Give Minecraft more memory: MultiMC

Just the Dungeon Train instance (recommended)

1. Select the Dungeon Train instance and click Edit Instance.
2. Open Settings, then the Java tab.
3. Tick the Memory box. This lets the instance use its own value.
4. Set Maximum memory allocation to 6144 MiB (6 GB).

Every instance (global setting)

1. Click Settings in the top bar.
2. Open Java.
3. Under Memory, set Maximum memory allocation to 6144 MiB.

Then close the game and launch the instance again. The new memory only applies from the next launch.

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Other launchers: see Give Minecraft more memory.