184 lines
5.8 KiB
Diff
184 lines
5.8 KiB
Diff
From 536edd67109df5e0cdb2c4ee759e9bade7976367 Mon Sep 17 00:00:00 2001
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From: Eric Dumazet <edumazet@google.com>
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Date: Sat, 12 May 2012 03:32:13 +0000
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Subject: [PATCH 37/41] codel: use Newton method instead of sqrt() and divides
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As Van pointed out, interval/sqrt(count) can be implemented using
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multiplies only.
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http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Iterative_methods_for_reciprocal_square_roots
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This patch implements the Newton method and reciprocal divide.
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Total cost is 15 cycles instead of 120 on my Corei5 machine (64bit
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kernel).
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There is a small 'error' for count values < 5, but we don't really care.
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I reuse a hole in struct codel_vars :
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- pack the dropping boolean into one bit
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- use 31bit to store the reciprocal value of sqrt(count).
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Suggested-by: Van Jacobson <van@pollere.net>
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Signed-off-by: Eric Dumazet <edumazet@google.com>
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Cc: Dave Taht <dave.taht@bufferbloat.net>
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Cc: Kathleen Nichols <nichols@pollere.com>
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Cc: Tom Herbert <therbert@google.com>
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Cc: Matt Mathis <mattmathis@google.com>
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Cc: Yuchung Cheng <ycheng@google.com>
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Cc: Nandita Dukkipati <nanditad@google.com>
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Cc: Stephen Hemminger <shemminger@vyatta.com>
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Signed-off-by: David S. Miller <davem@davemloft.net>
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---
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include/net/codel.h | 68 ++++++++++++++++++++++++++++-----------------------
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1 file changed, 37 insertions(+), 31 deletions(-)
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--- a/include/net/codel.h
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+++ b/include/net/codel.h
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@@ -46,6 +46,7 @@
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#include <linux/skbuff.h>
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#include <net/pkt_sched.h>
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#include <net/inet_ecn.h>
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+#include <linux/reciprocal_div.h>
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/* Controlling Queue Delay (CoDel) algorithm
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* =========================================
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@@ -123,6 +124,7 @@ struct codel_params {
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* entered dropping state
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* @lastcount: count at entry to dropping state
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* @dropping: set to true if in dropping state
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+ * @rec_inv_sqrt: reciprocal value of sqrt(count) >> 1
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* @first_above_time: when we went (or will go) continuously above target
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* for interval
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* @drop_next: time to drop next packet, or when we dropped last
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@@ -131,7 +133,8 @@ struct codel_params {
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struct codel_vars {
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u32 count;
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u32 lastcount;
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- bool dropping;
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+ bool dropping:1;
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+ u32 rec_inv_sqrt:31;
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codel_time_t first_above_time;
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codel_time_t drop_next;
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codel_time_t ldelay;
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@@ -158,11 +161,7 @@ static void codel_params_init(struct cod
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static void codel_vars_init(struct codel_vars *vars)
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{
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- vars->drop_next = 0;
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- vars->first_above_time = 0;
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- vars->dropping = false; /* exit dropping state */
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- vars->count = 0;
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- vars->lastcount = 0;
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+ memset(vars, 0, sizeof(*vars));
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}
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static void codel_stats_init(struct codel_stats *stats)
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@@ -170,38 +169,37 @@ static void codel_stats_init(struct code
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stats->maxpacket = 256;
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}
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-/* return interval/sqrt(x) with good precision
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- * relies on int_sqrt(unsigned long x) kernel implementation
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+/*
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+ * http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Iterative_methods_for_reciprocal_square_roots
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+ * new_invsqrt = (invsqrt / 2) * (3 - count * invsqrt^2)
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+ *
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+ * Here, invsqrt is a fixed point number (< 1.0), 31bit mantissa)
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*/
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-static u32 codel_inv_sqrt(u32 _interval, u32 _x)
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+static void codel_Newton_step(struct codel_vars *vars)
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{
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- u64 interval = _interval;
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- unsigned long x = _x;
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-
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- /* Scale operands for max precision */
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+ u32 invsqrt = vars->rec_inv_sqrt;
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+ u32 invsqrt2 = ((u64)invsqrt * invsqrt) >> 31;
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+ u64 val = (3LL << 31) - ((u64)vars->count * invsqrt2);
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-#if BITS_PER_LONG == 64
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- x <<= 32; /* On 64bit arches, we can prescale x by 32bits */
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- interval <<= 16;
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-#endif
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+ val = (val * invsqrt) >> 32;
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- while (x < (1UL << (BITS_PER_LONG - 2))) {
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- x <<= 2;
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- interval <<= 1;
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- }
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- do_div(interval, int_sqrt(x));
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- return (u32)interval;
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+ vars->rec_inv_sqrt = val;
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}
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+/*
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+ * CoDel control_law is t + interval/sqrt(count)
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+ * We maintain in rec_inv_sqrt the reciprocal value of sqrt(count) to avoid
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+ * both sqrt() and divide operation.
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+ */
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static codel_time_t codel_control_law(codel_time_t t,
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codel_time_t interval,
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- u32 count)
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+ u32 rec_inv_sqrt)
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{
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- return t + codel_inv_sqrt(interval, count);
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+ return t + reciprocal_divide(interval, rec_inv_sqrt << 1);
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}
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-static bool codel_should_drop(struct sk_buff *skb,
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+static bool codel_should_drop(const struct sk_buff *skb,
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unsigned int *backlog,
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struct codel_vars *vars,
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struct codel_params *params,
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@@ -274,14 +272,16 @@ static struct sk_buff *codel_dequeue(str
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*/
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while (vars->dropping &&
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codel_time_after_eq(now, vars->drop_next)) {
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- if (++vars->count == 0) /* avoid zero divides */
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- vars->count = ~0U;
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+ vars->count++; /* dont care of possible wrap
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+ * since there is no more divide
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+ */
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+ codel_Newton_step(vars);
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if (params->ecn && INET_ECN_set_ce(skb)) {
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stats->ecn_mark++;
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vars->drop_next =
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codel_control_law(vars->drop_next,
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params->interval,
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- vars->count);
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+ vars->rec_inv_sqrt);
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goto end;
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}
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qdisc_drop(skb, sch);
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@@ -296,7 +296,7 @@ static struct sk_buff *codel_dequeue(str
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vars->drop_next =
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codel_control_law(vars->drop_next,
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params->interval,
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- vars->count);
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+ vars->rec_inv_sqrt);
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}
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}
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}
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@@ -319,12 +319,18 @@ static struct sk_buff *codel_dequeue(str
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if (codel_time_before(now - vars->drop_next,
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16 * params->interval)) {
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vars->count = (vars->count - vars->lastcount) | 1;
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+ /* we dont care if rec_inv_sqrt approximation
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+ * is not very precise :
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+ * Next Newton steps will correct it quadratically.
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+ */
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+ codel_Newton_step(vars);
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} else {
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vars->count = 1;
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+ vars->rec_inv_sqrt = 0x7fffffff;
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}
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vars->lastcount = vars->count;
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vars->drop_next = codel_control_law(now, params->interval,
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- vars->count);
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+ vars->rec_inv_sqrt);
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}
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end:
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return skb;
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