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Re: [Xen-devel] Xen 4.3 development update

At 11:34 -0400 on 03 Apr (1364988853), 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:
> >>> 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?

Yes, 4.2 is definitely slower.  A compile test on a 4-vcpu VM that takes
about 12 minutes before this locking change takes more than 20 minutes
on the current tip of xen-unstable (I gave up at 22 minutes and rebooted
to test something else).


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