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

[Andres Lagar-Cavilla <andreslc@xxxxxxxxxxxxxx>]

On Apr 25, 2013, at 10:00 AM, George Dunlap <george.dunlap@xxxxxxxxxxxxx> wrote:

> On 04/25/2013 02:51 PM, Pasi Kärkkäinen wrote:
>> 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?
> 
> Do you want to compare 4.2.2 to 4.2.1, or 4.3?
> 
> The changeset in question was included in the initial release of 4.2, so 
> unless you think it's been fixed since, I would expect 4.2 to have this 
> regression.

I believe you will see this 4.2 onwards. 4.2 includes the rwlock optimization. 
Nothing has been added to the tree in that regard recently.

Andres
> 
> -George
> 


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