linux-kernel - Re: [PATCH v3] Add generic exponentially weighted moving average (EWMA) function

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Date:	Thu, 21 Oct 2010 12:04:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Bruno Randolf <br1@...fach.org>
Cc:	randy.dunlap@...cle.com, akpm@...ux-foundation.org,
	kevin.granade@...il.com, Lars_Ericsson@...ia.com,
	blp@...stanford.edu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] Add generic exponentially weighted moving average
 (EWMA) function

On Thu, 2010-10-21 at 14:40 +0900, Bruno Randolf wrote:
> On Thu October 21 2010 00:03:43 Peter Zijlstra wrote:
> > On Wed, 2010-10-20 at 17:23 +0900, Bruno Randolf wrote:
> > > +/**
> > > + * ewma_add() - Exponentially weighted moving average (EWMA)
> > > + * @avg: Average structure
> > > + * @val: Current value
> > > + *
> > > + * Add a sample to the average.
> > > + */
> > > +struct ewma*
> > > +ewma_add(struct ewma *avg, const unsigned int val)
> > > +{
> > > +       avg->internal = avg->internal  ?
> > > +               (((avg->internal * (avg->weight - 1)) +
> > > +                       (val * avg->factor)) / avg->weight) :
> > > +               (val * avg->factor);
> > > +       return avg;
> > > +}
> > > +EXPORT_SYMBOL(ewma_add);
> > 
> > How can it be a weighted avg if each sample has the same weight?
> 
> by applying the weight again and again, we get an exponential weighting.
> 
> http://en.wikipedia.org/wiki/Exponentially_weighted_moving_average

Ah, thanks. It might be worth adding some of that explanation to the
actual comment.

I think the naming is somewhat unfortunate since a weighted average
makes me think of:

       \Sum w_i * v_i
 wa = ---------------
         \Sum w_i

Instead of the described algorithm.
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