64-bit Kernel Question

Adam Williamson awilliam at redhat.com
Mon Mar 9 18:14:46 UTC 2009

On Mon, 2009-03-09 at 17:56 +0000, Bill Crawford wrote:
> On Monday 09 March 2009 17:33:19 Matej Cepl wrote:
> > On 2009-03-09, 17:00 GMT, Adam Williamson wrote:
> > > Running x86-64 basically uses twice as much RAM for any given
> > > workload as running x86-32 would.
> >
> > Sorry, but that's just not true -- the only data types which are
> > twice as big on x86_64 than on i386 are pointers and (I believe)
> > long int. Every other primitive C type else remains the same in
> > size. There was a long discussion of this on fedora-devel (I
> > believe) couple of weeks ago.
> ... and with pointers being bigger, there's a higher waste from alignment 
> restrictions, padding, etc., malloc() internal data structures are bigger, 
> libraries need more space for writable mappings like the procedure linkage 
> table, virtual function tables and so on. There's a fair bit extra usage that's 
> not immediately visible, in other words.

Sorry, I may well be wrong on the exact quantity there, I should've said
I wasn't 100% definite on that. It's what I'd been told by several
people prior to switching, and it more or less matches what I see on
this system after switching (700MB vs. 1.3GB in typical use), but I'm
absolutely happy to be corrected on the issue :)
Adam Williamson
Fedora QA Community Monkey
IRC: adamw | Fedora Talk: adamwill AT fedoraproject DOT org

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