From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arnd Hannemann Subject: Re: [PATCH] tcp: bound RTO to minimum Date: Thu, 25 Aug 2011 10:46:53 +0200 Message-ID: <4E560BFD.5020301@arndnet.de> References: <1314226834.6797.5.camel@edumazet-laptop> <1314229310-8074-1-git-send-email-hagen@jauu.net> <1314250134.6797.24.camel@edumazet-laptop> <4033BFEE-C432-4D94-8372-BA166AF2AA26@comsys.rwth-aachen.de> <1314260805.2387.11.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Alexander Zimmermann , Yuchung Cheng , Hagen Paul Pfeifer , netdev , Lukowski Damian To: Eric Dumazet Return-path: Received: from mail2.unitix.de ([176.9.2.175]:43515 "EHLO mail2.unitix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751837Ab1HYIzj (ORCPT ); Thu, 25 Aug 2011 04:55:39 -0400 In-Reply-To: <1314260805.2387.11.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC> Sender: netdev-owner@vger.kernel.org List-ID: Hi, Am 25.08.2011 10:26, schrieb Eric Dumazet: > Le jeudi 25 ao=C3=BBt 2011 =C3=A0 09:28 +0200, Alexander Zimmermann a= =C3=A9crit : >> Hi Eric, >> >> Am 25.08.2011 um 07:28 schrieb Eric Dumazet: >=20 >>> Real question is : do we really want to process ~1000 timer interru= pts >>> per tcp session, ~2000 skb alloc/free/build/handling, possibly ~100= 0 ARP >>> requests, only to make tcp revover in ~1sec when connectivity retur= ns >>> back. This just doesnt scale. >> >> maybe a stupid question, but 1000?. With an minRTO of 200ms and a ma= ximum >> probing time of 120s, we 600 retransmits in a worst-case-senario >> (assumed that we get for every rot retransmission an icmp). No? >=20 > Where is asserted the "max probing time of 120s" ?=20 >=20 > It is not the case on my machine : > I have way more retransmits than that, even if spaced by 1600 ms >=20 > 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) =3D 48 > 07:16:13.389417 select(7, [3 4], [], NULL, NULL) =3D 1 (in [3]) > 07:31:39.901311 read(3, 0xff8c4c90, 8192) =3D -1 EHOSTUNREACH (No rou= te to host) >=20 > Old kernels where performing up to 15 retries, doing exponential back= off. >=20 > 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=3D15) and do= ing 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