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


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