linux-kernel - Re: RFC: Link Time Optimization support for the kernel

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Message-ID: <20120820074835.GA6710@gmail.com>
Date:	Mon, 20 Aug 2012 09:48:35 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org, mmarek@...e.cz,
	linux-kbuild@...r.kernel.org, JBeulich@...e.com,
	akpm@...ux-foundation.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: RFC: Link Time Optimization support for the kernel

* Andi Kleen <andi@...stfloor.org> wrote:

> This rather large patchkit enables gcc Link Time Optimization (LTO) 
> support for the kernel. 
> 
> With LTO gcc will do whole program optimizations for
> the whole kernel and each module. This increases compile time,
> but can generate faster code.

By how much does it increase compile time?

How much faster does kernel code get?

Last time I checked LTO optimizations (half a year ago) it 
resulted in significantly slower build times.

I tried out and measured the LTO speedups and was less than 
impressed by them - a lot of build time increase for not much 
increase in performance. There was also visible, ongoing 
maintenance cost.

The combination of these seemed like a show-stopper.

It's obviously an optimization feature we should consider, but 
we really need hard numbers to make a cost/benefit analysis.

Thanks,

	Ingo
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