Richard Jones
2006-Feb-25 18:48 UTC
[Xen-users] Xen, SMP and Opterons with different stepping
We have a single Opteron server running Xen. For various reasons we need to add a second CPU. Although we can get the same model Opteron, it''s unlikely that it will have the same stepping. Is this going to be a problem, or do we have to buy matched Opterons? Can we avoid this by somehow pinning the domains to CPUs so that they will never unexpectedly move to a different stepping CPU? Rich. -- Richard Jones, CTO Merjis Ltd. Merjis - web marketing and technology - http://merjis.com Team Notepad - intranets and extranets for business - http://team-notepad.com _______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users
Petersson, Mats
2006-Feb-27 10:45 UTC
RE: [Xen-users] Xen, SMP and Opterons with different stepping
> -----Original Message----- > From: xen-users-bounces@lists.xensource.com > [mailto:xen-users-bounces@lists.xensource.com] On Behalf Of > Richard Jones > Sent: 25 February 2006 18:48 > To: xen-users@lists.xensource.com > Subject: [Xen-users] Xen, SMP and Opterons with different stepping > > > We have a single Opteron server running Xen. For various > reasons we need to add a second CPU. Although we can get the > same model Opteron, it''s unlikely that it will have the same stepping. > > Is this going to be a problem, or do we have to buy matched Opterons? > > Can we avoid this by somehow pinning the domains to CPUs so > that they will never unexpectedly move to a different stepping CPU?I recommend that you use the same stepping and speed on both CPU''s. Yes, I know, it''s a pain to have it that wat, but it assures that the CPU''s really are doing the same thing when they are supposed to - as a representative of AMD I can''t recommend you do anything else. Chances are also that the BIOS actually requires this to match... As to Xen, if you pin to one CPU, then that guest will run on that CPU. However, Xen kernel itself will run on both CPU''s (potentially simultaneously), which is where the worry could be - there are mechanisms in Xen that rely on the processor doing things in a particular fashion, and although this is guaranteed by the processor architecture, there''s no guarantee that mixes of steppings will not have subtle changes that means that one CPU doesn''t quite understand what the other one is doing - which would cause VERY interesting effects. This said, I think the likelyhood of such changes are actually quite small - it''s just that if you''re relying on the processor to do things in a particular way, it doesn''t take much to make things "not work" - even speeding up/slowing down a particular instruction by a single clock-cycle could cause things to start going wrong - something that DOES happen from one revision to another. -- Mats> > Rich. > > -- > Richard Jones, CTO Merjis Ltd. > Merjis - web marketing and technology - http://merjis.com > Team Notepad - intranets and extranets for business - > http://team-notepad.com_______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users