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Re: [PlanetCCRMA] Music Spin



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On 19/07/10 18:57, Fernando Lopez-Lezcano wrote:
> On Mon, 2010-07-19 at 13:46 -0300, Bernardo Barros wrote:
>> > For this low-level task some engineering would be required? I'm not
>> > this kind of guy who know much about tasks scheduling and priorities
>> > (kernel level) but some people told me that real-time kernels patches
>> > could slow down Fedora and I should not use it. 
>
> Did those people by any chance provide you with _numbers_ that tell you
> exactly what part of the kernel is impacted and by how much? (and, who
> are they?) My guess is probably not. 
> 
> Yes, perhaps the rt patches could affect performance. Maybe. In some
> subsystems (disk i/o, network come to mind). Maybe by a few %, in some
> specific situations. I don't think the impact will be something you will
> notice unless you measure it. 

To put things a bit straight.  Yes, RT kernel will affect performance.
Point.

Will you notice it effectively on "normal desktop tasks"?  Probably not
much, but it depends on how your system is saturated, tuned and what
kind of loads you have running.  Real Time is not Real Fast and never
will be.  A real time kernel will not give any fair scheduling to your
system, basically.

All tasks (processes) which is set to use the SCHED_FIFO or SCHED_RR
scheduling, even on the lowest priority will preempt and run it first
over, ignoring all other SCHED_OTHER (normal) processes.  SCHED_FIFO and
SCHED_RR tasks with the highest priority will be processed before all
other tasks, whenever they receive signals which requires these
processes to run.  This is the core nature of a real time system.

So if you don't have many SCHED_FIFO or SCHED_RR tasks, and that they
are light - you most probably will not notice anything at all.  But in
the moment you begin with heavy I/O dependent operations or other
operations which requires processing time and the kernel have given them
SCHED_FIFO scheduling and a rather high priority (which is not
uncommon), it will slow down all the other running tasks.  But this
behaviour is possible to tune!

So why would you want to run a real time kernel instead of a so called
"real fast" kernel (normal kernels)?  Latency determination.  A real
time kernel will strive to give you a predictable system latency, no
matter how loaded your computer is.  Which is a key-point for audio and
video - you don't want the sound or video stream to be delayed because
cron and updatedb wants to run.

Some relevant information can be found here:

<http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_MRG/1.2/html/Realtime_Installation_Guide/chap-Realtime_Installation_Guide-Why_Use_RT_to_Optimize_Latency.html>


<http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_MRG/1.2/html/Realtime_Reference_Guide/index.html>


<http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_MRG/1.2/html/Realtime_Tuning_Guide/index.html>


kind regards,

David Sommerseth
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