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lib: Improve EWMA efficiency by using bitshifts

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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: Bruno Randolf <br1@einfach.org>

--

v2:	use log2.h functions
Signed-off-by: John W. Linville <linville@tuxdriver.com>
This commit is contained in:
Bruno Randolf 2010-12-02 19:50:37 +09:00 committed by John W. Linville
parent 5dcc03fe29
commit af55688435
2 changed files with 13 additions and 11 deletions
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4
include/linux/average.h
View File

@ -1,8 +1,6 @@
#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 */

20
lib/average.c
View File

@ -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);