Re: measuring and/or dealing with "idleness" and variable frequency

[David Ahern <dsahern@gmail.com>]

On 6/6/13 11:46 AM, Rick Jones wrote:
> I am coming to "perf" (record -e cycles and report) from an old-time
> background where one could see the "idle routine" in a profile and know
> how "idle" (in terms of classic reporting a la top) a CPU was.  Back
> then the frequency was fixed, the idle loop was always "running" when
> the CPU was idle and there were no hardware threads.  Life was simple
> and good.

You could use the scheduling events or context-switches to know when a 
CPU is idle. Idle time for a CPU is one of the stats my perf-timehist 
command shows. I just dumped it to LKML:
    https://lkml.org/lkml/2013/6/7/656

It's an RFC from an inclusion upstream but it has been used for over 2 
years.

David


>
> Now I have to realign to the present and am wondering where/how to get
> started.  I've looked at https://perf.wiki.kernel.org/index.php/Tutorial
>
> which talks about:
>
>> By default, perf record uses the cycles event as the sampling event.
>> This is a generic hardware event that is mapped to a
>> hardware-specific PMU event by the kernel. For Intel, it is mapped to
>> UNHALTED_CORE_CYCLES. This event does not maintain a constant
>> correlation to time in the presence of CPU frequency scaling. Intel
>> provides another event, called UNHALTED_REFERENCE_CYCLES but this
>> event is NOT currently available with perf_events.
>>
>> On AMD systems, the event is mapped to CPU_CLK_UNHALTED and this
>> event is also subject to frequency scaling. On any Intel or AMD
>> processor, the cycle event does not count when the processor is idle,
>> i.e., when it calls mwait().
>
> And I am wondering if those are all still true.
>
> I am running a 3.5.0 kernel in an Ubuntu environment (3.5.0-23-generic
> #35~precise1-Ubuntu SMP), and among the things I wish to do is sanity
> check the Idle % being reported by top on one system which is passing
> traffic back and forth between two others.  Compared to the CPU
> utilization reported by top on those two systems, the reports for this
> one look extraordinarily low.
>
> The CPU involved here is:
>
> Intel(R) Xeon(R) CPU E5-2670 0 @ 2.60GHz
>
> I am also interested in seeing how the CPU consumption changes as I
> tweak the setup of the middle box, does it go more or less idle which
> routines have their consumption change etc etc.  and I am wondering if I
> can get that all with just perf or if I have to correlate things between
> multiple tools.
>
> thanks, and happy benchmarking,
>
> rick jones
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