scheduler/SCHED_FIFO behaviour

[Arun Srinivas]

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 <[EMAIL PROTECTED]>
To: Arun Srinivas <[EMAIL PROTECTED]>
CC: [EMAIL PROTECTED], 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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