RE: [Xen-users] Q: NUMA and XEN

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Ulrich Windl
> Sent: 09 October 2006 07:10
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] Q: NUMA and XEN
> 
> Hi,
> 
> I have a simple question, that's maybe an FAQ: The XEN 
> hypervisor only reports one 
> zonelist during boot, while a normal kernel will report two 
> for a two-node NUMA 
> machine. Now I'm wondering if I create two VMs with roughly 
> half of the RAM 
> (amount one node has), will XEN be clever or dumb when 
> assigning RAM to those VMs?

Dumb. There's no guarantee that any domain will get any particular
memory, or even that the memory is contiguous within a lump. Until the
actual code to support NUMA is in place, you're just as well off letting
the two DomU's switch between CPU's as they like - that will at least
average out, rather than being "always bad" (if you're unlucky). 

Of course, on a two-node Opteron system, I doubt that you'll be
measuring a huge difference unless you're running some HPC application -
in which case you should consider "not using Xen", as HPC on Xen isn't
the most optimal solution [lots of things are slowed down a little bit
by the extra layering - which isn't exactly great for your average HPC
application]. 
> 
> XEN will be clever if it takes the RAM from one node, 
> preferrably that where the 
> assigned CPU is, and it will be dumb if it assigns just any 
> RAM, causing a 
> slowdown of RAM access.

Since Xen isn't aware of which RAM belongs to which processor, it will
be completely random whether you get "good" or "bad" result. 

--
Mats
> 
> Regards,
> Ulrich
> 
> 
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