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@xxxxxxxxxxxx> 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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