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    SubjectRe: NO_HZ migration of TCP ack timers

    Hi Andi,

    > If the nohz balancer CPU is otherwise idle, shouldn't it have enough
    > cycles to handle acks for everyone? Is the problem the cache line
    > transfer time?

    Yeah, I think the timer spinlock on the nohz balancer cpu ends up being a
    global lock for every other cpu trying to migrate their ack timers to it.

    > Sounds like something that should be controlled by the cpufreq governour's
    > idle predictor? Only migrate if predicted idle time is long enough.
    > It's essentially the same problem as deciding how deeply idle to put
    > a CPU. Heavy measures only pay off if the expected time is long enough.

    Interesting idea, it seems like we do need a better understanding of
    how idle a cpu is, not just that it is idle when mod_timer is called.

    Anton


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