linux-kernel - Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

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Message-ID: <20150222082228.GB22972@gmail.com>
Date:	Sun, 22 Feb 2015 09:22:28 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Rik van Riel <riel@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

* Ingo Molnar <mingo@...nel.org> wrote:

>   - Do you have enough RAM that there's essentially no IO
>     in the system worth speaking of? Do you have enough RAM
>     to copy a whole kernel tree to /tmp/linux/ and do the
>     measurement there, on ramfs?

Doing that will also pin down the page cache: kernel build 
times are very sensitive to the page cache layout, and once 
a page cache layout is established on systems with lots of 
RAM it does not tend to be flushed out. But the next bootup 
will generate another random page cache layout - which 
makes inter-kernel kernel build times comparisons much 
noiser than the run-to-run numbers suggest.

So to get more precise measurements a 'pinned' page cache 
layout and the dynamic debug switch you implemented is very 
helpful and --repeat stddev will be pretty representative 
of the true noise of the measurement.

Thanks,

	Ingo
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