linux-kernel - Re: [RFC] PyTimechart

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Date:	Wed, 12 May 2010 20:23:21 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
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


* Steven Rostedt <rostedt@...dmis.org> wrote:

> 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.

i know, but most people still use /debug/tracing/ bits not trace-cmd.

> > 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.

Make sure you measure it in the context of a full app like PyTimechart.

You can measure the overhead using perf stat ;-)

	Ingo
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