From: Mel Gorman <mgorman@techsingularity.net>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>, Phil Auld <pauld@redhat.com>,
Valentin Schneider <valentin.schneider@arm.com>,
Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
Quentin Perret <quentin.perret@arm.com>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Morten Rasmussen <Morten.Rasmussen@arm.com>,
Hillf Danton <hdanton@sina.com>, Parth Shah <parth@linux.ibm.com>,
Rik van Riel <riel@surriel.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] sched, fair: Allow a small degree of load imbalance between SD_NUMA domains v2
Date: Wed, 8 Jan 2020 18:03:17 +0000 [thread overview]
Message-ID: <20200108180317.GM3466@techsingularity.net> (raw)
In-Reply-To: <20200108164657.GA16425@linaro.org>
On Wed, Jan 08, 2020 at 05:46:57PM +0100, Vincent Guittot wrote:
> > Allowing just 1 extra task would work for netperf in some cases except when
> > softirq is involved. It would partially work for IO on ext4 as it's only
> > communicating with one journal thread but a bit more borderline for XFS
> > due to workqueue usage. XFS is not a massive concern in this context as
> > the workqueue is close to the IO issuer and short-lived so I don't think
> > it would crop up much for load balancing unlike ext4 where jbd2 could be
> > very active.
> >
> > If v4 of the patch fails to meet approval then I'll try a patch that
>
> My main concern with v4 was the mismatch between the computed value and the goal to not overload the LLCs
>
Fair enough.
> > allows a hard-coded imbalance of 2 tasks (one communicating task and
>
> If there is no good way to compute the allowed imbalance, a hard coded
> value of 2 is probably simple value to start with
Indeed.
>
> > one kthread) regardless of NUMA domain span up to 50% of utilisation
>
> Are you sure that it's necessary ? This degree of imbalance already applies only if the group has spare capacity
>
> something like
>
> + /* Consider allowing a small imbalance between NUMA groups */
> + if (env->sd->flags & SD_NUMA) {
> +
> + /*
> + * Until we found a good way to compute an acceptable
> + * degree of imbalance linked to the system topology
> + * and that will not impatc mem bandwith and latency,
> + * let start with a fixed small value.
> + */
> + imbalance_adj = 2;
> +
> + /*
> + * Ignore small imbalances when the busiest group has
> + * low utilisation.
> + */
> + env->imbalance -= min(env->imbalance, imbalance_adj);
> + }
>
This is more or less what I had in mind with the exception that the "low
utilisation" part of the comment would go away. The 50% utilisation may
be unnecessary and was based simply on the idea that at that point memory
bandwidth, HT considerations or both would also be dominating factors. I
can leave out the check and add it in as a separate patch if proven to
be necessary.
--
Mel Gorman
SUSE Labs
next prev parent reply other threads:[~2020-01-08 18:03 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-20 8:42 [PATCH] sched, fair: Allow a small degree of load imbalance between SD_NUMA domains v2 Mel Gorman
2019-12-20 12:40 ` Valentin Schneider
2019-12-20 14:22 ` Mel Gorman
2019-12-20 15:32 ` Valentin Schneider
2019-12-21 11:25 ` Mel Gorman
2019-12-22 12:00 ` Srikar Dronamraju
2019-12-23 13:31 ` Vincent Guittot
2019-12-23 13:41 ` Vincent Guittot
2020-01-03 14:31 ` Mel Gorman
2020-01-06 13:55 ` Vincent Guittot
2020-01-06 14:52 ` Mel Gorman
2020-01-07 8:38 ` Vincent Guittot
2020-01-07 9:56 ` Mel Gorman
2020-01-07 11:17 ` Vincent Guittot
2020-01-07 11:56 ` Mel Gorman
2020-01-07 16:00 ` Vincent Guittot
2020-01-07 20:24 ` Mel Gorman
2020-01-08 8:25 ` Vincent Guittot
2020-01-08 8:49 ` Mel Gorman
2020-01-08 13:18 ` Peter Zijlstra
2020-01-08 14:03 ` Mel Gorman
2020-01-08 16:46 ` Vincent Guittot
2020-01-08 18:03 ` Mel Gorman [this message]
2020-01-07 11:22 ` Peter Zijlstra
2020-01-07 11:42 ` Mel Gorman
2020-01-07 12:29 ` Peter Zijlstra
2020-01-07 12:28 ` Peter Zijlstra
2020-01-07 19:26 ` Phil Auld
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