linux-kernel - Re: [PATCH 2/4 v2] sched/fair: reduce minimal imbalance threshold

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Message-ID: <jhjd02cmu3i.mognet@arm.com>
Date:   Wed, 23 Sep 2020 15:43:45 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, linux-kernel@...r.kernel.org, pauld@...hat.com,
        hdanton@...a.com
Subject: Re: [PATCH 2/4 v2] sched/fair: reduce minimal imbalance threshold


On 21/09/20 08:24, Vincent Guittot wrote:
> The 25% default imbalance threshold for DIE and NUMA domain is large
> enough to generate significant unfairness between threads. A typical
> example is the case of 11 threads running on 2x4 CPUs. The imbalance of
> 20% between the 2 groups of 4 cores is just low enough to not trigger
> the load balance between the 2 groups. We will have always the same 6
> threads on one group of 4 CPUs and the other 5 threads on the other
> group of CPUS. With a fair time sharing in each group, we ends up with
> +20% running time for the group of 5 threads.
>
> Consider decreasing the imbalance threshold for overloaded case where we
> use the load to balance task and to ensure fair time sharing.
>
> Acked-by: Hillf Danton <hdanton@...a.com>
> Reviewed-by: Phil Auld <pauld@...hat.com>
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>

Reviewed-by: Valentin Schneider <valentin.schneider@....com>