Summary
There is perhaps a common impression that 64bit is generally always better than 32bit software on PCs. This tends to be based on:
- 64bit obviously provides sufficient virtual address space to allow all RAM in any PC to be easily addressable – 32bit runs into problems with modern PCs.
- AMD64 provides twice the number of GPRs over IA32.
- AMD64 provides RIP relative addressing mode, to ease implementation of PI code.
This impression is not quite correct though. There are costs to 64bit. Particularly in increased memory usage, which can be of consequence certain environments, such as hosting VMs. More surprisingly, even the common wisdom that 64bit improves performance can have the odd exception.
All this means that there is value in running a 32bit userspace, even on 64bit kernels on 64bit hardware. Only those applications which require 64bit pointers, or have an actual demonstrable performance benefit from 64bit word sizes need be run as 64bit. Doing so can allow significant amounts of memory can be saved (30%, 60%, sometimes more).
Paul Jakma's Blog 