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(a)gurulabs.com>
Senior Instructor
Guru Labs, L.C. [
http://www.GuruLabs.com/ ]
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