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

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Message-ID: <5319BEC0.4050006@nod.at>
Date:	Fri, 07 Mar 2014 13:42:40 +0100
From:	Richard Weinberger <richard@....at>
To:	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Borislav Petkov <bp@...en8.de>,
	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



Am 07.03.2014 13:39, schrieb Austin S Hemmelgarn:
> 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.

*might*
We still need a sane proof.

Thanks,
//richard
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