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Date: Tue, 4 Aug 2009 16:38:12 +0200
From: Ingo Molnar <mingo@elte.hu>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
       Steven Rostedt <rostedt@goodmis.org>, Zhaolei <zhaolei@cn.fujitsu.com>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/3] Add walltime support for ring-buffer
Message-ID: <20090804143812.GJ7746@elte.hu>
References: <20090728105931.D801.A69D9226@jp.fujitsu.com> <20090803072255.GB27581@elte.hu> <20090803182412.CC15.A69D9226@jp.fujitsu.com>
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* 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