Messages in this thread | | | Date | Tue, 10 Dec 2013 23:20:01 +0800 | From | Alex Shi <> | Subject | Re: [question] sched: idle_avg and migration latency |
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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. > > 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 >
-- Thanks Alex
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