linux-kernel - Re: [patch] CFS scheduler, v3

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Date:	Sat, 21 Apr 2007 20:37:06 +1000
From:	Peter Williams <pwil3058@...pond.net.au>
To:	Ingo Molnar <mingo@...e.hu>
CC:	William Lee Irwin III <wli@...omorphy.com>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Con Kolivas <kernel@...ivas.org>,
	Nick Piggin <npiggin@...e.de>, Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Willy Tarreau <w@....eu>, Gene Heskett <gene.heskett@...il.com>
Subject: Re: [patch] CFS scheduler, v3

Ingo Molnar wrote:
> * Peter Williams <pwil3058@...pond.net.au> wrote:
> 
>> I retract this suggestion as it's a very bad idea.  It introduces the 
>> possibility of starvation via the poor sods at the bottom of the queue 
>> having their "on CPU" forever postponed and we all know that even the 
>> smallest possibility of starvation will eventually cause problems.
>>
>> I think there should be a rule: Once a task is on the queue its "on 
>> CPU" time is immutable.
> 
> Yeah, fully agreed. Currently i'm using the simple method of 
> p->nice_offset, which plainly just moves the per nice level areas of the 
> tree far enough apart (by a constant offset) so that lower nice levels 
> rarely interact with higher nice levels. Lower nice levels never truly 
> starve because rq->fair_clock increases deterministically and currently 
> the fair_key values are indeed 'immutable' as you suggest.
> 
> In practice they can starve a bit when one renices thousands of tasks, 
> so i was thinking about the following special-case: to at least make 
> them easily killable: if a nice 0 task sends a SIGKILL to a nice 19 task 
> then we could 'share' its p->wait_runtime with that nice 19 task and 
> copy the signal sender's nice_offset. This would in essence pass the 
> right to execute over to the killed task, so that it can tear itself 
> down.
> 
> This cannot be used to gain an 'unfair advantage' because the signal 
> sender spends its own 'right to execute on the CPU', and because the 
> target task cannot execute any user code anymore when it gets a SIGKILL.
> 
> In any case, it is clear that rq->raw_cpu_load should be used instead of 
> rq->nr_running, when calculating the fair clock, but i begin to like the 
> nice_offset solution too in addition of this: it's effective in practice 
> and starvation-free in theory, and most importantly, it's very simple. 
> We could even make the nice offset granularity tunable, just in case 
> anyone wants to weaken (or strengthen) the effectivity of nice levels. 
> What do you think, can you see any obvious (or less obvious) 
> showstoppers with this approach?

I haven't had a close look at it but from the above description it 
sounds an order of magnitude more complex than I thought it would be. 
The idea of different nice levels sounds like a recipe for starvation to 
me (if it works the way it sounds like it works).

I guess I'll have to spend more time reading the code because I don't 
seem to be able to make sense of the above description in any way that 
doesn't say "starvation here we come".

Peter
-- 
Peter Williams                                   pwil3058@...pond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce
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