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

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Date:	Thu, 25 Aug 2011 09:28:42 +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

Hi Eric,

Am 25.08.2011 um 07:28 schrieb Eric Dumazet:

> Le mercredi 24 août 2011 à 18:50 -0700, Yuchung Cheng a écrit :
>> On Wed, Aug 24, 2011 at 4:41 PM, Hagen Paul Pfeifer <hagen@...u.net> wrote:
>>> Check if calculated RTO is less then TCP_RTO_MIN. If this is true we
>>> adjust the value to TCP_RTO_MIN.
>>> 
>> but tp->rttvar is already lower-bounded via tcp_rto_min()?
>> 
>> static inline void tcp_set_rto(struct sock *sk)
>> {
>> ...
>> 
>>  /* NOTE: clamping at TCP_RTO_MIN is not required, current algo
>>   * guarantees that rto is higher.
>>   */
>>  tcp_bound_rto(sk);
>> }
> 
> Yes, and furthermore, we also limit ICMP rate, so in in my tests, I
> reach in a few rounds icsk_rto > 1sec
> 
> 07:16:13.010633 IP 10.2.1.2.59352 > 10.2.1.1.ssh: P 3833540215:3833540263(48) ack 2593537670 win 305
> 07:16:13.221111 IP 10.2.1.2.59352 > 10.2.1.1.ssh: P 0:48(48) ack 1 win 305 
> 07:16:13.661151 IP 10.2.1.2.59352 > 10.2.1.1.ssh: P 0:48(48) ack 1 win 305 
> 07:16:14.541153 IP 10.2.1.2.59352 > 10.2.1.1.ssh: P 0:48(48) ack 1 win 305 
> 07:16:16.301152 IP 10.2.1.2.59352 > 10.2.1.1.ssh: P 0:48(48) ack 1 win 305 
> <from this point, icsk_rto=1.76sec >
> 07:16:18.061158 IP 10.2.1.2.59352 > 10.2.1.1.ssh: P 0:48(48) ack 1 win 305 
> 07:16:19.821158 IP 10.2.1.2.59352 > 10.2.1.1.ssh: P 0:48(48) ack 1 win 305 
> 07:16:21.581018 IP 10.2.1.2.59352 > 10.2.1.1.ssh: P 0:48(48) ack 1 win 305 
> 07:16:23.341156 IP 10.2.1.2.59352 > 10.2.1.1.ssh: P 0:48(48) ack 1 win 305 
> 07:16:25.101151 IP 10.2.1.2.59352 > 10.2.1.1.ssh: P 0:48(48) ack 1 win 305 
> 07:16:26.861155 IP 10.2.1.2.59352 > 10.2.1.1.ssh: P 0:48(48) ack 1 win 305 
> 07:16:28.621158 IP 10.2.1.2.59352 > 10.2.1.1.ssh: P 0:48(48) ack 1 win 305 
> 07:16:30.381152 IP 10.2.1.2.59352 > 10.2.1.1.ssh: P 0:48(48) ack 1 win 305 
> 07:16:32.141157 IP 10.2.1.2.59352 > 10.2.1.1.ssh: P 0:48(48) ack 1 win 305 
> 
> 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?

> 
> On a server handling ~1.000.000 (long living) sessions, using
> application side keepalives (say one message sent every minute on each
> session), a temporary connectivity disruption _could_ makes it enter a
> critical zone, burning cpu and memory.
> 
> It seems TCP-LCD (RFC6069) depends very much on ICMP being rate limited.

This is right. We assume that a server/router sends only icmps when they
have free cycles.

> 
> I'll have to check what happens on multiple sessions : We might have
> cpus fighting on a single inetpeer and throtle, thus allowing backoff to
> increase after all. 
> 
> 
> 
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//
// 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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