[Bloat] Thoughts on Stochastic Fair Blue

[Dave Taht]

Can we try to keep coding stuff on bloat-devel? The traffic level on
bloat is (wonderfully) huge as it is, but a goodly percentage of the
audience can't deal at this level.

Eric had also sent me an interesting attempt at a 4 band pfifo_fast,
which explicitly prioritized ecn-enabled packets, I was wondering if
he could resend?

How does all these stuff cope with encapsulated (vpn/6in4) packets? UDP packets?

On Thu, Mar 24, 2011 at 9:32 AM, Eric Dumazet <eric.dumazet at gmail.com> wrote:
> Le jeudi 24 mars 2011 à 14:40 +0200, Jonathan Morton a écrit :
>
>> Finally, it might also be interesting and useful to add bare-bones ECN
>> support to the existing "dumb" qdiscs, such as SFQ and the FIFO
>> family.  Simply start marking (and dropping non-supporting flows) when
>> the queue is more than half full.
>
> Three months ago, I played with a SFQ patch to add ECN support, based on
> delay of packet in queue.
>
> http://www.spinics.net/lists/netdev/msg151594.html
>
> This patch is a hack of course (units are jiffies ticks, not ms)
>
>
> --------------------------------------
>
> Here is the POC patch I am currently testing, with a probability to
> "early drop" a packet of one percent per ms (HZ=1000 here), only if
> packet stayed at least 4 ms on queue.
>
> Of course, this only apply where SFQ is used, with known SFQ limits :)
>
> The term "early drop" is a lie. RED really early mark/drop a packet at
> enqueue() time, while I do it at dequeue() time [since I need to compute
> the delay]. But effect is the same on sent packets. This might use a bit
> more memory, but no more than current SFQ [and only if flows dont react
> to mark/drops]
>
> insmod net/sched/sch_sfq.ko red_delay=4
>
> By the way, I do think we should lower SFQ_DEPTH a bit and increase
> SFQ_SLOTS by same amount. Allowing 127 packets per flow seems not
> necessary in most situations SFQ might be used.
>
>  net/sched/sch_sfq.c |   37 +++++++++++++++++++++++++++++++++----
>  1 files changed, 33 insertions(+), 4 deletions(-)
>
> diff --git a/net/sched/sch_sfq.c b/net/sched/sch_sfq.c
> index d54ac94..4f958e3 100644
> --- a/net/sched/sch_sfq.c
> +++ b/net/sched/sch_sfq.c
> @@ -24,6 +24,8 @@
>  #include <net/ip.h>
>  #include <net/netlink.h>
>  #include <net/pkt_sched.h>
> +#include <net/inet_ecn.h>
> +#include <linux/moduleparam.h>
>
>
>  /*     Stochastic Fairness Queuing algorithm.
> @@ -86,6 +88,10 @@
>  /* This type should contain at least SFQ_DEPTH + SFQ_SLOTS values */
>  typedef unsigned char sfq_index;
>
> +static int red_delay; /* default : no RED handling */
> +module_param(red_delay, int, 0);
> +MODULE_PARM_DESC(red_delay, "mark/drop packets if they stay in queue longer than red_delay ticks");
> +
>  /*
>  * We dont use pointers to save space.
>  * Small indexes [0 ... SFQ_SLOTS - 1] are 'pointers' to slots[] array
> @@ -391,6 +397,7 @@ sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch)
>
>        sch->qstats.backlog += qdisc_pkt_len(skb);
>        slot_queue_add(slot, skb);
> +       qdisc_skb_cb(skb)->timestamp = jiffies;
>        sfq_inc(q, x);
>        if (slot->qlen == 1) {          /* The flow is new */
>                if (q->tail == NULL) {  /* It is the first flow */
> @@ -402,11 +409,8 @@ sfq_enqueue(struct sk_buff *skb, struct Qdisc *sch)
>                q->tail = slot;
>                slot->allot = q->scaled_quantum;
>        }
> -       if (++sch->q.qlen <= q->limit) {
> -               sch->bstats.bytes += qdisc_pkt_len(skb);
> -               sch->bstats.packets++;
> +       if (++sch->q.qlen <= q->limit)
>                return NET_XMIT_SUCCESS;
> -       }
>
>        sfq_drop(sch);
>        return NET_XMIT_CN;
> @@ -432,6 +436,7 @@ sfq_dequeue(struct Qdisc *sch)
>        sfq_index a, next_a;
>        struct sfq_slot *slot;
>
> +restart:
>        /* No active slots */
>        if (q->tail == NULL)
>                return NULL;
> @@ -455,12 +460,36 @@ next_slot:
>                next_a = slot->next;
>                if (a == next_a) {
>                        q->tail = NULL; /* no more active slots */
> +                       /* last packet queued, dont even try to apply RED */
>                        return skb;
>                }
>                q->tail->next = next_a;
>        } else {
>                slot->allot -= SFQ_ALLOT_SIZE(qdisc_pkt_len(skb));
>        }
> +       if (red_delay) {
> +               long delay = jiffies - qdisc_skb_cb(skb)->timestamp;
> +
> +               if (delay >= red_delay) {
> +                       long Px = delay * (0xFFFFFF / 100); /* 1 percent per jiffy */
> +                       if ((net_random() & 0xFFFFFF) < Px) {
> +                               if (INET_ECN_set_ce(skb)) {
> +                                       /* no ecnmark counter yet :) */
> +                                       sch->qstats.overlimits++;
> +                               } else {
> +                                       /* penalize this flow : we drop the
> +                                        * packet while we changed slot->allot
> +                                        */
> +                                       kfree_skb(skb);
> +                                       /* no early_drop counter yet :) */
> +                                       sch->qstats.drops++;
> +                                       goto restart;
> +                               }
> +                       }
> +               }
> +       }
> +       sch->bstats.bytes += qdisc_pkt_len(skb);
> +       sch->bstats.packets++;
>        return skb;
>  }
>
>
>
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-- 
Dave Täht
SKYPE: davetaht
US Tel: 1-239-829-5608
http://the-edge.blogspot.com