netdev - Re: [PATCH] tcp: bound RTO to minimum

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Date:	Thu, 25 Aug 2011 10:44:30 +0200
From:	Alexander Zimmermann <alexander.zimmermann@...sys.rwth-aachen.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Yuchung Cheng <ycheng@...gle.com>,
	Hagen Paul Pfeifer <hagen@...u.net>,
	netdev <netdev@...r.kernel.org>,
	Hannemann Arnd <arnd@...dnet.de>,
	Lukowski Damian <damian@....rwth-aachen.de>
Subject: Re: [PATCH] tcp: bound RTO to minimum


Am 25.08.2011 um 10:26 schrieb Eric Dumazet:

> Le jeudi 25 août 2011 à 09:28 +0200, Alexander Zimmermann a écrit :
>> Hi Eric,
>> 
>> Am 25.08.2011 um 07:28 schrieb Eric Dumazet:
> 
>>> Real question is : do we really want to process ~1000 timer interrupts
>>> per tcp session, ~2000 skb alloc/free/build/handling, possibly ~1000 ARP
>>> requests, only to make tcp revover in ~1sec when connectivity returns
>>> back. This just doesnt scale.
>> 
>> maybe a stupid question, but 1000?. With an minRTO of 200ms and a maximum
>> probing time of 120s, we 600 retransmits in a worst-case-senario
>> (assumed that we get for every rot retransmission an icmp). No?
> 
> Where is asserted the "max probing time of 120s" ? 
> 
> It is not the case on my machine :
> I have way more retransmits than that, even if spaced by 1600 ms
> 
> 07:16:13.389331 write(3, "\350F\235JC\357\376\363&\3\374\270R\21L\26\324{\37p\342\244i\304\356\241I:\301\332\222\26"..., 48) = 48
> 07:16:13.389417 select(7, [3 4], [], NULL, NULL) = 1 (in [3])
> 07:31:39.901311 read(3, 0xff8c4c90, 8192) = -1 EHOSTUNREACH (No route to host)
> 
> Old kernels where performing up to 15 retries, doing exponential backoff.

Yes I know. And in combination with RFC6069 we have to convert this
See Section 7.1

and

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=6fa12c85031485dff38ce550c24f10da23b0adaa

Is the transformation broken? Damian?


> 
> Now its kind of unlimited, according to experimental results.

Ok, unlimited is not what I expect...


> 
> 
> 

//
// Dipl.-Inform. Alexander Zimmermann
// Department of Computer Science, Informatik 4
// RWTH Aachen University
// Ahornstr. 55, 52056 Aachen, Germany
// phone: (49-241) 80-21422, fax: (49-241) 80-22222
// email: zimmermann@...rwth-aachen.de
// web: http://www.umic-mesh.net
//

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