Can't Allocate More RAM to Minecraft

To those having similar issues in the future:

Typing cat /proc/meminfo into the command line will inform you how much RAM you have free on your computer. Oftentimes there's just a lack of memory, meaning you need to either close some programs or upgrade your RAM.

If you definitely have enough memory (at least 1GB more than you want to dedicate), then adding -d64 to the same place where you put -Xmx and -Xms will force Minecraft to use Java x64 rather than any Java x86 that somehow snuck onto your machine.

Furthermore, you can try adding the 64-bit java to your computer's PATH:

1. Right click on My Computer/Computer (on win10 simply enter settings and search "system settings" then skip to step 4)
2. Go to Properties
3. Go to Advanced system settings (or just Advanced)
4. Click Environment variables in the bottom right.
5. Add the location of the folder that javaw.exe is in to the PATH by clicking Path then clicking "Edit", then clicking "New" and adding it there - ensure there is a semicolon (;) at the end, or it won't work. (Note: If the PATH variable doesn't exist among "User variables" click "New", and set the following: Variable name = PATH, Variable value = location of the folder javaw.exe is in (this is often Program Files).)

If the issue persists beyond that, you don't have 64-bit Java installed.

-Xmx is for the maximum heap space. It doesn't tell Java this is what should be started with, but is the maximum. -Xms is for the minimum, so what the JVM starts with. But really, what it starts with is a much smaller concern than the maximum.

That being said, if you observe that the issue comes when you go beyond 4GB RAM, that is a clear indicator that what you have is a 32-bit program. Whatever you did to ensure you use 64-bit Java can't have been right.