netdev - Re: [PATCH 2/3] net: TCP thin linear timeouts

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Message-ID: <4AE9B9BA.7020207@gmail.com>
Date:	Thu, 29 Oct 2009 16:50:18 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	apetlund@...ula.no
CC:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Arnd Hannemann <hannemann@...s.rwth-aachen.de>,
	Netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, shemminger@...tta.com,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH 2/3] net: TCP thin linear timeouts

apetlund@...ula.no a écrit :
>> Andreas Petlund a écrit :

>> There should be a limit to linear timeouts, to say ... no more than 6
> retransmits
>> (eventually tunable), then switch to exponential backoff. Maybe your
> patch
>> already implement such heuristic ?
>>
> 
> The limitation you suggest to the linear timeouts makes very good sense.
> Our experiments performed on the Internet indicate that it is extremely
> rare that more than 6 retransmissions are needed to recover. It is not
> included in the current patch, so I will include this in the next
> iteration.
> 
>> True link collapses do happen, it would be good if not all streams
> wakeup
>> in the same
>> second and make recovery very slow.
>>
> 
> Each stream will have its own schedule for wakeup, so such events will
> still be subject to coincidence. The timer granularity of the TCP wakeup
> timer will also influence how many streams will wake at the same time. The
> experiments we have performed on severely congested bottlenecks (link
> above) indicate that the modifications will not create a large negative
> effect. In fact, when goodput is drastically reduced due to severe
> overload, regular TCP and the LT and dupACK modifications seem to perform
> nearly identically. Other scenarios may exist where different effects can
> be observed, and I am open to suggestions for further testing.
> 
>> Thats too easy to accept possibly dangerous features with the excuse of
> saying
>> "It wont be used very much", because you cannot predict the future.
> 
> I agree that it is no argument to say that it won't be used much; indeed,
> my hope is that it will be used much. However, our experiments indicate no
> negative effects while showing a large improvement on retransmission
> latency for the scenario in question. I therefore think that the option
> for such an improvement should be made available for time-dependent
> thin-stream applications.
> 

Thanks ! I must say I am very interested by these experiments, I am looking
forward your next iteration.

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