linux-kernel - Re: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]

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Date:	Tue, 17 Apr 2007 11:24:22 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	William Lee Irwin III <wli@...omorphy.com>
Cc:	Davide Libenzi <davidel@...ilserver.org>,
	Nick Piggin <npiggin@...e.de>,
	Peter Williams <pwil3058@...pond.net.au>,
	Mike Galbraith <efault@....de>,
	Con Kolivas <kernel@...ivas.org>, ck list <ck@....kolivas.org>,
	Bill Huey <billh@...ppy.monkey.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]


* William Lee Irwin III <wli@...omorphy.com> wrote:

> [...] Also rest assured that the tone of the critique is not hostile, 
> and wasn't meant to sound that way.

ok :) (And i guess i was too touchy - sorry about coming out swinging.)

> Also, given the general comments it appears clear that some 
> statistical metric of deviation from the intended behavior furthermore 
> qualified by timescale is necessary, so this appears to be headed 
> toward a sort of performance metric as opposed to a pass/fail test 
> anyway. However, to even measure this at all, some statement of 
> intention is required. I'd prefer that there be a Linux-standard 
> semantics for nice so results are more directly comparable and so that 
> users also get similar nice behavior from the scheduler as it varies 
> over time and possibly implementations if users should care to switch 
> them out with some scheduler patch or other.

yeah. If you could come up with a sane definition that also translates 
into low overhead on the algorithm side that would be great! The only 
good generic definition i could come up with (nice levels are isolated 
buckets with a constant maximum relative percentage of CPU time 
available to every active bucket) resulted in having a per-nice-level 
array of rbtree roots, which did not look worth the hassle at first 
sight :-)

until now the main approach for nice levels in Linux was always: 
"implement your main scheduling logic for nice 0 and then look for some 
low-overhead method that can be glued to it that does something that 
behaves like nice levels". Feel free to turn that around into a more 
natural approach, but the algorithm should remain fairly simple i think.

	Ingo
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