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Date: Thu, 11 Jul 2002 08:27:13 +0200 (CEST)
From: Ingo Molnar <mingo@elte.hu>
Reply-To: Ingo Molnar <mingo@elte.hu>
To: "Kevin O'Connor" <kevin@koconnor.net>
Cc: linux-kernel@vger.kernel.org
Subject: Re: O(1) batch scheduler
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On Tue, 9 Jul 2002, Kevin O'Connor wrote:

> I looked through your sched-2.5.25-A5 patch, and I'm confused by the
> idle_count array. It calculates the idle average of the last 9 seconds -
> but why not just use a weighted average. A weighted average is going to
> be very close to the true average, and where it differs the weighted
> average should be preferable.

i agree, the hybrid weighted average you suggest is the right solution
here, because the sampling in that case has a fixed frequency which is
HZ-independent. I've applied your patch to my tree.

the problem with a pure weighted average (ie. no ->idle_count, just a
weighted average calculated in the scheduler tick) is that with HZ=1000
and a 32-bit word length the sampling gets too inaccurate. For the average
to be meaningful it needs to be at least 'a few seconds worth' - which is
'a few thousands of events' - the rounding errors are pretty severe in
that case.

(a good example where a running average has fundamental accuracy problem
is the ->sleep_avg sampling. The frequency of wakeups/sleep events can be
almost arbitrarily high, destroying the accuracy of a weighted average.)

	Ingo

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