linux-kernel - Re: [performance problem] CONFIG_DYNAMIC_FTRACE: Increased kernel build times

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Date:	Fri, 23 Oct 2009 12:37:46 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	John Kacur <jkacur@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [performance problem] CONFIG_DYNAMIC_FTRACE: Increased kernel
	build times


* John Kacur <jkacur@...hat.com> wrote:

> 2009/10/23 Ingo Molnar <mingo@...e.hu> wrote:
> > 
> > Steve,
> > 
> > i did an 64-bit allyesconfig build test (with debug_info disabled), with
> > and without CONFIG_DYNAMIC_FTRACE set.
> > 
> > The build time results are:
> > 
> >  -CONFIG_DYNAMIC_FTRACE:   266.30
> >  +CONFIG_DYNAMIC_FTRACE:   296.07 (+11.2%)
> > 
> > So we get more than 10% build time overhead.
> > 
> > That's quite high - higher than what i remember having measured
> > originally when the first iteration of the mcount code went in.
> > 
> > This extra overhead comes from the mcount post-processing script
> > (scripts/recordmcount.pl) that runs objcopy as well and is written in
> > Perl. I think this whole angle needs to be improved - 10% is way too
> > high of a price to pay.
> 
> Hi Ingo, do you still have the profiling data to share with us? [...]

no finegrained info - i did a perf stat run.

> [...] Was the problem more in objcopy / objdump, rather than in the 
> perl script itself?

in top i saw perl pop up most of the time, that's where i'd suspect a 
good portion of the overhead.

	Ingo
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