netdev - Re: txqueuelen has wrong units; should be time

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Message-ID: <20110228203105.13466xdjy27nr5wk@hayate.sektori.org>
Date:	Mon, 28 Feb 2011 20:31:05 +0200
From:	Jussi Kivilinna <jussi.kivilinna@...et.fi>
To:	Eric Dumazet <eric.dumazet@...il.com>
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

Quoting Eric Dumazet <eric.dumazet@...il.com>:

> 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...
>

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

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