Message-ID: <46247BE7.2070307@bigpond.net.au>
Date:	Tue, 17 Apr 2007 17:48:55 +1000
From:	Peter Williams <pwil3058@...pond.net.au>
To:	Nick Piggin <npiggin@...e.de>
CC:	Mike Galbraith <efault@....de>, Con Kolivas <kernel@...ivas.org>,
	Ingo Molnar <mingo@...e.hu>, ck list <ck@....kolivas.org>,
	Bill Huey <billh@...ppy.monkey.org>,
	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]

Nick Piggin wrote:
> On Tue, Apr 17, 2007 at 04:23:37PM +1000, Peter Williams wrote:
>> Nick Piggin wrote:
>>> And my scheduler for example cuts down the amount of policy code and
>>> code size significantly.
>> Yours is one of the smaller patches mainly because you perpetuate (or 
>> you did in the last one I looked at) the (horrible to my eyes) dual 
>> array (active/expired) mechanism.
> 
> Actually, I wasn't comparing with other out of tree schedulers (but it
> is good to know mine is among the smaller ones). I was comparing with
> the mainline scheduler, which also has the dual arrays.
> 
> 
>>  That this idea was bad should have 
>> been apparent to all as soon as the decision was made to excuse some 
>> tasks from being moved from the active array to the expired array.  This 
> 
> My patch doesn't implement any such excusing.
> 
> 
>> essentially meant that there would be circumstances where extreme 
>> unfairness (to the extent of starvation in some cases) -- the very 
>> things that the mechanism was originally designed to ensure (as far as I 
>> can gather).  Right about then in the development of the O(1) scheduler 
>> alternative solutions should have been sought.
> 
> Fairness has always been my first priority, and I consider it a bug
> if it is possible for any process to get more CPU time than a CPU hog
> over the long term. Or over another task doing the same thing, for
> that matter.
> 
> 
>> Other hints that it was a bad idea was the need to transfer time slices 
>> between children and parents during fork() and exit().
> 
> I don't see how that has anything to do with dual arrays.

It's totally to do with the dual arrays.  The only real purpose of the 
time slice in O(1) (regardless of what its perceived purpose was) was to 
control the switching between the arrays.

> If you put
> a new child at the back of the queue, then your various interactive
> shell commands that typically do a lot of dependant forking get slowed
> right down behind your compile job. If you give a new child its own
> timeslice irrespective of the parent, then you have things like 'make'
> (which doesn't use a lot of CPU time) spawning off lots of high
> priority children.

This is an artefact of trying to control nice using time slices while 
using them for controlling array switching and whatever else they were 
being used for.  Priority (static and dynamic) is the the best way to 
implement nice.

> 
> You need to do _something_ (Ingo's does). I don't see why this would
> be tied with a dual array. FWIW, mine doesn't do anything on exit()
> like most others, but it may need more tuning in this area.
> 
> 
>> This disregard for the dual array mechanism has prevented me from 
>> looking at the rest of your scheduler in any great detail so I can't 
>> comment on any other ideas that may be in there.
> 
> Well I wasn't really asking you to review it. As I said, everyone
> has their own idea of what a good design does, and review can't really
> distinguish between the better of two reasonable designs.
> 
> A fair evaluation of the alternatives seems like a good idea though.
> Nobody is actually against this, are they?

No.  It would be nice if the basic ideas that each scheduler tries to 
implement could be extracted and explained though.  This could lead to a 
melding of ideas that leads to something quite good.

> 
> 
>>> I haven't looked at Con's ones for a while,
>>> but I believe they are also much more straightforward than mainline...
>> I like Con's scheduler (partly because it uses a single array) but 
>> mainly because it's nice and simple.  However, his earlier schedulers 
>> were prone to starvation (admittedly, only if you went out of your way 
>> to make it happen) and I tried to convince him to use the anti 
>> starvation mechanism in my SPA schedulers but was unsuccessful.  I 
>> haven't looked at his latest scheduler that sparked all this furore so 
>> can't comment on it.
> 
> I agree starvation or unfairness is unacceptable for a new scheduler.
> 
> 
>>> For example, let's say all else is equal between them, then why would
>>> we go with the O(logN) implementation rather than the O(1)?
>> In the highly unlikely event that you can't separate them on technical 
>> grounds, Occam's razor recommends choosing the simplest solution. :-)
> 
> O(logN) vs O(1) is technical grounds.

In that case I'd go O(1) provided that the k factor for the O(1) wasn't 
greater than O(logN)'s k factor multiplied by logMaxN.

> 
> But yeah, see my earlier comment: simplicity would be a factor too.

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

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