[Wylug-discuss] Linux on Intel 64-Bit Xeon

[John Hodrien]

On Thu, 23 Jun 2005, Neil Pilgrim wrote:

> Phil Foxton wrote:
>> On Thu, 2005-06-23 at 12:46 +0100, Dave Fisher wrote:
> [...]
>>>  2.  Isn't AMD's X86_64 capable of running 32-bit x86 code, and
>>>  therefore "32-bit compatible"? Whereas there never was a 32-bit
>>>  version of Itanium, so it its "32-bit _in_compatible"? What have I
>>>  failed to grasp?
>> 
>> Yes you are correct, but the box in question is a 64 bit Xeon (following
>> me so far?) which will run in 64 AND 32 bit mode BUT not at the same
>> time.  There are ways around this, one is to have the box dual boot (not
>> really an option with a server, I am sure you will agree), one is to use
>> virtualisation (Xen) to create a 32 bit virtual machine on the 64bit
>> server and OS to allow the 32 bits apps to run.
>
> So are you saying that the EM64T intel chips don't allow running in 32- and 
> 64-bits 'simultaneously'? I'm intrigued because I run some 32-bit apps here 
> on my athlon64 running debian, in an i386 chroot (with i386 debs), at the 
> 'same' time as the rest of the system is running in 64-bit mode, ie. with the 
> 64-bit debs. Are you saying that while the EM64T is a compatible arch to amd 
> 64-bit, it can't do this?

I think this is a misunderstanding of how it all works.  Benchmarks on our
EM64T box show that 32bit code can run a squidgeon (new SI unit) faster in
64bit mode than 32bit.  Nothing that I'd bother about, but certainly not
slower.

> I thought the only software emulation required was if you run 32-bit on 
> Itanium?

Indeed.

> If I follow what you're saying, then I'm glad I didn't buy an EM64T chip!

I wouldn't be glad at all.  You don't lose in any way by having the
extensions, as you're always free to not use them.  The performance of a large
memory machine does however suck (as I understand things) on EM64T compared to
AMD64.

jh

-- 
"Tact in audacity consists in knowing how far we may go too far."
                                                      -- Jean Cocteau