linux-kernel - Re: [PATCH] sched/fair: improve spreading of utilization

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Message-ID: <20200313102626.GY12561@hirez.programming.kicks-ass.net>
Date:   Fri, 13 Mar 2020 11:26:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: improve spreading of utilization

On Thu, Mar 12, 2020 at 05:54:29PM +0100, Vincent Guittot wrote:
> During load_balancing, a group with spare capacity will try to pull some
> utilizations from an overloaded group. In such case, the load balance
> looks for the runqueue with the highest utilization. Nevertheless, it
> should also ensure that there are some pending tasks to pull otherwise
> the load balance will fail to pull a task and the spread of the load will
> be delayed.
> 
> This situation is quite transient but it's possible to highlight the
> effect with a short run of sysbench test so the time to spread task impacts
> the global result significantly.
> 
> Below are the average results for 15 iterations on an arm64 octo core:
> sysbench --test=cpu --num-threads=8  --max-requests=1000 run
> 
>                            tip/sched/core  +patchset
> total time:                172ms           158ms
> per-request statistics:
>          avg:                1.337ms         1.244ms
>          max:               21.191ms        10.753ms
> 
> The average max doesn't fully reflect the wide spread of the value which
> ranges from 1.350ms to more than 41ms for the tip/sched/core and from
> 1.350ms to 21ms with the patch.
> 
> Other factors like waiting for an idle load balance or cache hotness
> can delay the spreading of the tasks which explains why we can still
> have up to 21ms with the patch.

Thanks!