Re: Reduce scheduling interval on Linux...

[Simon Wright <simon@pushface.org>]

Peter <peter@atterfjall.pp.se> writes:

> On Red Hat linux a delay 0.0; would mean that process (thread) will be
> out of processing for 10 (ms) by default. This is a kernel
> configuration, and a good value in many cases. For my application I
> would like to reduce this time to maybe 1 (ms), or try to optimise it
> to the perfect value for the application. Yes, I know I am trying to
> make a RTOS of something that is not, but this is really the only
> thing missing, and the advantages of having a full system at least for
> development makes me rather accept 10 (ms) that to have to cross
> compile all the time.
> 
> Has anyone been changing this and could point me in the right
> direction. Tried to do this once on Red Hat 7.0....but only got
> confused by the parameter names and the comments around them in the
> kernel.... Are you aware of any other full systems, beeing it Linux,
> BSD or whatever were this value is lower or easily could be changed ?

I don't have the source installed at the moment, but on the 2.0
kernels you could change the tick time by altering include/asm/param.h
to set HZ to 1000.

I think you also have to change kernel/sched.c in sys_nanosleep() --
there's a busy wait for sleeps (as root) of 2 mS or less.

Then (just) rebuild and go!

We had machines that stayed up for (a few) weeks with this setting.

NB1: if you have a loop with a delay, the maximum repetion rate you
will get is half the jiffy frequency -- HZ=1000 gives 500 loops/sec.

NB2: the real reason stock Linux isn't a RTOS isn't the 10 mS clock --
VxWorks has a 60 Hz clock out of the box -- rather, it's the
unpredictability of kernel scheduling.