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Date: Sat, 12 May 2012 15:32:13 +0200
From: Eric Dumazet <eric.dumazet@...il.com>
To: David Miller <davem@...emloft.net>
Cc: Dave Taht <dave.taht@...ferbloat.net>,
netdev <netdev@...r.kernel.org>,
Kathleen Nichols <nichols@...lere.com>,
Van Jacobson <van@...lere.net>, codel@...ts.bufferbloat.net,
Yuchung Cheng <ycheng@...gle.com>,
Matt Mathis <mattmathis@...gle.com>,
Tom Herbert <therbert@...gle.com>,
Stephen Hemminger <shemminger@...tta.com>,
Nandita Dukkipati <nanditad@...gle.com>
Subject: [PATCH net-next] codel: use Newton method instead of sqrt() and
divides
From: Eric Dumazet <edumazet@...gle.com>
As Van pointed out, interval/sqrt(count) can be implemented using
multiplies only.
http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Iterative_methods_for_reciprocal_square_roots
This patch implements the Newton method and reciprocal divide.
Total cost is 15 cycles instead of 120 on my Corei5 machine (64bit
kernel).
There is a small 'error' for count values < 5, but we don't really care.
I reuse a hole in struct codel_vars :
- pack the dropping boolean into one bit
- use 31bit to store the reciprocal value of sqrt(count).
Suggested-by: Van Jacobson <van@...lere.net>
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Dave Taht <dave.taht@...ferbloat.net>
Cc: Kathleen Nichols <nichols@...lere.com>
Cc: Tom Herbert <therbert@...gle.com>
Cc: Matt Mathis <mattmathis@...gle.com>
Cc: Yuchung Cheng <ycheng@...gle.com>
Cc: Nandita Dukkipati <nanditad@...gle.com>
Cc: Stephen Hemminger <shemminger@...tta.com>
---
include/net/codel.h | 68 ++++++++++++++++++++++--------------------
1 file changed, 37 insertions(+), 31 deletions(-)
diff --git a/include/net/codel.h b/include/net/codel.h
index bce2cef..bd8747c 100644
--- a/include/net/codel.h
+++ b/include/net/codel.h
@@ -46,6 +46,7 @@
#include <linux/skbuff.h>
#include <net/pkt_sched.h>
#include <net/inet_ecn.h>
+#include <linux/reciprocal_div.h>
/* Controlling Queue Delay (CoDel) algorithm
* =========================================
@@ -123,6 +124,7 @@ struct codel_params {
* entered dropping state
* @lastcount: count at entry to dropping state
* @dropping: set to true if in dropping state
+ * @rec_inv_sqrt: reciprocal value of sqrt(count) >> 1
* @first_above_time: when we went (or will go) continuously above target
* for interval
* @drop_next: time to drop next packet, or when we dropped last
@@ -131,7 +133,8 @@ struct codel_params {
struct codel_vars {
u32 count;
u32 lastcount;
- bool dropping;
+ bool dropping:1;
+ u32 rec_inv_sqrt:31;
codel_time_t first_above_time;
codel_time_t drop_next;
codel_time_t ldelay;
@@ -158,11 +161,7 @@ static void codel_params_init(struct codel_params *params)
static void codel_vars_init(struct codel_vars *vars)
{
- vars->drop_next = 0;
- vars->first_above_time = 0;
- vars->dropping = false; /* exit dropping state */
- vars->count = 0;
- vars->lastcount = 0;
+ memset(vars, 0, sizeof(*vars));
}
static void codel_stats_init(struct codel_stats *stats)
@@ -170,38 +169,37 @@ static void codel_stats_init(struct codel_stats *stats)
stats->maxpacket = 256;
}
-/* return interval/sqrt(x) with good precision
- * relies on int_sqrt(unsigned long x) kernel implementation
+/*
+ * http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Iterative_methods_for_reciprocal_square_roots
+ * new_invsqrt = (invsqrt / 2) * (3 - count * invsqrt^2)
+ *
+ * Here, invsqrt is a fixed point number (< 1.0), 31bit mantissa)
*/
-static u32 codel_inv_sqrt(u32 _interval, u32 _x)
+static void codel_Newton_step(struct codel_vars *vars)
{
- u64 interval = _interval;
- unsigned long x = _x;
+ u32 invsqrt = vars->rec_inv_sqrt;
+ u32 invsqrt2 = ((u64)invsqrt * invsqrt) >> 31;
+ u64 val = (3LL << 31) - ((u64)vars->count * invsqrt2);
- /* Scale operands for max precision */
-
-#if BITS_PER_LONG == 64
- x <<= 32; /* On 64bit arches, we can prescale x by 32bits */
- interval <<= 16;
-#endif
+ val = (val * invsqrt) >> 32;
- while (x < (1UL << (BITS_PER_LONG - 2))) {
- x <<= 2;
- interval <<= 1;
- }
- do_div(interval, int_sqrt(x));
- return (u32)interval;
+ vars->rec_inv_sqrt = val;
}
+/*
+ * CoDel control_law is t + interval/sqrt(count)
+ * We maintain in rec_inv_sqrt the reciprocal value of sqrt(count) to avoid
+ * both sqrt() and divide operation.
+ */
static codel_time_t codel_control_law(codel_time_t t,
codel_time_t interval,
- u32 count)
+ u32 rec_inv_sqrt)
{
- return t + codel_inv_sqrt(interval, count);
+ return t + reciprocal_divide(interval, rec_inv_sqrt << 1);
}
-static bool codel_should_drop(struct sk_buff *skb,
+static bool codel_should_drop(const struct sk_buff *skb,
unsigned int *backlog,
struct codel_vars *vars,
struct codel_params *params,
@@ -274,14 +272,16 @@ static struct sk_buff *codel_dequeue(struct Qdisc *sch,
*/
while (vars->dropping &&
codel_time_after_eq(now, vars->drop_next)) {
- if (++vars->count == 0) /* avoid zero divides */
- vars->count = ~0U;
+ vars->count++; /* dont care of possible wrap
+ * since there is no more divide
+ */
+ codel_Newton_step(vars);
if (params->ecn && INET_ECN_set_ce(skb)) {
stats->ecn_mark++;
vars->drop_next =
codel_control_law(vars->drop_next,
params->interval,
- vars->count);
+ vars->rec_inv_sqrt);
goto end;
}
qdisc_drop(skb, sch);
@@ -296,7 +296,7 @@ static struct sk_buff *codel_dequeue(struct Qdisc *sch,
vars->drop_next =
codel_control_law(vars->drop_next,
params->interval,
- vars->count);
+ vars->rec_inv_sqrt);
}
}
}
@@ -319,12 +319,18 @@ static struct sk_buff *codel_dequeue(struct Qdisc *sch,
if (codel_time_before(now - vars->drop_next,
16 * params->interval)) {
vars->count = (vars->count - vars->lastcount) | 1;
+ /* we dont care if rec_inv_sqrt approximation
+ * is not very precise :
+ * Next Newton steps will correct it quadratically.
+ */
+ codel_Newton_step(vars);
} else {
vars->count = 1;
+ vars->rec_inv_sqrt = 0x7fffffff;
}
vars->lastcount = vars->count;
vars->drop_next = codel_control_law(now, params->interval,
- vars->count);
+ vars->rec_inv_sqrt);
}
end:
return skb;
--
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