linux-kernel - Re: txqueuelen has wrong units; should be time

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Date:	Mon, 28 Feb 2011 14:10:40 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jussi Kivilinna <jussi.kivilinna@...et.fi>
Cc:	Albert Cahalan <acahalan@...il.com>,
	Mikael Abrahamsson <swmike@....pp.se>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	netdev@...r.kernel.org
Subject: Re: txqueuelen has wrong units; should be time

Le lundi 28 février 2011 à 13:43 +0200, Jussi Kivilinna a écrit :
> Quoting Eric Dumazet <eric.dumazet@...il.com>:
> 
> > Le dimanche 27 février 2011 à 12:55 +0200, Jussi Kivilinna a écrit :
> >> Quoting Albert Cahalan <acahalan@...il.com>:
> >>
> >> > On Sun, Feb 27, 2011 at 2:54 AM, Eric Dumazet  
> >> <eric.dumazet@...il.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...



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