Messages in this thread | | | Date | Mon, 28 Feb 2011 13:43:38 +0200 | From | Jussi Kivilinna <> | Subject | Re: txqueuelen has wrong units; should be time |
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Quoting Eric Dumazet <eric.dumazet@gmail.com>:
> Le dimanche 27 février 2011 à 12:55 +0200, Jussi Kivilinna a écrit : >> Quoting Albert Cahalan <acahalan@gmail.com>: >> >> > On Sun, Feb 27, 2011 at 2:54 AM, Eric Dumazet >> <eric.dumazet@gmail.com> wrote: >> >> Le dimanche 27 février 2011 à 08:02 +0100, Mikael Abrahamsson a écrit : >> >>> On Sun, 27 Feb 2011, Albert Cahalan wrote: >> >>> >> >>> > Nanoseconds seems fine; it's unlikely you'd ever want >> >>> > more than 4.2 seconds (32-bit unsigned) of queue. >> > ... >> >> Problem is some machines have slow High Resolution timing services. >> >> >> >> _If_ we have a time limit, it will probably use the low resolution (aka >> >> jiffies), unless high resolution services are cheap. >> > >> > As long as that is totally internal to the kernel and never >> > getting exposed by some API for setting the amount, sure. >> > >> >> I was thinking not having an absolute hard limit, but an EWMA based one. >> > >> > The whole point is to prevent stale packets, especially to prevent >> > them from messing with TCP, so I really don't think so. I suppose >> > you do get this to some extent via early drop. >> >> I made simple hack on sch_fifo with per packet time limits >> (attachment) this weekend and have been doing limited testing on >> wireless link. I think hardlimit is fine, it's simple and does >> somewhat same as what packet(-hard)limited buffer does, drops packets >> when buffer is 'full'. My hack checks for timed out packets on >> enqueue, might be wrong approach (on other hand might allow some more >> burstiness). >> > > > Qdisc should return to caller a good indication packet is queued or > dropped at enqueue() time... not later (aka : never) > > Accepting a packet at t0, and dropping it later at t0+limit without > giving any indication to caller is a problem. > > This is why I suggested using an EWMA plus a probabilist drop or > congestion indication (NET_XMIT_CN) to caller at enqueue() time. > > The absolute time limit you are trying to implement should be checked at > dequeue time, to cope with enqueue bursts or pauses on wire. >
Would it be better to implement this as generic feature instead of qdisc specific? Have qdisc_enqueue_root do ewma check:
static inline int qdisc_enqueue_root(struct sk_buff *skb, struct Qdisc *sch) { qdisc_skb_cb(skb)->pkt_len = skb->len; if (likely(!sch->use_timeout)) { ewma_ok: return qdisc_enqueue(skb, sch) & NET_XMIT_MASK; }
status = qdisc_check_ewma_status() if (status == ok) goto ewma_ok;
if (status == overlimits) ...drop...
if (status == congestion) { ret = qdisc_enqueue(skb, sch) & NET_XMIT_MASK; return (ret == success) ? NET_XMIT_CN : ret; } }
And add qdisc_dequeue_root:
static inline struct sk_buff *qdisc_dequeue_root(struct Qdisc *sch) { skb = sch->dequeue(sch);
if (skb && unlikely(sch->use_timeout)) qdisc_update_ewma(skb);
return skb; }
Then user could specify any qdisc to use timeout or not with tc. Maybe go even as far as have some default timeout for default qdisc(?)
-Jussi
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