forked from mirrors/linux
[PKT_SCHED]: Generic RED layer
Extracts the RED algorithm from sch_red.c and puts it into include/net/red.h for use by other RED based modules. The statistics are extended to be more fine grained in order to differ between probability/forced marks/drops. We now reset the average queue length when setting new parameters, leaving it might result in an unreasonable qavg for a while depending on the value of W. Signed-off-by: Thomas Graf <tgraf@suug.ch> Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
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Thomas Graf
Thomas Graf
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include/net/red.h
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include/net/red.h
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#ifndef __NET_SCHED_RED_H
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#define __NET_SCHED_RED_H
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#include <linux/config.h>
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#include <linux/types.h>
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#include <net/pkt_sched.h>
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#include <net/inet_ecn.h>
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#include <net/dsfield.h>
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/* Random Early Detection (RED) algorithm.
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=======================================
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Source: Sally Floyd and Van Jacobson, "Random Early Detection Gateways
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for Congestion Avoidance", 1993, IEEE/ACM Transactions on Networking.
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This file codes a "divisionless" version of RED algorithm
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as written down in Fig.17 of the paper.
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Short description.
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------------------
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When a new packet arrives we calculate the average queue length:
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avg = (1-W)*avg + W*current_queue_len,
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W is the filter time constant (chosen as 2^(-Wlog)), it controls
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the inertia of the algorithm. To allow larger bursts, W should be
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decreased.
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if (avg > th_max) -> packet marked (dropped).
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if (avg < th_min) -> packet passes.
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if (th_min < avg < th_max) we calculate probability:
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Pb = max_P * (avg - th_min)/(th_max-th_min)
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and mark (drop) packet with this probability.
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Pb changes from 0 (at avg==th_min) to max_P (avg==th_max).
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max_P should be small (not 1), usually 0.01..0.02 is good value.
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max_P is chosen as a number, so that max_P/(th_max-th_min)
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is a negative power of two in order arithmetics to contain
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only shifts.
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Parameters, settable by user:
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-----------------------------
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qth_min - bytes (should be < qth_max/2)
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qth_max - bytes (should be at least 2*qth_min and less limit)
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Wlog - bits (<32) log(1/W).
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Plog - bits (<32)
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Plog is related to max_P by formula:
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max_P = (qth_max-qth_min)/2^Plog;
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F.e. if qth_max=128K and qth_min=32K, then Plog=22
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corresponds to max_P=0.02
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Scell_log
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Stab
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Lookup table for log((1-W)^(t/t_ave).
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NOTES:
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Upper bound on W.
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-----------------
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If you want to allow bursts of L packets of size S,
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you should choose W:
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L + 1 - th_min/S < (1-(1-W)^L)/W
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th_min/S = 32 th_min/S = 4
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log(W) L
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-1 33
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-2 35
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-3 39
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-4 46
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-5 57
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-6 75
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-7 101
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-8 135
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-9 190
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etc.
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*/
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#define RED_STAB_SIZE 256
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#define RED_STAB_MASK (RED_STAB_SIZE - 1)
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struct red_stats
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{
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u32 prob_drop; /* Early probability drops */
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u32 prob_mark; /* Early probability marks */
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u32 forced_drop; /* Forced drops, qavg > max_thresh */
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u32 forced_mark; /* Forced marks, qavg > max_thresh */
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u32 pdrop; /* Drops due to queue limits */
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u32 other; /* Drops due to drop() calls */
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u32 backlog;
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};
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struct red_parms
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{
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/* Parameters */
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u32 qth_min; /* Min avg length threshold: A scaled */
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u32 qth_max; /* Max avg length threshold: A scaled */
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u32 Scell_max;
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u32 Rmask; /* Cached random mask, see red_rmask */
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u8 Scell_log;
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u8 Wlog; /* log(W) */
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u8 Plog; /* random number bits */
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u8 Stab[RED_STAB_SIZE];
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/* Variables */
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int qcount; /* Number of packets since last random
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number generation */
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u32 qR; /* Cached random number */
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unsigned long qavg; /* Average queue length: A scaled */
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psched_time_t qidlestart; /* Start of current idle period */
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};
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static inline u32 red_rmask(u8 Plog)
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{
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return Plog < 32 ? ((1 << Plog) - 1) : ~0UL;
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}
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static inline void red_set_parms(struct red_parms *p,
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u32 qth_min, u32 qth_max, u8 Wlog, u8 Plog,
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u8 Scell_log, u8 *stab)
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{
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/* Reset average queue length, the value is strictly bound
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* to the parameters below, reseting hurts a bit but leaving
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* it might result in an unreasonable qavg for a while. --TGR
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*/
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p->qavg = 0;
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p->qcount = -1;
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p->qth_min = qth_min << Wlog;
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p->qth_max = qth_max << Wlog;
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p->Wlog = Wlog;
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p->Plog = Plog;
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p->Rmask = red_rmask(Plog);
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p->Scell_log = Scell_log;
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p->Scell_max = (255 << Scell_log);
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memcpy(p->Stab, stab, sizeof(p->Stab));
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}
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static inline int red_is_idling(struct red_parms *p)
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{
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return !PSCHED_IS_PASTPERFECT(p->qidlestart);
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}
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static inline void red_start_of_idle_period(struct red_parms *p)
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{
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PSCHED_GET_TIME(p->qidlestart);
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}
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static inline void red_end_of_idle_period(struct red_parms *p)
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{
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PSCHED_SET_PASTPERFECT(p->qidlestart);
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}
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static inline void red_restart(struct red_parms *p)
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{
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red_end_of_idle_period(p);
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p->qavg = 0;
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p->qcount = -1;
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}
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static inline unsigned long red_calc_qavg_from_idle_time(struct red_parms *p)
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{
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psched_time_t now;
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long us_idle;
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int shift;
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PSCHED_GET_TIME(now);
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us_idle = PSCHED_TDIFF_SAFE(now, p->qidlestart, p->Scell_max);
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/*
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* The problem: ideally, average length queue recalcultion should
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* be done over constant clock intervals. This is too expensive, so
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* that the calculation is driven by outgoing packets.
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* When the queue is idle we have to model this clock by hand.
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*
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* SF+VJ proposed to "generate":
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*
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* m = idletime / (average_pkt_size / bandwidth)
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*
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* dummy packets as a burst after idle time, i.e.
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*
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* p->qavg *= (1-W)^m
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*
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* This is an apparently overcomplicated solution (f.e. we have to
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* precompute a table to make this calculation in reasonable time)
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* I believe that a simpler model may be used here,
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* but it is field for experiments.
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*/
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shift = p->Stab[(us_idle >> p->Scell_log) & RED_STAB_MASK];
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if (shift)
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return p->qavg >> shift;
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else {
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/* Approximate initial part of exponent with linear function:
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*
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* (1-W)^m ~= 1-mW + ...
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*
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* Seems, it is the best solution to
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* problem of too coarse exponent tabulation.
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*/
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us_idle = (p->qavg * us_idle) >> p->Scell_log;
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if (us_idle < (p->qavg >> 1))
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return p->qavg - us_idle;
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else
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return p->qavg >> 1;
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}
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}
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static inline unsigned long red_calc_qavg_no_idle_time(struct red_parms *p,
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unsigned int backlog)
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{
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/*
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* NOTE: p->qavg is fixed point number with point at Wlog.
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* The formula below is equvalent to floating point
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* version:
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*
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* qavg = qavg*(1-W) + backlog*W;
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*
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* --ANK (980924)
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*/
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return p->qavg + (backlog - (p->qavg >> p->Wlog));
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}
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static inline unsigned long red_calc_qavg(struct red_parms *p,
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unsigned int backlog)
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{
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if (!red_is_idling(p))
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return red_calc_qavg_no_idle_time(p, backlog);
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else
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return red_calc_qavg_from_idle_time(p);
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}
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static inline u32 red_random(struct red_parms *p)
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{
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return net_random() & p->Rmask;
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}
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static inline int red_mark_probability(struct red_parms *p, unsigned long qavg)
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{
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/* The formula used below causes questions.
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OK. qR is random number in the interval 0..Rmask
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i.e. 0..(2^Plog). If we used floating point
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arithmetics, it would be: (2^Plog)*rnd_num,
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where rnd_num is less 1.
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Taking into account, that qavg have fixed
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point at Wlog, and Plog is related to max_P by
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max_P = (qth_max-qth_min)/2^Plog; two lines
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below have the following floating point equivalent:
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max_P*(qavg - qth_min)/(qth_max-qth_min) < rnd/qcount
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Any questions? --ANK (980924)
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*/
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return !(((qavg - p->qth_min) >> p->Wlog) * p->qcount < p->qR);
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}
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enum {
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RED_BELOW_MIN_THRESH,
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RED_BETWEEN_TRESH,
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RED_ABOVE_MAX_TRESH,
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};
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static inline int red_cmp_thresh(struct red_parms *p, unsigned long qavg)
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{
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if (qavg < p->qth_min)
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return RED_BELOW_MIN_THRESH;
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else if (qavg >= p->qth_max)
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return RED_ABOVE_MAX_TRESH;
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else
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return RED_BETWEEN_TRESH;
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}
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enum {
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RED_DONT_MARK,
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RED_PROB_MARK,
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RED_HARD_MARK,
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};
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static inline int red_action(struct red_parms *p, unsigned long qavg)
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{
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switch (red_cmp_thresh(p, qavg)) {
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case RED_BELOW_MIN_THRESH:
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p->qcount = -1;
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return RED_DONT_MARK;
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case RED_BETWEEN_TRESH:
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if (++p->qcount) {
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if (red_mark_probability(p, qavg)) {
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p->qcount = 0;
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p->qR = red_random(p);
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return RED_PROB_MARK;
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}
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} else
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p->qR = red_random(p);
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return RED_DONT_MARK;
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case RED_ABOVE_MAX_TRESH:
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p->qcount = -1;
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return RED_HARD_MARK;
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}
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BUG();
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return RED_DONT_MARK;
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}
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#endif
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