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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