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

From: Steven Rostedt <rostedt@goodmis.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
	Zhaolei <zhaolei@cn.fujitsu.com>, Ingo Molnar <mingo@elte.hu>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/3] Add walltime support for ring-buffer
Date: Mon, 27 Jul 2009 22:19:47 -0400 (EDT)	[thread overview]
Message-ID: <alpine.DEB.2.00.0907272218480.30108@gandalf.stny.rr.com> (raw)
In-Reply-To: <20090728015351.GF5147@nowhere>

On Tue, 28 Jul 2009, Frederic Weisbecker wrote:
> > 
> > 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?

This should be something in one of the trace clock sources. It can perhaps 
have a helper function called by gettimeofday or something, but it should 
be something that gets plugged into the ring buffer and not something that 
needs to modify any of its code.

-- Steve