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

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Date:	Thu, 24 Apr 2014 07:55:35 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Michael Opdenacker <michael.opdenacker@...e-electrons.com>
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

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".

- Josh Triplett
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