RE: [Xen-users] 32bit vs 64bit memory usage question

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Jamie Burns
> Sent: 05 February 2007 16:31
> To: Ulrich Windl; xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] 32bit vs 64bit memory usage question
> 
> Thanks for your insight.
> 
> But nothing else in my Debian install uses up double memory.
> 
> Only XEN.
> 
> Why only XEN?

There's no sensible reason for Xen using twice as much memory in itself.
However, as stated elsewhere, page-table-entries are 64-bit (as they are
for PAE!), and that may account for SOME of the difference. If you
compare 32-bit PAE with your previous measurements, how does it stand? 

The other difference that I see is that x86-64 has a bigger heap, but
that's only 4MB, so that wouldn't match what you're seeing. 

Of course, there's a little bit extra code in the code dealing with
page-tables due to 4-level page-tables, as this is not present in other
models of Xen. But since all the CODE in Xen is about 600K [64-bit
build] ("latest" unstable), it doesn't account for the difference you're
seeing. 

--
Mats


> 
> ----- Original Message ----- 
> From: "Ulrich Windl" <ulrich.windl@xxxxxxxxxxxxxxxxxxxx>
> To: <xen-users@xxxxxxxxxxxxxxxxxxx>
> Sent: Monday, February 05, 2007 3:28 PM
> Subject: Re: [Xen-users] 32bit vs 64bit memory usage question
> 
> 
> > On 5 Feb 2007 at 10:00, Tom Miller wrote:
> >
> > [...]
> >> 64 bit pointers and 64 bit memory address slots are twice as big as
> >> corresponding 32 bit ones.   So unless you have 4 GB of 
> memory or more,
> >> using 64bit software is actually slower for the system, not faster.
> >
> > Are you sure? You are not saying that 64-bit programs are 
> twice as large 
> > as 32-bit
> > programs, do you? Besides of that I'd expect a 64-bit CPU 
> to load a 64 bit 
> > word
> > from memory in the same speed as a 32-bit CPU would load a 
> 32-bit word. 
> > Also these
> > AMD/Intel CPUs seem to have additional registers in 64bit mode.
> >
> > I think the memory overhead comes from (As Andrew S 
> Tanenbaum explained in 
> > one of
> > his books about 10 years ago) the fact that the tables to 
> manage the huge 
> > 64 bit
> > virtual address space are a great deal larger than those for 32-bit.
> >
> > Finally (as in HP-UX 11), a 64 bit OS does not mean that 
> every program is 
> > a 64-bit
> > executable: a 32-bit Shell, Perl and Emacs is sufficient 
> for most users 
> > I'd think
> > ;-)
> >
> > Regards,
> > Ulrich
> >
> >
> >>
> >>
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> >
> >
> >
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> > 
> 
> 
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