From: Rik van Riel <riel@redhat.com>
To: Vincent Guittot <vincent.guittot@linaro.org>,
peterz@infradead.org, mingo@kernel.org,
linux-kernel@vger.kernel.org
Cc: linaro-kernel@lists.linaro.org
Subject: Re: [PATCH] sched: fix spurious active migration
Date: Tue, 30 Sep 2014 06:07:48 -0400 [thread overview]
Message-ID: <542A80F4.7020704@redhat.com> (raw)
In-Reply-To: <1412066468-4340-1-git-send-email-vincent.guittot@linaro.org>
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On 09/30/2014 04:41 AM, Vincent Guittot wrote:
> Since commit caeb178c60f4 ("sched/fair: Make
> update_sd_pick_busiest() ...") sd_pick_busiest returns a group that
> can be neither imbalanced nor overloaded but is only more loaded
> than others. This change has been introduced to ensure a better
> load balance in system that are not overloaded but as a side
> effect, it can also generate useless active migration between
> groups.
>
> Let take the example of 3 tasks on a quad cores system. We will
> always have an idle core so the load balance will find a busiest
> group (core) whenever an ILB is triggered and it will force an
> active migration (once above nr_balance_failed threshold) so the
> idle core becomes busy but another core will become idle. With the
> next ILB, the freshly idle core will try to pull the task of a busy
> CPU. The number of spurious active migration is not so huge in quad
> core system because the ILB is not triggered so much. But it
> becomes significant as soon as you have more than one sched_domain
> level like on a dual cluster of quad cores where the ILB is
> triggered every tick when you have more than 1 busy_cpu
>
> We need to ensure that the migration generate a real improveùent
> and will not only move the avg_load imbalance on another CPU.
Good catch.
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Rik van Riel <riel@redhat.com>
- --
All rights reversed
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next prev parent reply other threads:[~2014-09-30 10:08 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-09-30 8:41 [PATCH] sched: fix spurious active migration Vincent Guittot
2014-09-30 9:38 ` Vincent Guittot
2014-09-30 10:07 ` Rik van Riel [this message]
2014-09-30 18:41 ` Peter Zijlstra
2014-10-01 7:02 ` Vincent Guittot
2014-10-01 13:38 ` [PATCH v2] " Vincent Guittot
2014-10-03 5:27 ` [tip:sched/core] sched: Improve sysbench performance by fixing " tip-bot for Vincent Guittot
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