Boot with reduced memory

It is fun to boot with less memory, set performance counters analyze them and ofcourse be surprised by the results. Interestingly, it is quite straight forward to do this on Linux and Windows.
In Linux: boot-time parameter mem=X, where X is the amount of memory[MB] you want Linux to recognize. You could alter it in grub as well.
In windows: bcdedit /set removememory X, where X is the amount of memory[MB] to be deducted from total available memory.

One reason to embark on such an activity is to estimate how much RAM would be optimal for your regular workloads.
Here are some references:
Linux
Windows.