netdev - Re: [PATCH] sch_red: fix red

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Message-ID: <20111207225747.GA4758@nuttenaction>
Date:	Wed, 7 Dec 2011 23:57:48 +0100
From:	Hagen Paul Pfeifer <hagen@...u.net>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Dave Taht <dave.taht@...il.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Thomas Graf <tgraf@...radead.org>,
	netdev <netdev@...r.kernel.org>, Jim Gettys <jg@...edesktop.org>
Subject: Re: [PATCH] sch_red: fix red_change

* Ilpo Järvinen | 2011-12-05 13:42:44 [+0200]:

>I disagree. If there's any slow starting flow that alone can fill the 
>bottleneck, anything significantly larger than RTT just harms. RED is 
>just "too slow" if you follow the recommended parametrization..
>
>In a core router you can probably get away with multiple RTTs, but near 
>edge that is a grave mistake due to how slow-start behaves. With average 
>based on many RTTs, RED still estimates that the link has low load while 
>congestion has escalated to higher dimensions due to slow start. As a 
>result, RED graciously falls back to tail-drop once the physical queue 
>runs out and the flows respond allowing the load to decrease. However, 
>finally RED reaches a state where it starts to "pro-actively" react to an 
>"incipient congestion"?!? :-/ => Problem is made worse by those extra 
>drops/marks happening too late.

But then one question Ilpo: drive IW10 or IW14 the behavior even worse?
Especially if n connections start almost simultanously? You did some analysis
on this topic.


HGN
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