automatic kernel renice'ing in 4.X bsd?

[William L. Sebok]

> > We have started to notice a problem with some
> > people's login csh being reniced. Since we always
> > have a few processes running, these people's terminal
> > appears to be dead.
> > 
> > By reniceing the csh to 0, the terminal becomes alive
> > again, and renice reports the old priority as 19.
> > 
> > Is there some thing in 4.1 bsd that renices people when
> > the system load gets too high?
> 
> In 4.2BSD,
> .... if a process accumulates enough CPU time, the system decides it's had
> enough favorable treatment and it's time to get tough with it.  I seem to
> remember something like this also being in 4.1BSD, so that's what may be
> happening.
> 	Guy Harris

Although at nice 19 I rather doubt that this is what is going on.

The automatic reniceing behavior of the kernel is something of which I
strongly approve.  Astrovax has often has a large component of long-running
background jobs.  Because I was convinced that a large number of nice +4 jobs
was still affecting interactive response I changed the automatic renice to +8.

The algorithm in a pseudo-C is:

	if ( (over 10 cpu minutes)
	  && (uid != 0)
	  && (niceness >= 0)
	  && (niceness < 6)
	 )
		niceness = 8;

In other words if the niceness is positive and less than 6 it is reset to 8.
The gap between 6 and 8 is to encourage people to nice their background jobs
at the very beginning by giving a two point penalty if they forget.

The exemption for processes of negative niceness is a not-very-satisfactory
sop for the people here doing image-processing.  These are jobs that are
mostly interactive but can use a large amount of cpu time.  A more
satisfactory algorithm I would like to implement would involve terminal idle
time.  If someone could give me some pointers how to do this in the kernel
without too much overhead I would appreciate it.  Admittedly I haven't really
done my homework yet to see if it would be feasable.
-- 
Bill Sebok			Princeton University, Astrophysics
{allegra,akgua,burl,cbosgd,decvax,ihnp4,noao,princeton,vax135}!astrovax!wls