Re: [Xen-devel] understanding __linear_l2_table and friends

[Andi Kleen <ak@xxxxxxx>]

On Sat, Apr 23, 2005 at 08:13:08AM -0700, Wim Coekaerts wrote:
> On Sat, Apr 23, 2005 at 05:08:27PM +0200, Andi Kleen wrote:
> > That is why the PAE effort seems so pointless to me. I estimate it will
> > take some months at least until it is stable and released, and at this time 
> > most of the new x86 world is x86-64 capable.
> > 
> > The only boxes for which PAE is needed are basically some old servers,
> > and these will be quickly replaced with new 64bit capable ones.
> 
> 
> sorry andi I disagree
> "some" is incorrect. there are huge huge numbers of servers outthere,
> you don't just replace them. many potential xen users probably have 100s
> of relatively recent x86 servers around.
> 
> one doesn't just replace servers. maybe at home, but not companies.
> if you have a server farm with 4000 systems, you don't just toss it.

> I think it's worth the effort

You toss it after 3-4 years at least. Lets say 3 years.

If you bought them in the last year you very likely already got them 64bit 
capable. Assuming it takes a year until PAE Xen is usable. They are at least 
two years old when PAE Xen runs on them. Gives 1 years of usable runtime. 

Not too much.

My impression is more that people want PAE Xen because 64bit Xen is not
quite ready yet, but I would not be surprised if 64bit Xen works
sooner than PAE Xen and then that would be obsolete. In general
from my experience working on PAE Linux I can say that the complexity
of handling more than 4GB RAM with less than 4GB address space
is often greatly underestimated. Linux took years before the many corner
cases were flushed out, and now it is somewhat fragile. Of course
Xen is simpler than Linux, but in many ways it has much less
infrastructure to deal with memory pressure so I would not be 
surprised if some stuff would be harder to handle. So the 1 year
estimate for it running well might be optimistic.

Making 64bit Xen run well is probably easier, even if it needs
more changes and some hacks now.

-Andi

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