Re: [PATCH 2/4] sched/fair: Drop out incomplete current period when sched averages accrue

[Vincent Guittot]

On 13 April 2016 at 17:28, Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
> On 13 April 2016 at 17:13, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
>> On 10/04/16 23:36, Yuyang Du wrote:
>>> In __update_load_avg(), the current period is never complete. This
>>> basically leads to a slightly over-decayed average, say on average we
>>> have 50% current period, then we will lose 1.08%(=(1-0.5^(1/64)) of
>>> past avg. More importantly, the incomplete current period significantly
>>> complicates the avg computation, even a full period is only about 1ms.
>>>
>>> So we attempt to drop it. The outcome is that for any x.y periods to
>>> update, we will either lose the .y period or unduely gain (1-.y) period.
>>> How big is the impact? For a large x (say 20ms), you barely notice the
>>> difference, which is plus/minus 1% (=(before-after)/before). Moreover,
>>> the aggregated losses and gains in the long run should statistically
>>> even out.
>>>
>>
>> For a periodic task, the signals really get much more unstable. Even for
>> a steady state (load/util related) periodic task there is a meander
>> pattern which depends on if we for instance hit a dequeue (decay +
>> accrue) or an enqueue (decay only) after the 1ms has elapsed.
>>
>> IMHO, 1ms is too big to create signals describing task and cpu load/util
>> signals given the current scheduler dynamics. We simply see too many
>> signal driving points (e.g. enqueue/dequeue) bailing out of
>> __update_load_avg().
>>
>> Examples of 1 periodic task pinned to a cpu on an ARM64 system, HZ=250
>> in steady state:
>>
>> (1) task runtime = 100us period = 200us
>>
>>   pelt          load/util signal
>>
>>   1us:          488-491
>>
>>   1ms:          483-534
>>
>> We get ~2 dequeues (load/util example: 493->504) and ~2 enqueues
>> (load/util example: 496->483) in the meander pattern in the 1ms case.
>>
>> (2) task runtime = 100us period = 1000us
>>
>>   pelt          load/util signal
>>
>>   1us:          103-105
>>
>>   1ms:           84-145
>>
>> We get ~3-4 dequeues (load/util example: 104->124->134->140) and ~16-20
>> enqueues (load/util example: 137->134->...->99->97) in the meander
>> pattern in the 1ms case.
>
> yes, similarly i have some use cases with 2ms running task in a period
> of 5.12ms. it will be seen either as a 1ms running task or a 2ms

sorry, it's 5.242ms no 5.12ms

> running tasks depending on how the running is synced with the 1ms
> boundary
>
> so the load will vary between 197-215 up to 396-423 depending of when
> the 1ms boundary occurs in the 2ms running
>
> Vincent
>
>>
>> [...]