x86 vs. x86_64

[Alex Dean]

This has always confused me a bit, so it seems like a good time to ask.

I have a machine running 32-bit Ubuntu.  It has 4GB of RAM.  I'd like  
to use it as a host OS for several (maybe 6?) VM guests.  I thought it  
would make sense to add more RAM before doing this, but I wasn't sure  
if I'd be able to utilize the extra memory w/o changing to a 64-bit  
host OS.  From reading this thread, it sound like I wouldn't need to  
change over to a 64-bit host, since none of my guests would have  
processes accessing more than 4GB of RAM.  Is that correct?

The guests will be server OSs, not running any GUI applications, so I  
expect their resource utilization to be pretty slim.  I just need  
somewhere to test multiple-box HA cluster configurations, and I don't  
have the extra hardware lying around to do it with.

I'll probably use VMWare, but that's not settled.  If OpenVZ or  
VirtualBox might have advantages for this kind of thing, I'm  
interested in hearing about them. The host is Ubuntu, but the guests  
will be RHEL5, so I think that means I can't use OpenVZ since they'll  
have different kernel versions.  Someone correct me if I've got that  
wrong.

thanks,
alex

ps - Apologies if this is thread-hijacking.

On Mar 26, 2010, at 12:17 AM, Kurt Granroth wrote:

> Just to be pedantic; Matt is correct in his use of 'AND' with the  
> memory.  It is 'have more than 4GB RAM *AND* something that can use  
> more than 4GB in one process'.
>
> Basically, this is due to PAE.  The PAE extension (which all modern  
> CPUs support) allow the OS to access up to 64GB of memory, even on a  
> 32-bit x86 system.  There's no need for a 64-bit processor for that.
>
> HOWEVER, PAE doesn't permit a single process from mapping or  
> allocating more than 4GB.  That's where you'll need 64-bit processors.
>
> And if you don't know for sure that you need that capability... then  
> you definitely don't.  It's very very rare (unless, of course, you  
> are in the fields that do need that and then you'd know that).
>
> On 3/25/10 5:38 PM, Stephen wrote:
>> this is pretty accurate. if you are at the 4g limit or more then 64
>> bit. or if you are doing some really high end memory intensive
>> applications then you will have a need, otherwise 32 bit is the  
>> better
>> idea for compatibility and ease of use (some weirdness with flash in
>> 64 bit, solvable but you have some hoops to jump, there are a few
>> others like this)
>>
>> On Thu, Mar 25, 2010 at 3:32 PM, Matt  
>> Graham<danceswithcrows at usa.net>  wrote:
>>> From: Nathan England<nathan at paysonlinux.org>
>>>> Before I spend any time downloading and trying it myself, has any  
>>>> one taken
>>>> the time to compare performance differences in every day usage  
>>>> between
>>>> ubuntu x86 and the 64 bit version?
>>>
>>> There is essentially ZERO performance difference between 64- and  
>>> 32-bit Linux
>>> for normal user apps AFAICT on a Gentoo box.  Couple that with the  
>>> fact that
>>> at least one app (epsxe, PSX emulator) doesn't work at all on 64- 
>>> bit Linux,
>>> and it's a no-brainer:  Use 32-bit.
>>>
>>> The only reason that I can see to use 64-bit Linux is if you have  
>>> more than 4G
>>> RAM in your box AND you've got something that can benefit from  
>>> malloc()ing
>>> more than 4G of RAM in one process.  So if you're running a big DB  
>>> or a huge
>>> numerical simulation or something like that, go 64-bit.   
>>> Otherwise, go
>>> 32-bit.
>>>
>>> (Constructive criticism and flying attack porcupines welcomed,  
>>> since the above
>>> post may be full of bovine feces....)
>>>
>>> --
>>> Matt G / Dances With Crows
>>> The Crow202 Blog:  http://crow202.org/wordpress/
>>> There is no Darkness in Eternity/But only Light too dim for us to  
>>> see
>>>
>>>
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>>
>>
>>
>
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