linux-kernel - Re: [PATCH] Add option to build with -O3

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Date:	Fri, 07 Mar 2014 07:39:53 -0500
From:	Austin S Hemmelgarn <ahferroin7@...il.com>
To:	Richard Weinberger <richard.weinberger@...il.com>,
	Jon Ringle <jringle@...dpoint.com>
CC:	Greg KH <gregkh@...uxfoundation.org>,
	"Ringle, Jonathan" <Jonathan.Ringle@...dpoint.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Add option to build with -O3

On 2014-03-06 08:28, Richard Weinberger wrote:
> On Wed, Mar 5, 2014 at 6:37 AM, Jon Ringle <jringle@...dpoint.com>
> wrote:
>> 
>> 
>> On Wed, 5 Mar 2014, Greg KH wrote:
>> 
>>> On Tue, Mar 04, 2014 at 07:01:49PM -0500, Jon Ringle wrote:
>>>> +config CC_OPTIMIZE_FOR_SPEED +    bool "Optimze for speed
>>>> (-O3)" +    help +      Enabling this option will pass "-O3"
>>>> to gcc +      resulting in a larger kernel (but possibly
>>>> faster)
>>> 
>>> Are you sure about that?  Have you measured it?
>> 
>> I do know that there is an improvement performance-wise for my
>> particular use-case.
>> 
>> My target is an ARM board being built with gcc-4.8.2. My board
>> has on it a sc16is740 that is used as an RS-485 port. The
>> sc16is740 is on the i2c bus, so when an interrupt comes in to
>> indicate that there is data available to be read, I need to get
>> the data over the i2c bus. I do this on a kthread to do this
>> work. The i2c transactions (using i2c-davinci driver) are also 
>> interrupt driven. I was seeing a lot of lost packets when
>> receiving data at only 19200. Adding the -O3 compile option
>> helped in this regard in that I am now rarely seeing packet
>> loss.
> 
> Please also see: 
> http://www.gossamer-threads.com/lists/linux/kernel/1835761?do=post_view_threaded#1835761
>
> 
Ironically, combining these might achieve a significant performance
improvement over CONFIG_GENERIC_CPU and -O2.
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