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

KOSAKI Motohiro
2009-07-28T02:23:19Z

> 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?

It can. but I don't think it is sufficient.
On flight recorder use-case, The tracing run more than one year. And,
Administrator only see last N sec log. (previous logs were overwritten
by ring-buffer go-around)

Thanks.