Priority degradation and Niceness (A question)

[Russell J Fulton;ccc032u]

I am trying to set a batch system (Scott Presnell's excellent system) on a
4D/240s running Irix 3.3.2. The machine is heavily loaded with both 
interactive (editing, compiling etc) and long running batch jobs (some of 
which also need lots of memory) The aim of the exercise is to get the long
running jobs into the batch queues so that we can control the number and
type of jobs that are running. In particular we want to stop too many large
memory hungry jobs from starting together and degrading interactive response.
(We have about 1000 registered users so informal methods don't work very 
well.)

So much for the background. Now to my question:

I need to know a lot more about how the scheduler actually works. In particular
I need to understand how priority and niceness interact. I can't find anything
in TFM. I notice that the priority of long running jobs bounce around a lot
(from 100 up to 20 and then back to 90 and so on) The batch system allows me
to set non degrading priorities so I need to understand how these interact
with degrading priorities.

I am aware that there are not many SGI machines used in this way so
I would like to hear from other people who use their SGI machines as general
purpose computer to serve large numbers of Ascii terminals rather than 
dedicated graphics machines.

Thanks Russell.

-- 
Russell Fulton, Computer Center, University of Auckland, New Zealand.
<rj_fulton at aukuni.ac.nz>