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    SubjectRe: NO_HZ migration of TCP ack timers
    * Andi Kleen <andi@firstfloor.org> [2010-02-18 09:08:35]:

    > Anton Blanchard <anton@samba.org> writes:
    >
    > > echo 0 > /proc/sys/kernel/timer_migration
    > >
    > > makes the problem go away.
    > >
    > > I think the problem is the CPU is most likely to be idle when an rx networking
    > > interrupt comes in. It seems the wrong thing to do to migrate any ack timers
    > > off the current cpu taking the interrupt, and with enough networks we train
    > > wreck transferring everyones ack timers to the nohz load balancer cpu.
    >
    > 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?
    >
    > But yes if it's non idle the migration might need to spread out
    > to more CPUs.
    >
    > >
    > > What should we do? Should we use mod_timer_pinned here? Or is this an issue
    >
    > 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.
    >

    cpuidle infrastructure hs statistics about the idle times for
    all the cpus. Maybe we can look to use this infrastructure to decide
    whether to migrate timers or not?

    arun

    > > other areas might see (eg the block layer) and we should instead avoid
    > > migrating timers created out of interrupts.
    >
    > -Andi
    >
    > --
    > ak@linux.intel.com -- Speaking for myself only.


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