Re: [Xen-devel] Xen 4.3 development update / winxp AMD performance regression

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Wed, Apr 03, 2013 at 11:34:13AM -0400, Andres Lagar-Cavilla wrote:
> On Apr 3, 2013, at 6:53 AM, George Dunlap <george.dunlap@xxxxxxxxxxxxx> wrote:
> 
> > On 03/04/13 08:27, Jan Beulich wrote:
> >>>>> On 02.04.13 at 18:34, Tim Deegan <tim@xxxxxxx> wrote:
> >>> At 16:42 +0100 on 02 Apr (1364920927), Jan Beulich wrote:
> >>>>>>> On 02.04.13 at 16:07, George Dunlap <George.Dunlap@xxxxxxxxxxxxx> 
> >>>>>>> wrote:
> >>>>> * AMD NPT performance regression after c/s 24770:7f79475d3de7
> >>>>>   owner: ?
> >>>>>   Reference: http://marc.info/?l=xen-devel&m=135075376805215
> >>>> This is supposedly fixed with the RTC changes Tim committed the
> >>>> other day. Suravee, is that correct?
> >>> This is a separate problem.  IIRC the AMD XP perf issue is caused by the
> >>> emulation of LAPIC TPR accesses slowing down with Andres's p2m locking
> >>> patches.  XP doesn't have 'lazy IRQL' or support for CR8, so it takes a
> >>> _lot_ of vmexits for IRQL reads and writes.
> >> Ah, okay, sorry for mixing this up. But how is this a regression
> >> then?
> > 
> > My sense, when I looked at this back whenever that there was much more to 
> > this.  The XP IRQL updating is a problem, but it's made terribly worse by 
> > the changset in question.  It seemed to me like the kind of thing that 
> > would be caused by TLB or caches suddenly becoming much less effective.
> 
> The commit in question does not add p2m mutations, so it doesn't nuke the 
> NPT/EPT TLBs. It introduces a spin lock in the hot path and that is the 
> problem. Later in the 4.2 cycle we changed the common case to use an rwlock. 
> Does the same perf degradation occur with tip of 4.2?
> 

Adding Peter to CC who reported the original winxp performance 
problem/regression on AMD.

Peter: Can you try Xen 4.2.2 please and report if it has the performance 
problem or not? 

Thanks,

-- Pasi


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