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    SubjectRe: [Lse-tech] Re: CPU affinity & IPI latency
    On Sun, Jul 15, 2001 at 01:02:21PM -0700, Davide Libenzi wrote:
    > The problem of the current scheduler is that it acts like an infinite feedback
    > system.
    > When we're going to decide if we've to move a task we analyze the status at the
    > current time without taking in account the system status at previous time
    > values.
    > But the feedback we send ( IPI to move the task ) take a finite time to hit the
    > target CPU and, meanwhile, the system status changes.
    > So we're going to apply a feedback calculated in time T0 to a time Tn, and this
    > will result in system auto-oscillation that we perceive as tasks bouncing
    > between CPUs.
    > This is kind of electronic example but it applies to all feedback systems.
    > The solution to this problem, given that we can't have a zero feedback delivery
    > time, is :
    > 1) lower the feedback amount, that means, try to minimize task movements
    > 2) a low pass filter, that means, when we're going to decide the sort ( move )
    > of a task, we've to weight the system status with the one that it had
    > at previous time values

    Nice analysis. I think Mike's proposal of the 'saved_cpus_allowed' field
    to temporarily bind the task to the target would just act as such an low
    pass filter.

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