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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