Re: [RFC] tracing: Adding cgroup aware tracing functionality

From: Peter Zijlstra
Date: Sat Apr 09 2011 - 07:09:25 EST

On Fri, 2011-04-08 at 13:02 -0400, Steven Rostedt wrote:
> On Fri, 2011-04-08 at 16:07 +0200, Peter Zijlstra wrote:
> > > As you said perf has a lot of overhead due to data that it saves per
> > > event.
> >
> > Someday you should actually read the perf code before you say something.
> I have looked at the code although not as much recently, but I do plan
> on looking at it again in much more detail. But you are correct that I
> did not make this comment on the code itself, but on looking at the
> data:

> 1051776/8766 = 119

> 2957312/38268 = 77
> As you stated, I need to look more into the perf code (which I plan on
> doing), but it seems that perf adds 42 bytes more per event. Perhaps
> this is something we can fix.

Aside from the 8 byte header everything else is configurable with
PERF_SAMPLE_flags and probably has some overlap with stuff we also have
in the tracepoint data we then get through PERF_SAMPLE_RAW

> I'd love to make both perf and ftrace be
> able to limit its header. There's no reason to record the pid for every
> event if we don't need to. As well as the preempt count and interrupt
> status. But these are legacy from the latency tracer code from -rt.


> I think there's a lot of work that we can do make tracing in perf more
> compatible with the tracing features of ftrace. I did say the ugly word
> "roadmap" but perhaps it's just direction that we need. I feel we are
> all a bunch of cooks with their own taste, and we don't all like the
> spices used by each other.

Partly yeah, but there's also real functional differences, the last time
I profiled perf with perf (yay for recursion) we spend a lot of time in
conditionals. Due to the fact that perf is mainly sampling based (and
tracing being samples with period==1) and all that output
configurability there's a true forest of if statements to pass through
and I'm fairly sure we totally trash the branch predictor on that.

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