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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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