Path: utzoo!utgpu!jarvis.csri.toronto.edu!neat.cs.toronto.edu!lamy
Newsgroups: comp.unix.questions
From: lamy@cs.utoronto.ca (Jean-Francois Lamy)
Subject: Re: Nicer than nice!
Message-ID: <90Feb5.162123est.5069@neat.cs.toronto.edu>
Keywords: nice,speed,unix,sysv,priority
References: <9400@cat.UUCP> <1990Feb5.052345.2120@iwarp.intel.com> <707@umvlsi.ecs.umass.edu>
Date: 5 Feb 90 21:22:12 GMT
Lines: 30

someone wrote:
>> Is there a possibility to change the priority of a process more than a
>> 'nice -19' would do? I would like be able to start programs in background

to which Randal Schwartz replied:
>>the scheduler.  There isn't anything in off-the-shelf UNIX that tags the
>>process as "don't run this unless nobody else is doing much of anything."

and Uday Hegde confuses matters by saying:
>process with that nice value [-19] runs only when nothing else in the system 
>wants to.
>Correct me if I wrong.

Indeed.  The scheduler uses niceness as one of many factors in determining
priority.  In *practice*, several maximally niced jobs can still impact
significantly the system performance.  Recent SunOS releases seem especially
prone to this behaviour (I will dispense you from my pet theories on the topic
because I'd like to check them with the source first, should we *ever* receive
it).

Some Unix mutants like SGI's IRIX actually have a direct way to acheive what
the original poster wanted (an idle cycle soaker running at a priority far
below that assigned by normal scheduling).  This is one of the reasons SGIs
have been selling well in this neck of the woods (for the price, they make
excellent compute servers and we can accomodate both interactive symbolic
algebra and monster TeX jobs at the same time as neural network simulations
that run forever).

Jean-Francois Lamy               lamy@cs.utoronto.ca, uunet!cs.utoronto.ca!lamy
Department of Computer Science, University of Toronto, Canada M5S 1A4