sch_red: Adaptative RED AQM

Adaptative RED AQM for linux, based on paper from Sally FLoyd,
Ramakrishna Gummadi, and Scott Shenker, August 2001 :

http://icir.org/floyd/papers/adaptiveRed.pdf

Goal of Adaptative RED is to make max_p a dynamic value between 1% and
50% to reach the target average queue : (max_th - min_th) / 2

Every 500 ms:
 if (avg > target and max_p <= 0.5)
  increase max_p : max_p += alpha;
 else if (avg < target and max_p >= 0.01)
  decrease max_p : max_p *= beta;

target :[min_th + 0.4*(min_th - max_th),
          min_th + 0.6*(min_th - max_th)].
alpha : min(0.01, max_p / 4)
beta : 0.9
max_P is a Q0.32 fixed point number (unsigned, with 32 bits mantissa)

Changes against our RED implementation are :

max_p is no longer a negative power of two (1/(2^Plog)), but a Q0.32
fixed point number, to allow full range described in Adatative paper.

To deliver a random number, we now use a reciprocal divide (thats really
a multiply), but this operation is done once per marked/droped packet
when in RED_BETWEEN_TRESH window, so added cost (compared to previous
AND operation) is near zero.

dump operation gives current max_p value in a new TCA_RED_MAX_P
attribute.

Example on a 10Mbit link :

tc qdisc add dev $DEV parent 1:1 handle 10: est 1sec 8sec red \
   limit 400000 min 30000 max 90000 avpkt 1000 \
   burst 55 ecn adaptative bandwidth 10Mbit

# tc -s -d qdisc show dev eth3
...
qdisc red 10: parent 1:1 limit 400000b min 30000b max 90000b ecn
adaptative ewma 5 max_p=0.113335 Scell_log 15
 Sent 50414282 bytes 34504 pkt (dropped 35, overlimits 1392 requeues 0)
 rate 9749Kbit 831pps backlog 72056b 16p requeues 0
  marked 1357 early 35 pdrop 0 other 0

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 2 deletions

diff --git a/include/linux/pkt_sched.h b/include/linux/pkt_sched.h
index fb556dc594d3..e41e0d4de24b 100644
--- a/include/linux/pkt_sched.h
+++ b/include/linux/pkt_sched.h
@@ -181,6 +181,7 @@ enum {
 	TCA_RED_UNSPEC,
 	TCA_RED_PARMS,
 	TCA_RED_STAB,
+	TCA_RED_MAX_P,
 	__TCA_RED_MAX,
 };
 
@@ -194,8 +195,9 @@ struct tc_red_qopt {
 	unsigned char   Plog;		/* log(P_max/(qth_max-qth_min))	*/
 	unsigned char   Scell_log;	/* cell size for idle damping */
 	unsigned char	flags;
-#define TC_RED_ECN	1
-#define TC_RED_HARDDROP	2
+#define TC_RED_ECN		1
+#define TC_RED_HARDDROP		2
+#define TC_RED_ADAPTATIVE	4
 };
 
 struct tc_red_xstats {