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

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Date:	Thu, 19 Apr 2007 10:33:43 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Matt Mackall <mpm@...enic.com>,
	William Lee Irwin III <wli@...omorphy.com>,
	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@...r.kernel.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]

On Thu, Apr 19, 2007 at 08:38:10AM +0200, Ingo Molnar wrote:
> 
> * Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > > And yes, by fairly, I mean fairly among all threads as a base 
> > > resource class, because that's what Linux has always done
> > 
> > Yes, there are potential compatibility problems.  Example: a machine 
> > with 100 busy httpd processes and suddenly a big gzip starts up from 
> > console or cron.
> > 
> > Under current kernels, that gzip will take ages and the httpds will 
> > take a 1% slowdown, which may well be exactly the behaviour which is 
> > desired.
> > 
> > If we were to schedule by UID then the gzip suddenly gets 50% of the 
> > CPU and those httpd's all take a 50% hit, which could be quite 
> > serious.
> > 
> > That's simple to fix via nicing, but people have to know to do that, 
> > and there will be a transition period where some disruption is 
> > possible.
> 
> hmmmm. How about the following then: default to nice -10 for all 
> (SCHED_NORMAL) kernel threads and all root-owned tasks. Root _is_ 
> special: root already has disk space reserved to it, root has special 
> memory allocation allowances, etc. I dont see a reason why we couldnt by 
> default make all root tasks have nice -10. This would be instantly loved 
> by sysadmins i suspect ;-)

I have no problem with doing fancy new fairness classes and things.

But considering that we _need_ to have per-thread fairness and that
is also what the current scheduler has and what we need to do well for
obvious reasons, the best path to take is to get per-thread scheduling
up to a point where it is able to replace the current scheduler, then
look at more complex things after that.
 
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