linux-kernel - Re: [REPORT] cfs-v4 vs sd-0.44

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Message-ID: <20070423204431.GA1240@elte.hu>
Date:	Mon, 23 Apr 2007 22:44:31 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Nick Piggin <npiggin@...e.de>,
	Juliusz Chroboczek <jch@....jussieu.fr>,
	Con Kolivas <kernel@...ivas.org>, ck list <ck@....kolivas.org>,
	Bill Davidsen <davidsen@....com>, Willy Tarreau <w@....eu>,
	William Lee Irwin III <wli@...omorphy.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Williams <pwil3058@...pond.net.au>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Gene Heskett <gene.heskett@...il.com>
Subject: Re: [REPORT] cfs-v4 vs sd-0.44


* Ingo Molnar <mingo@...e.hu> wrote:

> (we obviously dont want to allow people to 'share' their loans with 
> others ;), nor do we want to allow a net negative balance. CFS is 
> really brutally cold-hearted, it has a strict 'no loans' policy - the 
> easiest economic way to manage 'inflation', besides the basic act of 
> not printing new money, ever.)

sorry, i was a bit imprecise here. There is a case where CFS can give 
out a 'loan' to tasks. The scheduler tick has a low resolution, so it is 
fundamentally inevitable [*] that tasks will run a bit more than they 
should, and at a heavy context-switching rates these errors can add up 
significantly. Furthermore, we want to batch up workloads.

So CFS has a "no loans larger than sched_granularity_ns" policy (which 
defaults to 5msec), and it captures these sub-granularity 'loans' with 
nanosec accounting. This too is a very sane economic policy and is 
anti-infationary :-)

	Ingo

[*] i fundamentally hate 'fundamentally inevitable' conditions so i 
    have plans to make the scheduler tick be fed from the rbtree and 
    thus become a true high-resolution timer. This not only increases 
    fairness (=='precision of scheduling') more, but it also decreases 
    the number of timer interrupts on a running system - extending 
    dynticks to sched-ticks too. Thomas and me shaped dynticks to enable 
    that in an easy way: the scheduler tick is today already a high-res 
    timer (but which is currently still driven via the jiffy mechanism).
-
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