[Fedora-xen] Heads-up: Requiring PAE for running Xen

[Lamont R. Peterson]

On Wednesday 17 May 2006 12:07pm, Jeremy Katz wrote:
> As we move forward with Xen enablement, there's a desire for
> being able to access more than 4 gigs of RAM on 32-bit Xen hosts.  The
> options for handling this are
> 1) Another kernel.  This is bad due to
>    a) we're running out of CD space already
>    b) keeping things matched up between the HV and the guest kernels
>    c) migration is worlds of pain with two types of kernels
> 2) Switch the 32-bit xen kernels to require PAE.  For most "current"
> non-laptop hardware, this is a non-issue.  It does mean that xen won't
> work a lot of earlier PentiumM laptops
> 3) Do nothing, tell people to use 64bit if they want more than 4 gigs of
> RAM
> 4) Make the PAE code handled at runtime.  This is a pretty non-trivial
> amount of work :)
>
> Given these, we're looking at going with #2 and thus only having Xen
> work on PAE-capable hardware in the development tree.  And we're
> planning to try to execute this switchover the beginning of next week.
> Note that this will not affect bare metal installs at all.

Personally, I like 4 better.  In addition, if someone wants to turn on PAE for 
their 32bit Xen boxes, it's not that hard to rebuild your own kernel RPMs.  
It only takes 6.5 hours on this old-school mobile P4 notebook, and on my Dual 
1.6GHz Athlon, it's about 1 hour to build a complete set of FC5 kernel 
packages.

However, I have another question about PAE; let's say I have a box with 2-4GB 
RAM and I run a non-PAE kernel (Xen or otherwise, but I am using Xen, of 
course :) ) ... what kind of performance hit should I expect to take from 
turning on PAE?  I've heard numbers around 3:1, before, but that doesn't 
quite sound right to me.

Now, let's say I scale up the RAM in that box to 8GB after turning on PAE.  
Will there be a difference in memory access performance compared to using PAE 
with 4GB or less?

What about if I two Xen guests that decide to start running some memory 
intensive operations and they just happen to be in different 4GB regions?  
i.e. guest A is in the "lower" 4GB and guest B is in the upper 4GB.  Would 
there be a difference if they were within 4GB of each other.

Lastly, are you talking about 16GB or 64GB support.  As I understand things, 
there are almost no 32bit processors (if any) that can actually use PAE to 
access RAM past 16GB.

Thanks.
-- 
Lamont R. Peterson <lamont at gurulabs.com>
Senior Instructor
Guru Labs, L.C. [ http://www.GuruLabs.com/ ]
GPG Key fingerprint: F98C E31A 5C4C 834A BCAB  8CB3 F980 6C97 DC0D D409
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