linux-kernel - Re: Unified tracing buffer

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Date:	Tue, 23 Sep 2008 11:02:09 -0400
From:	"Frank Ch. Eigler" <fche@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Masami Hiramatsu <mhiramat@...hat.com>,
	Martin Bligh <mbligh@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Steven Rostedt <rostedt@...dmis.org>, od@...ell.com,
	systemtap-ml <systemtap@...rces.redhat.com>
Subject: Re: Unified tracing buffer

Hi -

On Tue, Sep 23, 2008 at 11:36:26PM +0900, KOSAKI Motohiro wrote:
> > By the way, systemtap uses two modes;
> > 
> > - single-channel mode
> >  In this mode, all cpus share one buffer channel to write and read.
> >  each writer locks spinlock and write a probe-local data to buffer.
> > - per-cpu buffer mode [...]
> 
> I can't imazine a merit of the single-channel mode.
> Could you please explain it?

It could be a way of saving some memory and merging hassle for
low-throughput data.  (Remember that systemtap enables in-situ
analysis of events so that often only brief final results need be sent
along need be sent out.)  If timestampwise cross-cpu merging can be
done on demand by the hypothetical future buffer widget, then little
reason remains not to use it.

- FChE
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