Linuxppc-dev Digest, Vol 72, Issue 63

[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
>
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