RE: [Xen-users] Help Please! 64bit Xen/CentOS_64 on quad-core Opteron 12gig RAM

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

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Stephen Yum
> Sent: 06 August 2006 10:15
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] Help Please! 64bit Xen/CentOS_64 on 
> quad-core Opteron 12gig RAM
> 
> Thanks for suggestion. It just turned out that I set too little  
> memory dom0 - 64M. I upped it to 128M and it boots okay now, 
> although  
> in the 32-bit world, 64M for dom0 and domU's work just fine.

That's probably more because the 32-bit kernel is more optimised to run
on a small system (very few people with a small memory machine are
likely to have a 64-bit processor, so the "wasting memory" problems
aren't being so apparent). Also page-tables take up more space in
64-bit, a little bit more than twice the amount of space needed for
32-bit-non-PAE (twice the memory is accounted as PAE-mode which uses
64-bit page-table entries, vs. non-PAE-mode pagetable entries that use
32-bit entries. On top of thise, there is the fact that the page-table
is now 4-level, which means that there is more table to deal with, but
that's a relatively small amount - 4KB for every 512 x 2MB -> 1GB of
memory, so 12 x 4 -> 48KB for your entire 12GB of RAM [actually, with
multiple page-tables, it may be a few times it, but then every one of
those page-tables wouldn't have entries for all the memory, so although
it's hard to give an exact number, it's unlikely to be MUCH more than
4KB per gigabyte overall). 

> 
> In x86_64 based Xen, is the minimum memory for domains 128MB?

It's possibly somewhat lower. As discussed above, page tables takes 2x+
amount of space, but many other things may only be marginally larger
(data structures that contain a few pointers and many other things may
not grow more than a few percent). 

But if you have 12GB, I'm sure that 64MB +/- on the Dom0 isn't going to
matter. DomU will most likely require less space, as it's got fewer
drivers. How much less is harder to say - I haven't experimented to find
out... Also depends on whether you use the same or a different kernel in
DomU of course - if you use the same kernel, the difference is modules
that will not get loaded in DomU. 

> 
> And just out of curiosity, is there a big difference between Fedora  
> 64bit vs. CentOS 64bit? Not much I would think.

I would think so too. The main difference would be that Fedora comes
with a pre-built "know to work" Xen binary, so you don't need to build
your own... 

--
Mats
> 
> S
> 
> On Aug 6, 2006, at 2:01 AM, Jerone Young wrote:
> 
> > I would first suggest finding a Xen ready distribution and 
> not hacking
> > it your self to work the first time. Best to try Fedora 
> Core 5 x86-64.
> > Use the xen packages there (yum install xen kernel-xen0)  
> It will make
> > life easier. Then from their people can help you more.
> >
> >             
> > On Sat, 2006-08-05 at 23:10 -0700, Stephen Yum wrote:
> >> Hi all,
> >>
> >> I'm having a hell of a time trying to get 64 bit Xen on a machine
> >> with 2 x dual core Opteron system with 12GB of RAM.  No 
> matter what I
> >> do, the dom0 boot bombs out with kernel panic saying low memory out
> >> of space.
> >>
> >> This is my first foray into the 64 bit land. In the 32 bit world,
> >> everything worked okay, seemingly no matter what i do.
> >>
> >> Can someone tell me what I'm doing wrong here?
> >>
> >> Thanks in advance!!
> >>
> >> S
> >>
> >> _______________________________________________
> >> Xen-users mailing list
> >> Xen-users@xxxxxxxxxxxxxxxxxxx
> >> http://lists.xensource.com/xen-users
> >
> 
> 
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> 



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