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codel: use Newton method instead of sqrt() and divides 

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>
This commit is contained in:
Eric Dumazet 2012-05-12 03:32:13 +00:00 committed by David S. Miller
parent 470f16c83c
commit 536edd6710
1 changed files with 37 additions and 31 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

68
include/net/codel.h
View File

@ -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 */
val = (val * invsqrt) >> 32;
#if BITS_PER_LONG == 64
x <<= 32; /* On 64bit arches, we can prescale x by 32bits */
interval <<= 16;
#endif
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;