linux-kernel - Re: Benchmarks of kernel tracing options 2 (ftrace, lttng and perf)

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Message-ID: <AANLkTim9f2aNAaF8YgbOH3xKY9pjAtkHOoYVTv+XOE8W@mail.gmail.com>
Date:	Wed, 17 Nov 2010 15:22:15 -0800
From:	David Sharp <dhsharp@...gle.com>
To:	Douglas Santos <douglas.santos@...ymtl.ca>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ltt-dev@...ts.casi.polymtl.ca" <ltt-dev@...ts.casi.polymtl.ca>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michael Rubin <mrubin@...gle.com>
Subject: Re: Benchmarks of kernel tracing options 2 (ftrace, lttng and perf)

On Wed, Nov 17, 2010 at 2:56 PM, Douglas Santos
<douglas.santos@...ymtl.ca> wrote:
> Quoting Steven Rostedt <rostedt@...dmis.org>:
>> On Wed, 2010-11-17 at 16:31 -0500, Douglas Santos wrote:
>> > Hi all,
>> >
>> > This is a response to a benchmark, submitted a few weeks ago, comparing
>> kernel
>> > tracing options.
>> > http://lkml.org/lkml/2010/10/28/422
>> >
>> > We followed the methodology described in the link bellow,
>> > but using the shellscripts posted there to reproduce autotest scripts.
>> > http://lkml.org/lkml/2010/10/28/261
>> >
>> > We disabled the extra syscall tracing on lttng, for a fair comparison.
>> > http://lkml.org/lkml/2010/10/28/290
>> >
>> > Average results with tracing "on":
>> >
>> > lttng:  220 ns
>> > ftrace: 260 ns
>>
>> Heh, so ftrace got worse with the new kernel?

Steve, can you explain how you're drawing that conclusion? Did Douglas
run this benchmark before on a previous kernel (I didn't see it if
so)?

- You can't directly compare to my results because of different hardware.
- The methodology for lttng is different (syscall tracing was removed).
- My results were also on 2.6.36

> The previous bench was doing tracing "on" minus "off"
> average results. They also used autotest scripts, not sure if
> it does exactly the same thing.

I think the subtraction is important, or it is at least important to
see what the "off" result is as a baseline of comparison. Otherwise, a
huge portion of the measurement is the cost of making the syscall
itself.

>
> I'll check if we missed something.
>
>
>
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