On Tue, 2013-12-10 at 12:30 +0100, 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 ?
Yeah, it's a magic number. We used to use boot time measurements.
> 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's all about not beating living hell outta ourselves on every
micro-idle. As with all load balancing, it's usually too much balancing
that creates a problem. You need it, but it's really expensive, so less
is more.
> Then if there is no task to balance we will enter idle, thus we
> initialize the idle_stamp to the current clock.
>
> 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' ?
That has changed a little recently. I originally slammed avg_idle
itself straight to max to ensure that a bursty load would idle balance,
and not use stale data. If you start cross core switching at high
frequency, you'll still shut idle balancing quickly.
> 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 ?
Ingo's quick like bunny smooth average.
-Mike
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