linux-kernel - Re: [PATCH 0/1] init/Kconfig: improve CC_OPTIMIZE_FOR

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Date:	Thu, 24 Apr 2014 21:34:58 +0200
From:	Michael Opdenacker <michael.opdenacker@...e-electrons.com>
To:	Josh Triplett <josh@...htriplett.org>
CC:	akpm@...ux-foundation.org, paulmck@...ux.vnet.ibm.com,
	fweisbec@...il.com, eparis@...hat.com,
	paul.gortmaker@...driver.com, vapier@...too.org,
	kyungsik.lee@....com, jslaby@...e.cz, dwight.engen@...cle.com,
	pefoley2@...oley.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] init/Kconfig: improve CC_OPTIMIZE_FOR_SIZE documentation

Hi Josh,

Thank you for your review!

On 04/24/2014 04:55 PM, Josh Triplett wrote:
> On Thu, Apr 24, 2014 at 02:07:33PM +0200, Michael Opdenacker wrote:
>> This patch proposes to warn users about the negative performance
>> impact of CC_OPTIMIZE_FOR_SIZE (compiling with -Os instead of -O2).
>> For the moment, we are just advising users not to select this option,
>> but extra details would be useful.
>>
>> I made some experiments with this option. All of them yielded
>> degraded overall boot time. The kernel is smaller and hence is copied
>> and started earlier, but the benefits are quickly offset by the
>> slower kernel code. Even the total boot time is usually worse
>> than with a regular kernel.
>>
>> Here are my latest results, on Atmel SAMA5D3 Xplained (ARM),
>> Linux 3.10, gzip compressed kernel:
>>
>> Timestamp		O2		Os		Diff
>> Starting kernel 	4.307 s 	5.593 s 	-94 ms
>> Starting init		4.213 s 	5.549 s 	-44 ms
>> Login prompt		21.085 s	22.900 s 	+1.815 s 
> Did you get the times in the upper two rows transposed row/column?  Based
> on the diffs, I'd guess 4.213s goes in Os "Starting kernel", and 5.593s
> goes in O2 "Starting init".
Oops, yes I did. I shouldn't have tried to copy from a PDF file I
produced ;)

Thanks!

Michael.

-- 
Michael Opdenacker, CEO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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