[ltt-dev] Benchmarks of kernel tracing options 2 (ftrace, lttng and perf)

[dhsharp@google.com (David Sharp)]

On Wed, Nov 17, 2010 at 2:56 PM, Douglas Santos
<douglas.santos at polymtl.ca> wrote:
> Quoting Steven Rostedt <rostedt at goodmis.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.
>
>
>