From af5568843594fb71055debe36e521fa8072fcecc Mon Sep 17 00:00:00 2001 From: Bruno Randolf Date: Thu, 2 Dec 2010 19:50:37 +0900 Subject: 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: Bruno Randolf -- v2: use log2.h functions Signed-off-by: John W. Linville --- include/linux/average.h | 4 +--- lib/average.c | 20 ++++++++++++-------- 2 files changed, 13 insertions(+), 11 deletions(-) diff --git a/include/linux/average.h b/include/linux/average.h index 7706e40..c6028fd 100644 --- a/include/linux/average.h +++ b/include/linux/average.h @@ -1,8 +1,6 @@ #ifndef _LINUX_AVERAGE_H #define _LINUX_AVERAGE_H -#include - /* 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 */ diff --git a/lib/average.c b/lib/average.c index f1d1b46..5576c28 100644 --- a/lib/average.c +++ b/lib/average.c @@ -8,6 +8,7 @@ #include #include #include +#include /** * 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); -- cgit v1.1