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    SubjectRe: [RFC 0/5] sched: Add CPU rate caps
    Björn Steinbrink wrote:

    >>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.

    Yes, although the current thinking is that you need to set a special
    clone() flag (which may be restricted via capabilities such as
    CAP_SYS_RESOURCE) to set a new CPU scheduling namespace, so the workers
    will inherit the same scheduling ns and therefore be accounted against
    the one resource.

    Sorry if I'm replying out of context, I'll catch up on this thread
    shortly. Thanks for including me.

    >>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.

    This is correct. Basically I read the (which explains Linux
    network schedulers etc) and the description of the Token Bucket
    Scheduler inspired me to write the same thing for CPU resources. It was
    originally developed for the 2.4 Alan Cox series kernels. The primary
    design guarantee of the scheduler is a low total performance impact -
    maximum CPU utilisation prioritisation and fairness a secondary
    concern. It was built with the idea that people wanting different sorts
    of scheduling policies could at least get a set of userland controls to
    implement their approach - to the limit of the effectiveness of task

    I most recently described this at, a
    lot of that thread is probably worth catching up on.

    It would be nice if we could somehow re-use the scheduling algorithms in
    use in the network space here, if it doesn't impact on performance.

    For instance, the "CBQ" network scheduler is the same approach as used
    in OpenVZ's CPU scheduler, and the classful Token Bucket Filter is the
    approach used in VServer. The "Sched_prio" and "Sched_hard" distinction
    in vserver could probably be compared to "Ingres Policing", where
    available CPU that could run a process instead sits idle - similar to
    the network world where data that has arrived is dropped to try to
    convince the application to throttle its network activity.

    As in the network space ( in this
    space we have a continual scale of possible implementations, marked by a
    highly efficient solution akin to "binding" at one end, and a
    virtualisation at the other. Each deliver guarantees most applicable to
    certain users or workloads.


    >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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      Last update: 2006-05-29 00:50    [W:0.036 / U:110.608 seconds]
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