execution time on RPI.

[Mohammad Hamad]

Hi Martin,

thank you for the explanation.

best,

On 3 August 2017 at 16:54, Martin Stein <martin.stein at ...1...>
wrote:

> Hi Mohammad,
>
> Am 02.08.2017 um 16:36 schrieb Mohammad Hamad:
> > I am trying to measure the execution time of a function on RPI model B.
> >
> > I used the "Trace::timestamp" function to calculate it. In the same
> > time, I used the elapsed_ms() function to compare with it.
> >
> > as I understood, the "Trace::timestamp" function gives the performance
> > counter.  Dose the elapsed_ms function provide the execution time of the
> > method in milliseconds?
>
> Yes, 'Trace::timestamp' returns the ARM performance-counter value on
> RPI. You can find the implementation details in [1]. And no,
> 'elapsed_ms()' does not return the execution time of any function but
> the time elapsed since the creation of the Timer::Connection on which
> you call it. More precisely the point in time when the Timer server
> initializes the 'elapsed_ms' counting for the Connection. So, the best
> way to use it would be by creating the delta of two subsequent calls.
>
> > During the measurements, I got some related numbers in somehow. For an
> > example, the average of  elapsed_ms() was 2,18 and the  average of
> > performance counter is 24825,02
> >
> > How could I convert the delta of the performance counter to
> > milliseconds? I guess I need to divide by the CPU_cycle.
>
> The performance counter on ARMv6 platforms like RPI is configured to
> count every 64th CPU cycle. So, the formula for the performance counter is:
>
> PERF_COUNTER = MS * 1000 * CPU_FREQ / 64
>
> And thus, the translation to milliseconds would look like this:
>
> MS = PERF_COUNTER * 64 / ( CPU_FREQ * 1000 )
>
> >  If yes, does the cpu_cycle specified or modified by the Genode? How can
> >  I get this value?
>
> AFAIK, the RPI model B has a CPU frequency of 700 Mhz and Genode does
> not adapt this. Only one sidenote at this point: Be aware that the ARM
> performance counter does not necesarily increase constantly over time.
> AFAIK, it does for the RPI, but there are counter examples. For
> instance, on all ARMv7 platforms, we use the WFI (wait for interrupt)
> instruction on idle [2] to save energy and thereby pause the counter for
> an unforeseeable amount of time. But, as I said, for RPI, I do not know
> any reason why the counter shouldn't be constant over time.
>
> I hope this helps you...
>
> Cheers,
> Martin
>
> [1] base-hw/src/core/spec/arm_v6/perf_counter.cc
> [2] base-hw/src/core/spec/arm_v7/cpu_support.h
>
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