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Date: Wed, 5 Mar 2014 02:32:26 -0500 (EST)
From: Jon Ringle <jon@ringle.org>
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To: Greg KH <gregkh@linuxfoundation.org>
cc: "Ringle, Jonathan" <Jonathan.Ringle@gridpoint.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] Add option to build with -O3
In-Reply-To: <20140305050927.GA8534@kroah.com>
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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?

(Resending this message, since it was "destroyed". Hopefully, this is now 
an an acceptable form :)

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