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

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Date:   Tue, 7 Jan 2020 10:43:08 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Hillf Danton <hdanton@...a.com>, Rik van Riel <riel@...riel.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Parth Shah <parth@...ux.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched, fair: Allow a small load imbalance between low
 utilisation SD_NUMA domains v3

On Tue, 7 Jan 2020 at 10:12, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> On Tue, Jan 07, 2020 at 09:51:11AM +0800, Hillf Danton wrote:
> >
> > Hi Folks
> >
> > On Mon, 06 Jan 2020 11:44:57 -0500 Rik van Riel wrote:
> > > On Mon, 2020-01-06 at 16:33 +0000, Mel Gorman wrote:
> > > > On Mon, Jan 06, 2020 at 10:47:18AM -0500, Rik van Riel wrote:
> > > > > > +                     imbalance_adj = (100 / (env->sd->imbalance_pct - 100)) - 1;
> > > > > > +
> > > > > > +                     /*
> > > > > > +                      * Allow small imbalances when the busiest group has
> > > > > > +                      * low utilisation.
> > > > > > +                      */
> > > > > > +                     imbalance_max = imbalance_adj << 1;
> > > > > > +                     if (busiest->sum_nr_running < imbalance_max)
> > > > > > +                             env->imbalance -= min(env->imbalance, imbalance_adj);
> > > > > > +             }
> > > > > > +
> > > > >
> > > > > Wait, so imbalance_max is a function only of
> > > > > env->sd->imbalance_pct, and it gets compared
> > > > > against busiest->sum_nr_running, which is related
> > > > > to the number of CPUs in the node?
> > > > >
> > > >
> > > > It's not directly related to the number of CPUs in the node. Are you
> > > > thinking of busiest->group_weight?
> > >
> > > I am, because as it is right now that if condition
> > > looks like it might never be true for imbalance_pct 115.
> > >
> > > Presumably you put that check there for a reason, and
> > > would like it to trigger when the amount by which a node
> > > is busy is less than 2 * (imbalance_pct - 100).
> >
> >
> > If three per cent can make any sense in helping determine utilisation
> > low then the busy load has to meet
> >
> >       busiest->sum_nr_running < max(3, cpus in the node / 32);
> >
>
> Why 3% and why would the low utilisation cut-off depend on the number of

But in the same way, why only 6 tasks ? which is the value with
default imbalance_pct ?
I expect a machine with 128 CPUs to have more bandwidth than a machine
with only 32 CPUs and as a result to allow more imbalance

Maybe the number of running tasks (or idle cpus) is not the right
metrics to choose if we can allow a small degree of imbalance because
this doesn't take into account it wether the tasks are long running or
short running ones

> CPUs in the node? That simply means that the cut-off scales to machine
> size and does not take into account any consideration between local memory
> latency and memory bandwidth.
>
> > And we can't skip pulling tasks from a numa node without comparing it
> > to the local load
> >
> >       local->sum_nr_running * env->sd->imbalance_pct <
> >       busiest->sum_nr_running * 100;
> >
> > with imbalance_pct taken into account.
> >
>
> Again, why? In this context, an imbalance has already been calculated
> and whether based on running tasks or idle CPUs, it's not a negative
> number. The imbalance_adj used as already accounted for imbalance_pct
> albeit not as a ratio as it's normally used.
>
> --
> Mel Gorman
> SUSE Labs