Linux 2.6 32-bit + BIGMEM versus Linux 2.6 64-bit

32-bit32bit-64bit64-bitlinuxperformance

I've been told that 32-bit Linux with a bigmem kernel (when the system has >=4GB RAM) will perform better than 64-bit Linux.

I'd love to define "perform better than" for you, but unfortunately the person who told me this didn't elaborate/justify.. hence this post!

Is there any truth to this at all? Any circumstance under which this would be true?

Thanks

Best Answer

in x86 processors, 64-bit code helps two ways:

  • bigger addresses let you access more memory directly (only relevant for processes that manage huge datasets)
  • more registers might let a compiler reduce memory accesses for tight variables (slight extra optimization, unnoticeable except in tight loops of highly optimized code).

and has the following cons:

  • more and bigger registers means more state to save/restore on every context switch.
  • bigger pointers means more RAM use and bigger structures, more data to read/write.

therefore, in a lot of cases, the best of both worlds is a 64-bit OS and 32-bit processes:

  • a 64-bit OS can handle lots of RAM, both to hold many processes and for big caches
  • 32-bit apps are limited to 2 or 3 GB RAM for each process, but it's enough for the vast majority of tasks.
  • no matter where in the 64-bit space a 32-bit task's RAM is located, it will only need 32-bit pointers to access its memory, so all pointers and data structures are still the smaller 32-bit variety.
  • a 32-bit task (process or thread) only have to save/restore the few and small registers available in 32-bit x86, the 64-bit Linux scheduler handles this case well.

but, in all, the advantage is seldom noticeable (just guessing it would be far less than 5%), so just go with 64-bit everywhere and get it all simpler.

the only case where i would definitely go with 32-on-64 is when doing OpenVZ-kind of isolation. That way each partition owner would make the most of the limited RAM he can access.

still, i don't know of any advantage of PAE over 64-bit (not even small pointers, since each PAE pointer has a 32-bit offset and an extra (up to 32-bit) 'start' (remember the segmented memory of 8086? what a load of bloat!)

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