Re: kvm ppc timing stats

[Hollis Blanchard <hollis_blanchard@mentor.com>]

Ah right, makes sense. I guess you'll send a patch soon. :)

Hollis Blanchard
Mentor Graphics, Embedded Systems Division


On 12/07/2010 02:32 PM, Yoder Stuart-B08248 wrote:
> I figure out what is going on here.   The timing stats convert
> exit durations into microseconds.   The problem is that any
> exits that take less than 1us will wind up having a duration
> of 0.  After to changing to count timebase ticks instead, the
> numbers add up.
>
> By the way-- there aren't any instructions accounted for,
> even though they don't cover 100% of the handler-- any
> instructions that run before reading the timestamp will
> get accounted to the guest.
>
> Stuart
>
>> -----Original Message-----
>> From: Hollis Blanchard [mailto:hollis_blanchard@mentor.com]
>> Sent: Thursday, December 02, 2010 12:38 PM
>> To: Yoder Stuart-B08248
>> Cc: kvm-ppc@vger.kernel.org
>> Subject: Re: kvm ppc timing stats
>>
>> I don't think the numbers added up like that for us either. We treated
> them
>> as relative data, not absolute.
>>
>> I don't remember how early/late the timestamps were recorded, but
> obviously
>> they cannot cover 100% of the handler. As the number of exits
> increases,
>> those unaccounted-for instructions would add up... but I wouldn't
> expect
>> that to cause errors of the magnitude you're seeing.
>> Perhaps there is a more obvious problem with the way certain exit
> types are
>> recorded (or not recorded).
>>
>> Hollis Blanchard
>> Mentor Graphics, Embedded Systems Division
>>
>>
>> On 12/01/2010 08:18 PM, Yoder Stuart-B08248 wrote:
>>> Well, in the example I'm looking at, which runs for 30 seconds, the
>>> "sum" column of the stats adds up to
>>> about 10 seconds total.   So, there is 20 seconds
>>> unaccounted for.
>>>
>>> Interestingly enough, if I let the guest just sit idle for 30
> seconds,
>>> the stats do sum up to about 30 seconds.
>>>
>>> Will get to the bottom of it, but wanted to make sure I was not
>>> missing something obvious.
>>>
>>> Stuart
>>>
>>>
>>>> -----Original Message-----
>>>> From: Hollis Blanchard [mailto:hollis_blanchard@mentor.com]
>>>> Sent: Wednesday, December 01, 2010 6:27 PM
>>>> To: Yoder Stuart-B08248
>>>> Cc: kvm-ppc@vger.kernel.org
>>>> Subject: Re: kvm ppc timing stats
>>>>
>>>> Yes, your understanding is correct (barring any bugs, of course).
> Why
>>> do
>>>> you think "time is missing"?
>>>>
>>>> Hollis Blanchard
>>>> Mentor Graphics, Embedded Systems Division
>>>>
>>>>
>>>> On 12/01/2010 12:02 PM, Yoder Stuart-B08248 wrote:
>>>>> Hollis,
>>>>>
>>>>> Am looking at some performance data and want to make sure that
>>>>>
>>>>> I'm understanding things correctly with your
> CONFIG_KVM_EXIT_TIMING
>>>>> stuff.   If I reset the timing counters, run a workload
>>>>>
>>>>> under for a fixed duration (e.g. 30 seconds), and then look
>>>>>
>>>>> at the exit stats, I should see 30 seconds worth of time accounted
>>>>>
>>>>> for, correct?
>>>>>
>>>>> Right now I'm seeing a substantial amount of time "missing" and
>>>>>
>>>>> want to make sure I'm not missing something.
>>>>>
>>>>> Stuart
>>>>>
>