Re: txqueuelen has wrong units; should be time

[Jussi Kivilinna]

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
> 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@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/