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Commit af556884 authored by Bruno Randolf's avatar Bruno Randolf Committed by John W. Linville
Browse files

lib: Improve EWMA efficiency by using bitshifts 



Using bitshifts instead of division and multiplication should improve
performance. That requires weight and factor to be powers of two, but i think
this is something we can live with.

Thanks to Peter Zijlstra for the improved formula!

Signed-off-by: default avatarBruno Randolf <br1@einfach.org>

--

v2:	use log2.h functions
Signed-off-by: default avatarJohn W. Linville <linville@tuxdriver.com>
parent 5dcc03fe

include/linux/average.h

+1 −3
Original line number Diff line number Diff line 
#ifndef _LINUX_AVERAGE_H

#define _LINUX_AVERAGE_H



#include <linux/kernel.h>



/* Exponentially weighted moving average (EWMA) */



/* For more documentation see lib/average.c */
@@ -26,7 +24,7 @@ extern struct ewma *ewma_add(struct ewma *avg, unsigned long val); 
 */

static inline unsigned long ewma_read(const struct ewma *avg)

{

	return DIV_ROUND_CLOSEST(avg->internal, avg->factor);

	return avg->internal >> avg->factor;

}



#endif /* _LINUX_AVERAGE_H */

lib/average.c

+12 −8
Original line number Diff line number Diff line
@@ -8,6 +8,7 @@ 
#include <linux/module.h>

#include <linux/average.h>

#include <linux/bug.h>

#include <linux/log2.h>



/**

 * DOC: Exponentially Weighted Moving Average (EWMA)
@@ -24,18 +25,21 @@ 
 * ewma_init() - Initialize EWMA parameters

 * @avg: Average structure

 * @factor: Factor to use for the scaled up internal value. The maximum value

 *	of averages can be ULONG_MAX/(factor*weight).

 *	of averages can be ULONG_MAX/(factor*weight). For performance reasons

 *	factor has to be a power of 2.

 * @weight: Exponential weight, or decay rate. This defines how fast the

 *	influence of older values decreases. Has to be bigger than 1.

 *	influence of older values decreases. For performance reasons weight has

 *	to be a power of 2.

 *

 * Initialize the EWMA parameters for a given struct ewma @avg.

 */

void ewma_init(struct ewma *avg, unsigned long factor, unsigned long weight)

{

	WARN_ON(weight <= 1 || factor == 0);

	WARN_ON(!is_power_of_2(weight) || !is_power_of_2(factor));



	avg->weight = ilog2(weight);

	avg->factor = ilog2(factor);

	avg->internal = 0;

	avg->weight = weight;

	avg->factor = factor;

}

EXPORT_SYMBOL(ewma_init);
@@ -49,9 +53,9 @@ EXPORT_SYMBOL(ewma_init); 
struct ewma *ewma_add(struct ewma *avg, unsigned long val)

{

	avg->internal = avg->internal  ?

		(((avg->internal * (avg->weight - 1)) +

			(val * avg->factor)) / avg->weight) :

		(val * avg->factor);

		(((avg->internal << avg->weight) - avg->internal) +

			(val << avg->factor)) >> avg->weight :

		(val << avg->factor);

	return avg;

}

EXPORT_SYMBOL(ewma_add);