Re: [RFC 2/5] sched: Add CPU rate soft caps

[Con Kolivas <kernel@kolivas.org>]

On Friday 26 May 2006 14:20, Peter Williams wrote:
> This patch implements (soft) CPU rate caps per task as a proportion of a
> single CPU's capacity expressed in parts per thousand.  The CPU usage
> of capped tasks is determined by using Kalman filters to calculate the
> (recent) average lengths of the task's scheduling cycle and the time
> spent on the CPU each cycle and taking the ratio of the latter to the
> former.  To minimize overhead associated with uncapped tasks these
> statistics are not kept for them.
>
> Notes:
>
> 1. To minimize the overhead incurred when testing to skip caps processing
> for uncapped tasks a new flag PF_HAS_CAP has been added to flags.

[ot]I'm sorry to see an Australian adopt American spelling [/ot]

> 3. Enforcement of caps is not as strict as it could be in order to
> reduce the possibility of a task being starved of CPU while holding
> an important system resource with resultant overall performance
> degradation.  In effect, all runnable capped tasks will get some amount
> of CPU access every active/expired swap cycle.  This will be most
> apparent for small or zero soft caps.

The array swap happens very frequently if there are nothing but heavily cpu 
bound tasks, which is not an infrequent workload. I doubt the zero caps are 
very effective in that environment.

-- 
-ck