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

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Date:	Wed, 13 Apr 2016 17:28:18 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Dietmar Eggemann <dietmar.eggemann@....com>
Cc:	Yuyang Du <yuyang.du@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Benjamin Segall <bsegall@...gle.com>,
	Paul Turner <pjt@...gle.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Juri Lelli <juri.lelli@....com>
Subject: Re: [PATCH 2/4] sched/fair: Drop out incomplete current period when
 sched averages accrue

On 13 April 2016 at 17:13, Dietmar Eggemann <dietmar.eggemann@....com> 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
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

>
> [...]