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Date: Tue, 28 Jul 2009 03:53:52 +0200 From: Frederic Weisbecker <fweisbec@...il.com> To: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> Cc: Steven Rostedt <rostedt@...dmis.org>, Zhaolei <zhaolei@...fujitsu.com>, Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [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? Thanks, Frederic. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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