linux-kernel - RE: [PATCH][RSDL-mm 0/7] RSDL cpu scheduler for 2.6.21-rc3-mm2

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Date:	Tue, 13 Mar 2007 12:58:01 -0700
From:	"David Schwartz" <davids@...master.com>
To:	<jeremy@...p.org>
Cc:	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH][RSDL-mm 0/7] RSDL cpu scheduler for 2.6.21-rc3-mm2

> There's a distinction between giving it more cpu and giving it higher
> priority: the important part about having high priority is getting low
> latency access to the cpu when its needed.

I agree. Tasks that voluntarily relinquish their timeslices should get lower
latency compared to other processes at the same static priority.

> This really seems like the wrong approach to me.  The implication here
> and in other mails is that fairness is an inherently good thing which
> should obviously take preference over any other property.

Yes, that is the implication. The alternative to fairness is arbitrary
unfairness. "Rational unfairness" is a form of fairness.

> The old unix-style dynamic priority scheme was designed to give
> interactive processes high priorities, by using the observation that
> "interactive" means "spends a lot of time blocked waiting for input".
> That model of interactive is way too simple now, and the attempts to try
> an find an equivalent heuristic have been flawed and lead to - in some
> cases - wildly bad behaviours.  I'm guessing the emphasis on "fairness"
> is in reaction to this, which is fair enough.

I don't think it makes sense for the scheduler to look for some hint that
the user would prefer a task to get more CPU and try to give it more. That's
what 'nice' is for.

> But saying that the user needs to explicitly hold the schedulers hand
> and nice everything to tell it how to schedule seems to be an abdication
> of duty, an admission of failure.  We can't expect users to finesse all
> their processes with nice, and it would be a bad user interface to ask
> them to do so.

Then you will always get cases where the scheduler does not do what the user
wants because the scheduler does not *know* what the user wants. You always
have to tell a computer what you want it to do, and the best it can do is
faithfully follow your request.

I think it's completely irrational to ask for a scheduler that automatically
gives more CPU time to CPU hogs.

> And if someone/distro *does* go to all the effort of managing how to get
> all the processes at the right nice levels, you have this big legacy
> problem where you're now stuck keeping all those nice values meaningful
> as you continue to develop the scheduler.  Its bad enough to make them
> do the work in the first place, but its worse if they need to make it a
> kernel version dependent function.

I agree. I'm not claiming to have the perfect solution. Let's not let the
perfect be the enemy of the good though.

DS

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