsure, do not change it.
Optimize memory before launch
When enabled, GPCL reclaims some of the memory it uses before launching the game. It only cleans up the launcher's own process; it does not touch a running Minecraft, and it does not clear the sign-in state.
On a hard disk drive this step may cause a brief stutter, which is normal.