linux-kernel - Re: [PATCH] sched, fair: Allow a small degree of load imbalance between SD

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Date:   Wed, 18 Dec 2019 21:58:01 -0500
From:   Rik van Riel <riel@...riel.com>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, pauld@...hat.com,
        valentin.schneider@....com, srikar@...ux.vnet.ibm.com,
        quentin.perret@....com, dietmar.eggemann@....com,
        Morten.Rasmussen@....com, hdanton@...a.com, parth@...ux.ibm.com,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched, fair: Allow a small degree of load imbalance
 between SD_NUMA domains

On Wed, 2019-12-18 at 15:44 +0000, Mel Gorman wrote:

> +			/*
> +			 * Ignore imbalance unless busiest sd is close
> to 50%
> +			 * utilisation. At that point balancing for
> memory
> +			 * bandwidth and potentially avoiding
> unnecessary use
> +			 * of HT siblings is as relevant as memory
> locality.
> +			 */
> +			imbalance_max = (busiest->group_weight >> 1) -
> imbalance_adj;
> +			if (env->imbalance <= imbalance_adj &&
> +			    busiest->sum_nr_running < imbalance_max) {
> +				env->imbalance = 0;
> +			}
> +		}
>  		return;
>  	}

I can see how the 50% point is often great for HT,
but I wonder if that is also the case for SMT4 and
SMT8 systems...

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