Messages in this thread | | | Date | Mon, 28 Feb 2011 20:31:05 +0200 | From | Jussi Kivilinna <> | Subject | Re: txqueuelen has wrong units; should be time |
| |
Quoting Eric Dumazet <eric.dumazet@gmail.com>:
> Le lundi 28 février 2011 à 13:43 +0200, Jussi Kivilinna a écrit : >> 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: > > Problem is you can have several virtual queues in a qdisc. > > For example, pfifo_fast has 3 bands. You could have a global ewma with > high values, but you still want to let a high priority packet going > through... >
Ok. It would better to have ewma/timelimit at leaf qdisc. (Or have in-middle-qdisc handling ewma/timelimit for leaf qdisc, sch_timelimit)
-Jussi
-- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |