Re: txqueuelen has wrong units; should be time

From: Jussi Kivilinna
Date: Mon Feb 28 2011 - 13:31:29 EST

Quoting Eric Dumazet <eric.dumazet@xxxxxxxxx>:

Le lundi 28 fÃvrier 2011 Ã 13:43 +0200, Jussi Kivilinna a Ãcrit :
Quoting Eric Dumazet <eric.dumazet@xxxxxxxxx>:

> Le dimanche 27 fÃvrier 2011 Ã 12:55 +0200, Jussi Kivilinna a Ãcrit :
>> Quoting Albert Cahalan <acahalan@xxxxxxxxx>:
>> > On Sun, Feb 27, 2011 at 2:54 AM, Eric Dumazet
>> <eric.dumazet@xxxxxxxxx> wrote:
>> >> Le dimanche 27 fÃvrier 2011 Ã 08:02 +0100, Mikael Abrahamsson a Ãcrit :
>> >>> 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

Ok. It would better to have ewma/timelimit at leaf qdisc.
(Or have in-middle-qdisc handling ewma/timelimit for leaf qdisc, sch_timelimit)


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