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RE: [Xen-users] 32 bit versus 64 bit

To: "Mathias Diehl" <md@xxxxxxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-users] 32 bit versus 64 bit
From: "Petersson, Mats" <mats.petersson@xxxxxxx>
Date: Fri, 25 Nov 2005 15:33:51 +0100
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Thread-topic: [Xen-users] 32 bit versus 64 bit
> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Mathias Diehl
> Sent: 25 November 2005 16:01
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] 32 bit versus 64 bit
> Hi List,
> because I'm still quite new to Linux please kindly advise 
> about hardware.
> I need a new server and need to understand how xen will 
> handle 32 / 64 bit systems. 
> Main contraint is a 32 software to be running on a domU. 
> So - do I still need to run XEN on a 32bit system or will my 
> software (java application) also run on a 64bit system?. I 
> noticed that I will not be able to run a 32 domU on a 64 dom0
> any help would be appreciated.

Ok, there's three different things that matter here:
1. Xen with paravirtualization - You have to have the same memory-type
and bitness on Xen and the Linux kernel (or BSD kernel) that you run on
top of Xen. So 32-bit no PAE Xen means that your Linux kernels need to
be 32-bit no PAE. If you enable PAE or make it 64-bit in Xen, then your
Linux kernel needs to be PAE or 64-bit too. 

2. Xen with new hardware which supports HW virtualization - this mode
supports mixing & matching, at least "backwards" i.e. as long as the Xen
kernel is "better" than the Linux kernel you're fine. Better is ranked
32-bit no PAE < 32-bit PAE < 64-bit, so with a 64-bit Xen, you can run
any version of Linux 32-bit with or without PAE and 64-bit as well.
Obviously, Dom0 is still paravirtualized, so it needs to be 64-bit if
you run 64-bit Xen. 

3. OS 32-bit compatibility. For 99% of all software, Linux supports
32-bit applications under 64-bit OS, with a (fairly thin) thunking layer
that translates 32-bit parameters to 64-bit and adds flags to restrict
for instance memory allocations to return an address within the lower
4GB of the address space. 

So if you have a Java application that runs under a 32-bit Java VM (or
JIT), it would run just as well in a 64-bit OS as it does today in
32-bit OS, with the slight difference that the 64-bit OS will be able to
support 64-bit applications too, and more importantly for the 32-bit
apps, the OS runs a little bit faster for most system calls, and it can
handle huge amounts of memory (you could run several 32-bit apps all
using almost 4GB of memory each, at once). 

I would suggest that you try out your Java app on a 64-bit OS - whether
it's running under Xen or not shouldn't make a difference for it's
compatibility mode - and my guess is that it works just fine.


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