scheduler/SCHED_FIFO behaviour

["Arun Srinivas" <getarunsri@hotmail.com>]

I am scheduling 2 SCHED_FIFO processes and set them affinity( process A runs 
on processor 1 and process B runs on processor 2), on a HT processor.(I did 
this cause I wanted to run them together).Now, in schedule() I measure the 
timedifference between when they are scheduled. I found that when I 
introduce these 2 processes as SCHED_FIFO they are

1)scheduled only once and run till completion ( they running time is around 
2 mins.)
2)entire system appears frozen....no mouse/key presses detected until the 
processes exit.

> From what I observed does it mean that even the OS / interrupt handler does 
not occur during the entire period of time these real time processes run?? 
(as I said the processes run in minutes).
How can I verify that?

Thanks
Arun

> From: Steven Rostedt <rostedt@goodmis.org>
> To: Arun Srinivas <getarunsri@hotmail.com>
> CC: juhl-lkml@dif.dk, LKML <linux-kernel@vger.kernel.org>
> Subject: Re: sched /HT processor
> Date: Sun, 03 Apr 2005 19:08:06 -0400
>
> On Mon, 2005-04-04 at 04:22 +0530, Arun Srinivas wrote:
> > Thanks. yes, a reschedule may not take place after a ms, if the 
> currently
> > running task cannot be preempted by another task.
> >
> > (1) But, can a reschedule happen within a millisec (or once a process is
> > scheduled can schedule() be called before the next millisec.) ?
> >
>
> Yes.  For example: a high priority task may be waiting for some IO to
> come in. Right after the normal timer interrupt scheduled another task,
> the IO may come in and wake the high priority process up. This process
> will preempt the other task right away. (ie. less than 1 ms).
>
> > 2) Also in case argument (1) is not true, and I want rescheduling to be 
> done
> > (i.e., schedule() called) in less than 1 ms , can I directly change the 
> HZ
> > value in <asm-i386/param.h> and recompile my kernel so that my timer
> > interrupt will occur frequently?
> >
>
> Well, 1) is true, but you can also increase HZ over 1000 if you like,
> but that will usually cause more overhead, since, although a schedule
> may not take place every HZ, a timer interrupt will.
>
> -- Steve

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