mirror of https://github.com/hak5/openwrt-owl.git
186 lines
5.8 KiB
Diff
186 lines
5.8 KiB
Diff
From 4a8056dfeef49b306ad6af24a5563d7d6867aae0 Mon Sep 17 00:00:00 2001
|
|
From: Eric Dumazet <edumazet@google.com>
|
|
Date: Sat, 12 May 2012 03:32:13 +0000
|
|
Subject: [PATCH] codel: use Newton method instead of sqrt() and divides
|
|
|
|
commit 536edd67109df5e0cdb2c4ee759e9bade7976367 upstream.
|
|
|
|
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@pollere.net>
|
|
Signed-off-by: Eric Dumazet <edumazet@google.com>
|
|
Cc: Dave Taht <dave.taht@bufferbloat.net>
|
|
Cc: Kathleen Nichols <nichols@pollere.com>
|
|
Cc: Tom Herbert <therbert@google.com>
|
|
Cc: Matt Mathis <mattmathis@google.com>
|
|
Cc: Yuchung Cheng <ycheng@google.com>
|
|
Cc: Nandita Dukkipati <nanditad@google.com>
|
|
Cc: Stephen Hemminger <shemminger@vyatta.com>
|
|
Signed-off-by: David S. Miller <davem@davemloft.net>
|
|
---
|
|
include/net/codel.h | 68 ++++++++++++++++++++++++++++-----------------------
|
|
1 file changed, 37 insertions(+), 31 deletions(-)
|
|
|
|
--- 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 cod
|
|
|
|
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 code
|
|
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;
|
|
-
|
|
- /* Scale operands for max precision */
|
|
+ u32 invsqrt = vars->rec_inv_sqrt;
|
|
+ u32 invsqrt2 = ((u64)invsqrt * invsqrt) >> 31;
|
|
+ u64 val = (3LL << 31) - ((u64)vars->count * invsqrt2);
|
|
|
|
-#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(str
|
|
*/
|
|
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(str
|
|
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(str
|
|
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;
|