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

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