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Date: Wed, 24 Sep 2008 02:59:37 +0900 (JST)
From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Masami Hiramatsu <mhiramat@redhat.com>
Subject: Re: Unified tracing buffer
Cc: kosaki.motohiro@jp.fujitsu.com, Martin Bligh <mbligh@google.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Thomas Gleixner <tglx@linutronix.de>,
       Mathieu Desnoyers <compudj@krystal.dyndns.org>,
       Steven Rostedt <rostedt@goodmis.org>, od@novell.com,
       "Frank Ch. Eigler" <fche@redhat.com>,
       systemtap-ml <systemtap@sources.redhat.com>
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> > I can't imazine a merit of the single-channel mode.
> > Could you please explain it?
> 
> Actually, single-channel mode is for not-frequently event tracing.
> At least systemtap case, sometimes we just want to collect data
> and watch it periodically(as like as 'top'). Or, just monitoring
> errors as additional printk. in these cases, overhead is not so
> important.
> 
> I think the main reason of using single-channel mode is simplicity of
> userspace reader. We can use 'cat' or 'tail' to read the buffer on-line.
> I'm not sure how much overhead ftrace-like buffer merging routine has,
> but if kernel provides an interface which gives us single-merged buffer
> image(like ftrace buffer), we are grad to use it. :-)

Yup, I also think it is better.

Thanks.