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

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Date:	Tue, 4 Aug 2009 16:38:12 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Zhaolei <zhaolei@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] Add walltime support for ring-buffer


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

> > 
> > * KOSAKI Motohiro <kosaki.motohiro@...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
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