linux-kernel - Re: [RFC] PyTimechart

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Message-ID: <1273684511.27703.37.camel@gandalf.stny.rr.com>
Date:	Wed, 12 May 2010 13:15:11 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Pierre Tardy <tardyp@...il.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Tom Zanussi <tzanussi@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
	arjan@...radead.org, ziga.mahkovec@...il.com
Subject: Re: [RFC] PyTimechart

On Wed, 2010-05-12 at 18:59 +0200, Ingo Molnar wrote:
> * Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > On Wed, 2010-05-12 at 16:48 +0200, Frederic Weisbecker wrote:
> > > On Wed, May 12, 2010 at 03:37:27PM +0200, Pierre Tardy wrote:
> > 
> > > But we don't yet support trace_printk in perf. May be we could wrap
> > > them in trace events.
> > 
> > Hmm, do we really want to do that?
> > 
> > We really need to get the perf and ftrace trace buffers combined. I 
> > understand why perf chose to do the mmap buffers for the counting, but for 
> > live streaming, it is very inefficient compared to splice.
> 
> The thing is that for a very long time ftrace didnt have splice support and 
> survived just fine. Even today most of the ftrace usage isnt utilizing splice.

Actually, trace-cmd implements the splice interface and is used by
several people. I find myself using trace-cmd 90% of the time that I use
ftrace, specifically because of this speedup.

> 
> Yes, splice might help in some situations but on average it's an independent 
> speedup on the order of magnitude of a few percents, not a 'must have' item.

I'll have start running benchmarks to see what the actual speed up is.
I'm guessing it may be more than a few percent. It allows for zero copy
overhead and reuse of the data page.

-- Steve


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