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

[Björn Steinbrink <B.Steinbrink@gmx.de>]

On 2006.05.26 10:04:20 +0200, Mike Galbraith wrote:
> On Fri, 2006-05-26 at 14:20 +1000, Peter Williams wrote:
> > These patches implement CPU usage rate limits for tasks.
> > 
> > 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 killer problem I see with this approach is that it doesn't address
> the divide and conquer problem.  If a task is capped, and forks off
> workers, each worker inherits the total cap, effectively extending same.
> 
> IMHO, per task resource management is too severely limited in it's
> usefulness, because jobs are what need managing, and they're seldom
> single threaded.  In order to use per task limits to manage any given
> job, you have to both know the number of components, and manually
> distribute resources to each component of the job.  If a job has a
> dynamic number of components, it becomes impossible to manage.

Linux-VServer uses a token bucket scheduler (TBS) to limit cpu ressources
for processes in a "context". All processes in a context share one token
bucket, which has a set of parameters to tune scheduling behaviour.
As the token bucket is shared by a group of processes, and inherited by
child processes/threads, management is quite easy. And the parameters
can be tuned to allow different scheduling behaviours, like allowing a
process group to burst, ie. use as much cpu time as is available, after
being idle for some time, but being limited to X % cpu time on average.

I'm CC'ing Herbert and Sam on this as they can explain the whole thing a
lot better and I'm not familiar with implementation details.

Regards
Björn
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