Re: [Xen-devel] understanding __linear_l2_table and friends

[Gerd Knorr <kraxel@xxxxxxx>]

On Sat, Apr 23, 2005 at 05:28:26PM +0200, Andi Kleen wrote:
> If you bought them in the last year you very likely already got them 64bit 
> capable.

That the machines are 64bit capable doesn't mean that people will
actually run 64bit software on them.  Note that the very good backward
compatibility of x86_64 machines to 32bit software is one of the key
features leading to the success of the processors (lesson learned from
ia64 ;)

Not everyone will instantly switch over to 64bit software just because
the processor is able to do so, there are still way to much issues with
64bit Software.  Linux is way ahead compared to most other operating
systems, and still there are plenty of problems: OpenOffice is still
32bit, Firefox runs in 32bit much more stable than in 64bit, to name
just two prominent examples.  And with non-mainstream software it is
even more likely to trap into not-yet fixed 64bit bugs.

Nevertheless I don't expect 80% of the installations being PAE, thats
too much.  People will start using 64bit software, but I'm sure not
everybody will not instantly switch over to 64bit just because the
hardware can do that.  If it's only to reduce the maintainance work
in a data center with both 32 and 64bit capable machines ...

> 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.

> 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.

Well, after looking into Xen's mm code I'd say this is no problem for
Xen.  Xen basically delegates all that work to the guest operating
system, it simply doesn't has to deal with memory pressure issues.

> So the 1 year estimate for it running well might be optimistic.

I'd say it is pessimistic, but let's see ...

At the moment my pae xenlinux kernel doesn't survice paging_init() yet.
It seems to me that this piece of code already triggers almost
everything which must be touched for PAE support in xenlinux and xen
though, so I expect a dom0 multi-user boot isn't that far away once
paging_init() works fine ;)

  Gerd


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