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 <br1@einfach.org> -- v2: use log2.h functions Signed-off-by: John W. Linville <linville@tuxdriver.com>

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 <br1@einfach.org> -- v2: use log2.h functions Signed-off-by: John W. Linville <linville@tuxdriver.com>
Bruno Randolf · John W. Linville
1 parent 5dcc03fe29
Showing 2 changed files with 13 additions and 11 deletions Side-by-side Diff
include/linux/average.h
lib/average.c
 #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 @@
  */
 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 */
@@ -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 @@
 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);
1	1	#ifndef _LINUX_AVERAGE_H
2	2	#define _LINUX_AVERAGE_H
3	3
4		-#include <linux/kernel.h>
5		-
6	4	/* Exponentially weighted moving average (EWMA) */
7	5
8	6	/* For more documentation see lib/average.c */
...	...	@@ -26,7 +24,7 @@
26	24	*/
27	25	static inline unsigned long ewma_read(const struct ewma *avg)
28	26	{
29		- return DIV_ROUND_CLOSEST(avg->internal, avg->factor);
	27	+ return avg->internal >> avg->factor;
30	28	}
31	29
32	30	#endif /* _LINUX_AVERAGE_H */
...	...	@@ -8,6 +8,7 @@
8	8	#include <linux/module.h>
9	9	#include <linux/average.h>
10	10	#include <linux/bug.h>
	11	+#include <linux/log2.h>
11	12
12	13	/**
13	14	* DOC: Exponentially Weighted Moving Average (EWMA)
14	15
15	16
16	17
...	...	@@ -24,18 +25,21 @@
24	25	* ewma_init() - Initialize EWMA parameters
25	26	* @avg: Average structure
26	27	* @factor: Factor to use for the scaled up internal value. The maximum value
27		- * of averages can be ULONG_MAX/(factor*weight).
	28	+ * of averages can be ULONG_MAX/(factor*weight). For performance reasons
	29	+ * factor has to be a power of 2.
28	30	* @weight: Exponential weight, or decay rate. This defines how fast the
29		- * influence of older values decreases. Has to be bigger than 1.
	31	+ * influence of older values decreases. For performance reasons weight has
	32	+ * to be a power of 2.
30	33	*
31	34	* Initialize the EWMA parameters for a given struct ewma @avg.
32	35	*/
33	36	void ewma_init(struct ewma *avg, unsigned long factor, unsigned long weight)
34	37	{
35		- WARN_ON(weight <= 1 \|\| factor == 0);
	38	+ WARN_ON(!is_power_of_2(weight) \|\| !is_power_of_2(factor));
	39	+
	40	+ avg->weight = ilog2(weight);
	41	+ avg->factor = ilog2(factor);
36	42	avg->internal = 0;
37		- avg->weight = weight;
38		- avg->factor = factor;
39	43	}
40	44	EXPORT_SYMBOL(ewma_init);
41	45
...	...	@@ -49,9 +53,9 @@
49	53	struct ewma ewma_add(struct ewma avg, unsigned long val)
50	54	{
51	55	avg->internal = avg->internal ?
52		- (((avg->internal * (avg->weight - 1)) +
53		- (val * avg->factor)) / avg->weight) :
54		- (val * avg->factor);
	56	+ (((avg->internal << avg->weight) - avg->internal) +
	57	+ (val << avg->factor)) >> avg->weight :
	58	+ (val << avg->factor);
55	59	return avg;
56	60	}
57	61	EXPORT_SYMBOL(ewma_add);