Re: [RFC][PATCH] Make MAX_RT_PRIO and MAX_USER_RT_PRIO configurable

[Ingo Molnar <mingo@elte.hu>]

* Steven Rostedt <rostedt@goodmis.org> wrote:

> A colleague of mine, well actually the VP of my company of the time, 
> Doug Locke, gave me a perfect example.  If you have a program that 
> runs a nuclear power plant that needs to wake up and run 4 seconds 
> every 10 seconds, and on that same computer you have a program running 
> a washing machine that needs to wake up every 3 seconds and run for 
> one second (I'm using seconds just to make the example simple). Which 
> process gets the higher priority?  The answer is the washing machine.
> 
> Rational: If the power plant was higher priority, the washing machine 
> would fail almost every time, since the power plant program would run 
> for 4 seconds, and since the cycle of the washing machine is 3 
> seconds, it would fail everytime the nuclear power plant program ran.  
> Now if you have the washing machine run in it's cycle, the nuclear 
> power plant can easily make the 4 seconds ever 10 seconds, even when 
> it is interrupted by the washing machine.

nitpicking: i guess the answer also depends on what the precise 
requirement is. If the requirement is 'run for 4 seconds every 10 
seconds, uninterrupted, else the power plant melts down', i'd sure not 
make the washing machine process the higher priority one ;-)

(also, i'd give the power plant process higher priority even if the 
requirement is not as strict, just from a risk POV: what if the washing 
machine control program is buggy and got into an infinite loop 
somewhere.)

	Ingo