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

Ingo Molnar
2009-08-04T14:38:23Z

* KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

> >
> > * KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:
> >
> > > 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)
> >
> > That's definitely something we want to enable ASAP.
> >
> > I'm wondering why a single u64 timestamp is not enough to express
> > the full, absolute range of time elapsed since bootup, in
> > nanoseconds.
> >
> > That would make walltime a matter of pretty-printing only - we'd
> > have to convert the u64 nsec timestamp into a walltime format,
> > right?
> >
> > In fact we could change all the timestamps to be standardized along
> > 'nanoseconds elapsed since 1970' or so - not nanoseconds since the
> > last bootup. That still fits just fine within 64 bits - u64
> > nanoseconds has a scope of 500+ years.
>
> Ah, you are right. thanks correct me. but I think ntp updating
> time storing is necessary too.

Yeah. As long as we only save a single u64 into the trace it should
all be fine.

There could be periodic samples of <walltime, sched_clock()> saved
on a per cpu basis, and thus converting sched_clock() values into
walltime unit?

Ingo