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

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Message-ID: <AANLkTin_jY_J7tJseMLVmrQVVRezOVQTGCgB_6tefgXJ@mail.gmail.com>
Date:	Mon, 28 Feb 2011 16:51:29 -0500
From:	John Heffner <johnwheffner@...il.com>
To:	Bill Sommerfeld <wsommerfeld@...gle.com>
Cc:	Hagen Paul Pfeifer <hagen@...u.net>,
	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

Right... while I generally agree that a fixed-length drop-tail queue
isn't optimal, isn't this problem what the various AQM schemes try to
solve?

  -John


On Mon, Feb 28, 2011 at 12:20 PM, Bill Sommerfeld
<wsommerfeld@...gle.com> wrote:
> 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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