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

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Message-Id: <201010221025.30164.br1@einfach.org>
Date:	Fri, 22 Oct 2010 10:25:30 +0900
From:	Bruno Randolf <br1@...fach.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
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 Fri October 22 2010 10:11:38 KOSAKI Motohiro wrote:
> few additional reviewing comments is here.
> 
> > +struct ewma {
> > +	unsigned int internal;
> > +	unsigned int factor;
> > +	unsigned int weight;
> > +};
> 
> I think unsigned long is better because long is natual register size
> on both 32bit and 64bit arch.
> and, example, almost linux resource limit is using long or ulong. then
> uint may have overflow risk if we are using this one on 64bit arch.
> Does uint has any benefit? (note: scheduler loadavg has already used ulong)

You know more about this than me. I have no specific reason to use unsigned 
int. I'll change it to unsigned long, if that's better.

> > +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;
> 
> Hm, if ewma_add has this function prototype, we almost always need to
> typing "new = ewma_get(ewma_add(&ewma, val))". Is this intentional?
> if so, why?
>
> Why can't we simple do following?
> 
> unsigned long ewma_add(struct ewma *avg, const unsigned int val)
> {
> (snip)
> 	return ewma_get(avg);
> }

Hmm, I guess that depends on the way you want to use it. In my case, most of 
the times when I add a value to the average, I don't need to get the value. 
I'd call ewma_add() many more times than ewma_get(). Having the functions 
defined like this gives us the flexibility to choose and IMHO  
ewma_get(ewma_add(&ewma, val)) isn't so bad?

bruno
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