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

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Date:	Thu, 25 Aug 2011 10:46:53 +0200
From:	Arnd Hannemann <arnd@...dnet.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Alexander Zimmermann <alexander.zimmermann@...sys.rwth-aachen.de>,
	Yuchung Cheng <ycheng@...gle.com>,
	Hagen Paul Pfeifer <hagen@...u.net>,
	netdev <netdev@...r.kernel.org>,
	Lukowski Damian <damian@....rwth-aachen.de>
Subject: Re: [PATCH] tcp: bound RTO to minimum

Hi,

Am 25.08.2011 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.
> 
> Now its kind of unlimited, according to experimental results.

That shouldn't be. It should stop after the same time a TCP connection with an
RTO of Minimum RTO which is doing 15 retries (tcp_retries2=15) and doing exponential backoff.
So it should be around 900s*. But it could be that because of the icsk_retransmit wrapover
this doesn't work as expected.

* 200ms + 400ms + 800ms ...

Best regards,
Arnd
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