[CentOS] 32bit vs 64bit memory usage

Peter Kjellstrom cap at nsc.liu.se
Mon May 25 07:45:14 UTC 2009


On Thursday 21 May 2009, Joshua Baker-LePain wrote:
> On Thu, 21 May 2009 at 4:59pm, Robert Heller wrote
>
> > No, you are not wrong. All x86 flavered 64-bit processors will run as
> > 32-bit (i686) processors and when running in 32-bit mode are
> > effectively just a i686 as far as any 32-bit program can tell.  There
> > is no reason NOT to just install a straight 32-bit OS on such a machine
> > if there is less than 4gig of virtual memory and non-of the programms
> > being run has any reason to use the 64-bit address space.  Web hosting
>
> That's not strictly true.  On some x86_64 chips, there are extra registers
> which are only available when running in 64-bit mode.  Running without
> those registers can hamper performance, even if the program isn't using
> the larger address space.

There's a 2nd factor. On 32-bit you loose full flexibility memory wise when 
you pass 920-ish MB. After that you're split up into low-mem and high-mem. On 
64-bit, of course, all memory is low-mem.

/Peter

> This can make a big difference, e.g., in the 
> HPC space.  Web hosting, yeah, probably not so much.  But just saying
> "64bit iff >4GB RAM" doesn't tell the whole story.
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