NO_HZ migration of TCP ack timers
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From: Andi Kleen on 18 Feb 2010 11:10 > > > 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? Yes sorry I reallhy meant cpuidle when I wrote cpufreq. That's what I suggested too. But if the problem is lock contention on the target CPU that would still not completely solve it, just make it less frequent depending on the idle pattern. -Andi -- ak(a)linux.intel.com -- Speaking for myself only. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: David Miller on 26 Feb 2010 07:30
From: Anton Blanchard <anton(a)samba.org> Date: Thu, 18 Feb 2010 16:28:20 +1100 > 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. This migration against the very design of all of the TCP timers in the tree currently. For TCP, even when the timer is no longer needed, we don't cancel the timer. We do this in order to avoid touching the timer for the cancel from a cpu other than the one the timer was scheduled on. The timer therefore is always accessed, cache hot, locally to the cpu where it was scheduled. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |