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

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Message-ID: <AANLkTikje6rvjSfEA3CvUjdgFwzk-SBZDdO5kQvi6LZy@mail.gmail.com>
Date:	Mon, 28 Feb 2011 09:20:28 -0800
From:	Bill Sommerfeld <wsommerfeld@...gle.com>
To:	Hagen Paul Pfeifer <hagen@...u.net>
Cc:	Albert Cahalan <acahalan@...il.com>,
	Jussi Kivilinna <jussi.kivilinna@...et.fi>,
	Eric Dumazet <eric.dumazet@...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

On Mon, Feb 28, 2011 at 07:38, Hagen Paul Pfeifer <hagen@...u.net> wrote:
> On Sun, 27 Feb 2011 18:33:39 -0500, Albert Cahalan wrote:
>> I suppose there is a need to allow at least 2 packets despite any
>> time limits, so that it remains possible to use a traditional modem
>> even if a huge packet takes several seconds to send.
>
> That is a good point! We talk about as we may know every use case of
> Linux. But this is not true at all. One of my customer for example operates
> the Linux network stack functionality on top of a proprietary MAC/Driver
> where the current packet queue characteristic is just fine. The
> time-drop-approach is unsuitable because the bandwidth can vary in a small
> amount of time over a great range (0 till max. bandwidth). A sufficient
> buffering shows up superior in this environment (only IPv{4,6}/UDP).

The tension is between the average queue length and the maximum amount
of buffering needed.  Fixed-sized tail-drop queues -- either long, or
short -- are not ideal.

My understanding is that the best practice here is that you need
(bandwidth * path delay) buffering to be available to absorb bursts
and avoid drops, but you also need to use queue management algorithms
with ECN or random drop to keep the *average* queue length short;
unfortunately, researchers are still arguing about the details of the
second part...
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