> -----Original Message-----
> From: xen-users-bounces@lists.xensource.com
> [mailto:xen-users-bounces@lists.xensource.com] On Behalf Of
> Mathias Diehl
> Sent: 25 November 2005 16:01
> To: xen-users@lists.xensource.com
> 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.
--
Mats
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