Re: [Xen-devel] Xen 4.3 development update

[Tim Deegan <tim@xxxxxxx>]

At 16:20 +0100 on 25 Apr (1366906804), George Dunlap wrote:
> On Thu, Apr 4, 2013 at 4:23 PM, Tim Deegan <tim@xxxxxxx> wrote:
> > 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).
> 
> Tim,
> 
> Can you go into a bit more detail about what you complied on what kind of OS?

I was compiling on Win XP sp3, 32-bit, 1vcpu, 4G ram.  The compile was
the Windows DDK sample code. 

As I think I mentioned later, all my measurements are extremely suspect
as I was relying on guest wallclock time, and the 'before' case was
before the XP wallclock time was fixed. :(

> The VM was a Debian Wheezy VM, stock kernel (3.2), PVHVM mode, 1G of
> RAM, 4 vcpus, LVM-backed 8G disk.

I suspect the TPR access patterns of XP are not seen on linux; it's been
known for long enough now that it's super-slow on emulated platforms and
AFAIK it was only ever Windows that used the TPR so aggressively anyway.

Tim.

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