linux-kernel - Re: scheduler nice 19 versus 'idle' behavior / static low-priority scheduling

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Date:	Fri, 30 Jan 2009 02:59:09 -0500
From:	Nathanael Hoyle <nhoyle@...letech.com>
To:	Jan Engelhardt <jengelh@...ozas.de>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: scheduler nice 19 versus 'idle' behavior / static low-priority
	scheduling

On Fri, 2009-01-30 at 08:21 +0100, Jan Engelhardt wrote:
> On Friday 2009-01-30 07:40, Nathanael Hoyle wrote:
> >On Fri, 2009-01-30 at 07:16 +0100, Jan Engelhardt wrote:
> >
> >The one discussion I saw referencing SCHED_BATCH seemed to imply that it
> >was a non-standard kernel patch by Con Kolivas in one of his -ck
> >variants that never made it into mainline and is not being maintained.
> >Is this inaccurate?
> 
> The presence of SCHED_BATCH in linux/sched.h tells me it is available
> (on the other hand, SCHED_ISO, also from -ck, is only listed as a comment.)
> 

Fair enough. I should have re-checked recent sources after your mention
rather than going on the old thread I found.

> >I was unfamiliar with SCHED_IDLE. Having done a little Googling now, I
> >finally find reference to the man page for sched_setscheduler(2). This
> >appears that it is likely what I wanted.
> >
> >I think the information I had been able to find was somehwat out of
> >date.
> 
> The manpage does say it, but if your local distro does not 
> mention SCHED_BATCH/SCHED_IDLE, then that's a pretty sad distro.
> 
> The doc in sched_setschedule seems complete to me as of man-pages 3.13.
> 
> >Is there currently a standardized userspace tool to use to run a command
> >in order to alter its scheduling class? Obviously writing one would be
> >trivial, but didn't know if something like:
> 
> man chrt

The latest version of man chrt that I can find implies that it handles
SCHED_BATCH but not SCHED_IDLE. To that end, if anyone else is
interested, I have thrown together the above-suggested 'runidle' which
will invoke the passed command using the SCHED_IDLE scheduler; it's
nothing fancy.

I am running foldingathome under it at the moment, and it seems to be
improving the situation somewhat, but I still need/want to test with
Mike's referenced patches.

runidle.c:

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sched.h>
#include <linux/sched.h>

extern char **environ;

int
main(int argc, char **argv) {
  if(argc<2) {
    perror("Must specify at least one argument: the path to the program
to " \
      "execute. Additional arguments may be specified which will be
passed " \
      "to the called program.");
    return EXIT_FAILURE;
  }

  sched_setscheduler(0, SCHED_IDLE, NULL);

  if(argc==2) {
    if(execve(argv[1], NULL, environ) == -1) {
      perror("Failed to execve target!");
    }
  } else {
    if(execve(argv[1], argv+1, environ) == -1) {
      perror("Failed to execve target!");
    }
  }

  /* should be unreachable */
  return EXIT_FAILURE;
}

-Nathanael

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