linux-kernel - Re: Why not build kernel with -O3

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Message-ID: <55252B6B.7040907@nod.at>
Date:	Wed, 08 Apr 2015 15:21:47 +0200
From:	Richard Weinberger <richard@....at>
To:	Pengfei Yuan <0xcoolypf@...il.com>
CC:	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Why not build kernel with -O3

Am 08.04.2015 um 15:16 schrieb Pengfei Yuan:
> 2015-04-08 20:19 GMT+08:00 Richard Weinberger <richard.weinberger@...il.com>:
>> On Wed, Apr 8, 2015 at 3:00 AM, Pengfei Yuan <0xcoolypf@...il.com> wrote:
>>> Could you please provide some examples that I can investigate?
>>> Thanks!
>>
>> It would be awesome if you could find out which gcc optimizations
>> cause the speed up.
>> "gcc -c -Q -O3 --help=optimizers" will help you.
>>
> 
> This is really helpful.
> But I can only find very short description for each option from
> https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

Yeah, but if you know -fXY gives a nice speed up we can add it to our
CFLAGS if it makes sense and has not much downsides.
Blindly enabling -O3 can be dangerous as it might make the generated code
much bigger and the asm unreadable.

>> Please also double check your results.
>> You need do to multiple runs, etc...
>> Especially the redis speed up looks odd. Does redis really spend that much time
>> in the kernel?
> 
> Redis is special among the six applications because it is single-threaded.

Still it would be nice to now much more about the load and why -O3 helps.

Thanks,
//richard
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