Minecraft allocating RAM problem?

If I change mine to anything from Xmx1G, I get this error:

Error occurred during initialization of VM
Could not reserve enough space for 2097152KB object heap
Java HotSpot(TM) Client VM warning: Using incremental CMS is deprecated and will likely be removed in a future release

If I try to do something like Xmx8G, it says Java could not make the virtual machine, and it closes.

My computer has 16GB of RAM. Please help.

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