linux-kernel - Re: [PATCH 0/4] trace: profiling branches

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Date:	Sun, 30 Nov 2008 02:58:56 -0800
From:	Daniel Walker <dwalker@...o99.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Theodore Tso <tytso@....edu>,
	Arjan van de Ven <arjan@...radead.org>,
	Hua Zhong <hzhong@...il.com>
Subject: Re: [PATCH 0/4] trace: profiling branches

On Fri, 2008-11-21 at 02:12 -0500, Steven Rostedt wrote:
> Ingo,
> 
> The following patches clean up the unlikely/likely tracer. Namely
> it consolidates it into a single file called "profile_annotated_branch".
> 
> It also adds a new profiler. A true branch profiler that profiles all
> if() statements where the conditional is not a constant. It puts 
> a bit of overhead on the system, but the results seem pretty interesting.
> The results are placed in "profile_branch".
> 

I looked at the full version of this, and it looks really slow.. As I
recall the biggest problem with the -mm version was it's cacheline
bouncing (pointed out by Ingo), and yours doesn't _seem_ to fix that. In
fact your version looks a lot worse..

So really between the two if we want mainline likely profiling the -mm
version is a better choice.. The reason that version never went into
mainline is cause neither me or Andrew felt strongly that this was
useful in more than just -mm ..

If you look at the output from the profiling long enough it becomes
clear that it's frequently misleading .. In the short term a certain
branch might be likely, and in the long term it isn't.. So you can't
really blindly start converting the annotation..

I should also mention that I didn't write the -mm version alone, it was
an effort between three people me, Andrew, and Hua Zhong (CC added)..

Daniel

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