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

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Message-ID: <20130218081345.GA4157@linux.vnet.ibm.com>
Date:	Mon, 18 Feb 2013 13:43:45 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	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()

> 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?

-- 
Thanks and Regards
Srikar Dronamraju

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