Query regarding 2.6.335 RT[Ingo's] and Non-RT performance

[Manikandan Ramachandran]

> ------------------------------------------------------
> > Date: Thu, 12 Aug 2010 13:53:51 -0400
> > From: Jeff Angielski <jeff at theptrgroup.com>
> > To: linuxppc-dev at lists.ozlabs.org
> > Subject: Re: Query regarding 2.6.335 RT[Ingo's] and Non-RT performance
> > Message-ID: <4C64352F.4090005 at theptrgroup.com>
> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> >
> > On 08/11/2010 06:18 PM, Manikandan Ramachandran wrote:
> > > Hello All,
> > >      I created a very simple program which has higher priority than
> > > normal tasks and runs a tight loop. Under same test environment I ran
> > > this program on both non-rt and rt 2.6.33.5 kernel.  To my suprise I see
> > > that performance of non-RT kernel is better than RT. non-RT kernel took
> > > 3 sec and 366156 usec while RT kernel took about 3 sec and 418011
> > > usec.Can someone please explain why the performance of non-rt kernel is
> > > better than rt kernel? From the face of the test result, I feel RT has
> > > more overhead,Is there any configuration that I could do to bring down
> > > the overhead?
> >
> > Your "surprise" is due to your definition of "performance".
> >
> > The purpose of the -rt kernels is to reduce the kernel latency.  This is
> > important for servicing hardware.  Normal users find the -rt useful for
> > audio/video applications.  Engineering and scientific users find the -rt
> > beneficially for servicing hardware like sensors or control systems.
> >
> > If you are just trying to run calculations as fast as you can in user
> > space, you'd be better off using the non-rt variants.
> >
> >
> > --
> > Jeff Angielski
> > The PTR Group
> > www.theptrgroup.com



 Thanks for your response.

On one hand I hear that RT-kernel is meant for reducing kernel latency on
other hand I see that there is RT-kernel overhead. So what really RT-kernel
brings to system performance?

Actually I see that latency for higher priority is more or less same for
non-rt system.

One more thing, since irqs being threaded in RT, and with CFS scheduler in
2.6.33, wouldn't we bring down system performance as CFS is O(log(n))  in
nature?
 --
 Thanks,
 Manik