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

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Date:	Wed, 5 Mar 2014 00:37:56 -0500
From:	Jon Ringle <jringle@...dpoint.com>
To:	Greg KH <gregkh@...uxfoundation.org>
CC:	"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 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.

Jon

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