Messages in this thread | | | Date | Tue, 10 Dec 2013 20:07:44 +0100 | From | Daniel Lezcano <> | Subject | Re: [question] sched: idle_avg and migration latency |
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On 12/10/2013 04:20 PM, Alex Shi wrote: > CC to MikeG, he written this part. :) > I try to explain sth I know. I am sorry if my understanding incorrect. > > On 12/10/2013 07:30 PM, Daniel Lezcano wrote: >> >> Hi All, >> >> I am trying to understand how is computed the idle_avg and how it is >> used regarding the migration latency. >> >> 1. What is the sysctl_sched_migration_cost value ? It is initialized to >> 500000UL. Is it an arbitrarily chosen value ? Could it change depending >> on the hardware performances ? > > current sysctl_sched_mirgration_cost is 0.5ms, used to limit > overscheduling. Guess it is a kind of arbitrary. But it can be rewrite > at /proc/sys/kernel/sched_migration_cost_ns. > So if you find some new suitable value in particular scenario. guess > PeterZ like to modify it. :) > >> >> >> 2. The idle_balance function checks: >> >> if (this_rq->avg_idle < sysctl_sched_migration_cost) >> return 0; >> >> IIUC, it is not worth to migrate a task to this cpu as we expect to run >> another task before we can pull a task to the current cpu, right ? > > No, that used to prevent every idle_balance cause a task migration if > idle balance happens too much and too quick, -- frequency more than task > migration limitation. >> >> Then if there is no task to balance we will enter idle, thus we >> initialize the idle_stamp to the current clock. > > If we pulled task, we will restart frequency calculation by set > idle_stamp = 0; > or if new task adding this rq, allow more idle_balance.
Thanks Alex for the explanation.
>> When another task is woken up with the ttwu_do_wakeup, the duration of >> the idle time is computed in there: >> >> if (rq->idle_stamp) { >> u64 delta = rq_clock(rq) - rq->idle_stamp; >> u64 max = 2*sysctl_sched_migration_cost; >> >> if (delta > max) >> rq->avg_idle = max; >> else >> update_avg(&rq->avg_idle, delta); >> rq->idle_stamp = 0; >> } >> >> Why is the 'delta' leveraged by 'max' ? >> >> >> 3. And finally the function update_avg does: >> >> s64 diff = sample - *avg; >> *avg += diff >> 3; >> >> Why is diff >> 3 used instead of the number of values ? > > It is a kind of decay. but has no idea of why this value '3'. Guess > MikeG has some reason. >> >> Thanks in advance for any answers >> >> -- Daniel >> > >
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