[ltt-dev] Interpreting tracing results

[Mathieu Desnoyers]

* Akyurek, Ali (EXT) (ali.akyurek.ext at siemens.com) wrote:
> 
> Hi
> 
> So, it means that calculating a context switching time in arm is not sensible for now,huh?
> Because i found that 1220 ns.
> 
> And 1220 ns is just = [1 / (HZ << TRACE_CLOCK_SHIFT)]. 
> 
> // HZ is 100 in my system, and trace_clock_shift is 13
> 

Exactly. You cannot expect that kind of precision on ARM currently
without implementing a more precise trace-clock suited for your
sub-architecture, possibly based on a free timer.

Mathieu

> Thanks.
> 
> 
> -----Ursprüngliche Nachricht-----
> Von: Mathieu Desnoyers [mailto:compudj at krystal.dyndns.org] 
> Gesendet: Donnerstag, 15. Januar 2009 16:19
> An: Akyurek, Ali (EXT)
> Cc: ltt-dev at lists.casi.polymtl.ca
> Betreff: Re: [ltt-dev] Interpreting tracing results
> 
> * Akyurek, Ali (EXT) (ali.akyurek.ext at siemens.com) wrote:
> > 
> > Hi all,
> > 
> > In my arm machine, context switching takes 1220 or 1221 nanoseconds. 
> > i thought this time as (start of sched_schedule - start of
> > whatever_previous_event), Actually that is the time between different
> > PID values in rows.am i right?
> > 
> > and
> > 
> > Handling of interrupts (i take only the top handlers, not soft ones.)
> > takes 2441 nanoseconds.
> > i thought this time as (end of irq_exit - start of irq_entry), am i
> > right?
> > 
> > Why are these values multiple of 1220 nanoseconds? What is related to? 
> > 
> 
> Hi,
> 
> LTTng uses the generic trace clock for ARM by default. See
> include/asm-generic/trace-clock.h.
> 
> This clock has only the precision of HZ frequency, and I use an atomic
> counter in the LSBs to keep the events ordered.
> 
> Please have a look at the ARM-related messages in the past 2 weeks for
> implementations of sub-arch specific clock sources for ARM. I will
> integrate this kind of work soon after making sure it's smp-safe when it
> needs to.
> 
> Mathieu
> 
> > Thanks all.
> > 
> > _______________________________________________
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> > ltt-dev at lists.casi.polymtl.ca
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> > 
> 
> -- 
> Mathieu Desnoyers
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68