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Which OS should I choose for my VPS ?
On a low-end server (one virtualize 64bit core at 2.5Ghz and 512MB memory), does it make more sense to install 32bit or 64bit Linux? The 64bit installation will use up more resources on disk and memory, but how would they compare on performance?
Choose 32-bit if you're constrained by memory.
In most cases, there is very little difference in performance. This especially applies to virtual machines. However, with only 512 MB of RAM allocated, you'll be bumping up against the memory limit right away. Using a 32-bit OS with 32-bit apps will help this slightly.
Generally, on x86 hardware, you want to use the x86_64 instruction set when possible. Most importantly, this offers more general-purpose CPU registers, which are a tight resource in the Intel x86 architecture. This is probably enough of a gain to justify the additional memory use.
Additionally, with x86_64, you always have the ability to use the important-for-security NX bit. With 32-bit, you need a special "PAE" kernel or else that feature will have to be emulated in software, giving a performance hit. (So, if you do go 32-bit, make sure to use a PAE-enabled kernel.)
That said, the only real way to answer this question is to benchmark with your specific load.
