linux-kernel - Re: [PATCH 0/2] [GIT PULL] function profiler updates

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Date:	Thu, 26 Mar 2009 09:07:57 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH 0/2] [GIT PULL] function profiler updates


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

> 
> Ingo,
> 
> Please pull the latest rfc/tip/tracing/function-profiler tree, which can be found at:
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-2.6-trace.git
> rfc/tip/tracing/function-profiler
> 
> 
> Steven Rostedt (2):
>       tracing: remove on the fly allocator from function profiler
>       tracing: add average time in function to function profiler
> 
> ----
>  kernel/trace/ftrace.c |   97 +++++++++++++++++++++++++++++--------------------
>  1 files changed, 57 insertions(+), 40 deletions(-)
> -- 

Pulled, thanks Steve. The on-the-fly allocator was my main 
conceptual worry about this.

How useful this kind of profiling will end up being in practice 
remains to be seen - but it does have a handful of unique properties 
that cannot be dismissed.

It's main downside is its overhead - sampling based profilers are a 
lot more lightweight and allow unintrusive inspection of workloads.

The upside of it are all the things that this profiling overhead 
allows us to do:

 - 100% precise, non-sampling profiling

 - accurate profiles possible with very short, bursty workloads too

 - filtering capabilities of the tracing subsystem allow the 
   flexible shaping of the set of functions and the set of 
   conditions that are profiled: there's the regex filter and 
   there's also the tracepoint based filter expressions.

 - it also allowsuser-space driven filtering: if user-space wants to 
   profile just one specific functionality (say the function calls 
   of one specific library), they can use /debug/tracing/enabled
   fast toggle to turn profiling on/off runtime. This can be done at 
   a very high frequency as well.

 - developers can switch between profiling and tracing mode 
   seemlessly. This allows multiple 'dimensions' of the same 
   workload to be observed, via the same set of facilities.

	Ingo
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