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

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Date:	Tue, 28 Jul 2009 10:43:11 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	kosaki.motohiro@...fujitsu.com, Zhaolei <zhaolei@...fujitsu.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] Add walltime support for ring-buffer

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.


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