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

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Message-Id: <1256303931.20866.826.camel@gandalf.stny.rr.com>
Date:	Fri, 23 Oct 2009 09:18:51 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Frédéric Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [performance problem] CONFIG_DYNAMIC_FTRACE: Increased kernel
 build times

On Fri, 2009-10-23 at 09:45 +0200, Ingo Molnar 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.

Yeah, I know this is quite a hit, and to fix this is on my todo list.
What I plan on doing is checking if libelf is installed, and if it is,
then doing a C program that does the linking and checking. If not, it
defaults back to the perl code.

But I'm bug hunting right now, and that takes a higher priority.

-- Steve

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