Subject: Re: [vserver] Pros and Cons of 32/64bit guests on 64bit host
From: Martin <inkubus@interalpha.co.uk>
Date: Wed, 20 Aug 2008 23:15:10 +0100

On Wed, 2008-08-20 at 22:56 +0200, Oliver Welter wrote:
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> Hi All,
> 
> I am running vserver on 32bit boxes for years now. I am know replacing
> one group of nodes with nice Dual AMD Quad-Cores which run a 64bit
> debian host OS.
> 
> As I have to update the guests anyway (I used and will use gentoo for
> them), so I am considering to move them to 64bit, too. The guests are
> several LAMP servers, mysql and postgres and a set of qmail nodes.
> 
> The only issue that is an obvious go for 64bit would be memory segments
> >3GB, but I guess I wont have guests who use it. So, anybody here can
> give pros and cons for 32/64 bit guests?

64 bit host seems an obvious choice as then you can easily have either
32 bit or 64 bit guests.  Also, if you are looking at consolidation you
can buy a bigger host machine and make use of the extra RAM.

On x86*, 64 bit gives you extra address space and critically extra
registers, at the cost of larger executables and more meory used per
process.  If you have more than 4 Gb physical RAM then applications that
use a large amount of memory (i.e. databases) will likely run faster, as
probably will CPU intensive applications (the extra registers really
help the code generation in the compiler), however memory bandwidth /
latency sensitive apps will likley run slower as the amount of memory
they use is increased.  In short, it depends on your workload.  If you
workload is disk / I/O limited then it is unlikely to make much
difference.

At the operating system level you may find there are a few compatability
issues with 64 bit applications, particularly propriatory applications.
However 64 bit guests can run 32 bit libraries for backwards
compatability so this is normally not a problem.  I recall seeing
occasional problems of this kind a fw years ago but nothing since.

Personally - I'd run 64 bit host, build new guests as 64 bit and leave
old guests as 32 bit.

HTH

Cheers,
 - Martin