linux-kernel - Re: [RFC] sched: The removal of idle

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Date:	Tue, 19 Feb 2013 10:13:10 +0600
From:	Rakib Mullick <rakib.mullick@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Clark Williams <clark@...hat.com>,
	Andrew Theurer <habanero@...ibm.com>
Subject: Re: [RFC] sched: The removal of idle_balance()

On Mon, Feb 18, 2013 at 9:25 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Mon, 2013-02-18 at 13:43 +0530, Srikar Dronamraju wrote:
>> > The cache misses dropped by ~23% and migrations dropped by ~28%. I
>> > really believe that the idle_balance() hurts performance, and not just
>> > for something like hackbench, but the aggressive nature for migration
>> > that idle_balance() causes takes a large hit on a process' cache.
>> >
>> > Think about it some more, just because we go idle isn't enough reason to
>> > pull a runable task over. CPUs go idle all the time, and tasks are woken
>> > up all the time. There's no reason that we can't just wait for the sched
>> > tick to decide its time to do a bit of balancing. Sure, it would be nice
>> > if the idle CPU did the work. But I think that frame of mind was an
>> > incorrect notion from back in the early 2000s and does not apply to
>> > today's hardware, or perhaps it doesn't apply to the (relatively) new
>> > CFS scheduler. If you want aggressive scheduling, make the task rt, and
>> > it will do aggressive scheduling.
>> >
>>
>> How is it that the normal tick based load balancing gets it correctly while
>> the idle_balance gets is wrong?  Can it because of the different
>> cpu_idle_type?
>>
>
> Currently looks to be a fluke in my box, as this performance increase
> can't be duplicated elsewhere (yet). But from looking at my traces, it
> seems that my box does the idle balance at just the wrong time, and
> causes these issues.
>
A default hackbench run creates 400 tasks (10 * 40), on a i7 system (4
core, HT), idle_balance() shouldn't be in action, cause on a 8 cpu
system we're assigning 400 tasks. If idle_balance() comes in, that
means - we've done something wrong while distributing tasks among the
CPUs, that indicates a problem during fork/exec/wake balancing?

Thanks,
Rakib.
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