Re: [RFC 0/5] sched: Add CPU rate caps

[Peter Williams <pwil3058@bigpond.net.au>]

Con Kolivas wrote:
> On Friday 26 May 2006 14:20, Peter Williams wrote:
>> These patches implement CPU usage rate limits for tasks.
> 
> Nice :)

Thanks.

> 
>> Although the rlimit mechanism already has a CPU usage limit (RLIMIT_CPU)
>> it is a total usage limit and therefore (to my mind) not very useful.
>> These patches provide an alternative whereby the (recent) average CPU
>> usage rate of a task can be limited to a (per task) specified proportion
>> of a single CPU's capacity.  The limits are specified in parts per
>> thousand and come in two varieties -- hard and soft.
> 
> Why 1000?

Probably a hang over from a version where the units were proportion of a 
whole machine.  Percentage doesn't work very well if there are more than 
1 CPU in that case (especially if there are more than 100 CPUs :-)). 
But it's also useful to have the extra range if your trying to cap 
processes (or users) from outside the scheduler using these primitives.

> I doubt that degree of accuracy is possible in cpu accounting and 
> accuracy or even required. To me it would seem to make more sense to just be 
> a percentage.
> 

It's not meant to imply accuracy :-).  The main issue is avoiding 
overflow when doing the multiplications during the comparisons.

Peter
-- 
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce