To clarify,
I use KSysGuard in Linux Ubuntu (also available in Kubuntu) to tell me which software is using what amount of memory.
It has been my experience, if Java total ram use tries to exceed what I have installed, the software malfunctions or crashes the entire computer.
Of course my current computer is far more powerful then that these days.
OvermindDL1’s description is the correct way to allocate ram in Java configs for best operation of Minecraft.
However, total java ram use is always more than the actual software, because like many have said Java sucks.
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