Re: [PATCH 0/3] Add walltime support for ring-buffer

Frederic Weisbecker
2009-07-28T01:53:56Z

On Tue, Jul 28, 2009 at 10:43:11AM +0900, KOSAKI Motohiro wrote:
> Hi Zhaolei,
>
> > Hi Zhaolei,
> >
> > Could you not just save the wall time in some 64bit format within the
> > current timestamp? Right now there's three clocks that can be used by
> > ftrace. Two that you can really get two (I need to add access to the
> > third). The default clock is sched_clock. But there's an option to get to
> > global_clock:
> >
> > debug/tracing/options/global-clock
> >
> > # echo 1 > /debug/tracing/options/global-clock
> >
> > Will enable the global clock. Perhaps we can add a "wall clock" version
> > that will map the timestamps to the wall clock?
> >
> > I really hate adding any more fields to the ring buffer headers. That
> > takes away from the amount that you can record per page.
>
> I think wall-time recording on every event is too slow. slower tracer
> isn't useful.
> Instead, Can we make pseudo wall-time recording event periodically?
>
> In nowadays, many workload run on cluster machine environment. Then
> We often need compare different machines tracing log. it mean TSC isn't
> sufficient. but nobody want performance reduce.
> Fortunatelly, ntp time adjustment is not happend so frequently. we
> don't need wall-time on _every_ event.
>
> Thanks.

Yeah, that would really slow the tracing.

Can't we get a snapshot of the couple (walltime, cpu clock) once
at startup.
Then we can retrieve the walltime when cpu_clock was 0 and compute
the walltime for each traces at output time using the trace timestamp as
a delta?

Thanks,
Frederic.