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

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Date:	Sat, 21 Feb 2015 19:34:55 +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>
Subject: Re: [RFC PATCH] x86, fpu: Use eagerfpu by default on all CPUs

* Borislav Petkov <bp@...en8.de> wrote:

> plain 3.19:
>
>      234.681331200 seconds time elapsed                                          ( +-  0.15% )
>
> eagerfpu=ENABLE
>
>      234.066525648 seconds time elapsed                                          ( +-  0.19% )

hm, a win of more than 600 milliseconds - more than I'd 
have expected.

I really want to believe this, but I worry about the 
systematic boot-to-boot noise though, which cannot be 
eliminated via --repeat 10.

Would it be possible to add a simple runtime debug switch 
to switch between the two FPU modes dynamically via a 
sysctl?

It should not crash tasks I think if it's flipped around on 
a reasonably idle system, so should allow much better 
apples-to-apples comparisons with the same kind of page 
cache layout, etc.

Thanks,

	Ingo
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