From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-x22c.google.com (mail-pa0-x22c.google.com [IPv6:2607:f8b0:400e:c03::22c]) (using TLSv1 with cipher RC4-SHA (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority G2" (verified OK)) by huchra.bufferbloat.net (Postfix) with ESMTPS id CC14721F259 for ; Fri, 21 Mar 2014 10:51:53 -0700 (PDT) Received: by mail-pa0-f44.google.com with SMTP id bj1so2702745pad.17 for ; Fri, 21 Mar 2014 10:51:53 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=message-id:subject:from:to:cc:date:in-reply-to:references :content-type:content-transfer-encoding:mime-version; bh=NzSq1GogK3UxmhgUIGpRfXPLdRGzICduYAx9+3s1uOo=; b=dqptZNrn7MWpxMn6oMK7gx6rlJIqpu8XZorNknBBF+Namc01BDIdW45+WyMZb8mz0j ZEqNRS9QI8B9Fftagn17Cd6huDDduBUP+3VNQcUrZGv7JYMf4CtXQ/12pppJTHW5jhsT BnKrf1P+mpIvPRa4Qg6cTUlpEjq8zzy58hxogts0klAI5SpjWg/I1VNnql+zBYwIQ011 ZAZAgvHakbHN3YaMPN2re+ylniiiH2fKiyr8T9jk1LQnQoZDyLHSen2BqNSEE/oHsPDI fvWW+AhnKP6y5KkS20CEndwheXzbBNbxwW7Dx1nVnqNBXZDoP/ylttgLEd4oLXYikkbh Bl0g== X-Received: by 10.68.237.99 with SMTP id vb3mr55562145pbc.76.1395424313362; Fri, 21 Mar 2014 10:51:53 -0700 (PDT) Received: from [172.19.250.111] ([172.19.250.111]) by mx.google.com with ESMTPSA id qx11sm29359325pab.35.2014.03.21.10.51.52 for (version=SSLv3 cipher=RC4-SHA bits=128/128); Fri, 21 Mar 2014 10:51:52 -0700 (PDT) Message-ID: <1395424311.8701.20.camel@edumazet-glaptop2.roam.corp.google.com> From: Eric Dumazet To: renaud sallantin Date: Fri, 21 Mar 2014 10:51:51 -0700 In-Reply-To: References: <20140318145221.GA31327@sesse.net> <07BD4518-2A7E-4F43-8978-791E3B2BDA2A@cisco.com> <87eh1wc05c.fsf@toke.dk> <87a9ckbz1q.fsf@toke.dk> <1395358884.9114.102.camel@edumazet-glaptop2.roam.corp.google.com> <1395414379.6441.17.camel@edumazet-glaptop2.roam.corp.google.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3-0ubuntu6 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Cc: "Steinar H. Gunderson" , bloat Subject: Re: [Bloat] AQM creeping into L2 equipment X-BeenThere: bloat@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list List-Id: General list for discussing Bufferbloat List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 21 Mar 2014 17:51:54 -0000 On Fri, 2014-03-21 at 16:53 +0100, renaud sallantin wrote: > For our tests, we needed to adjust the "tcp_initial_quantum" in the > FQ, > but as you said, it is just a FQ parameter. > Yep, default ones are a compromise between performance and pacing accuracy. At 40Gbps speeds, it is a bit challenging. The consensus is that IW10 is adopted, meaning that we can send 10 MSS at whatever speed we want without knowing anything of the network conditions. If people want to play with other values, they have to change the route settings of their linux box, and fq parameters if they want. ip ro change default via 192.168.1.254 dev eth0 initcwnd 20 (As a matter of fact it seems some providers use higher values than IW10) > The "patch" we added, and once again, it was just a few lines, > enabled to set, via a sysctl parameter, the initial pacing value, > regardless of the RTT. > This can be valuable for different reasons: > o In case of long RTT, not set the pacing value is going to > introduce an un-necessary delay > (we aims to use this mechanism for satcom, so the delay could be > greater than 500ms) If you have a 500ms rtt, then you also want a bigger IW. Sending 10 MSS in the first RTT is going to be slow, no matter how you pace them. The first ACK wont come before 500 ms. > o In case of a wrong RTT measurement, i.e. an RTT measurement > that is higher that the real RTT (because of congestion for example), > you are going to have a wrong pacing evaluation... Well, if you have big rtt because of congestion, you exactly want to reduce the rate... rate = cwnd * mss / srtt And fq/pacing uses srtt, not rtt, so a single wrong rtt doesn't have a big impact (unless it is the first sample, as it will serve as the ewma initial value) You can not predict the network conditions just by studying the SYN/SYNACK/ACK initial messages. It gives a guess, but it is hard to send everything you want in a single RTT at 'optimal speed' Thats why it was so hard to decide the IW if you want an universal value. It depends on the state of the Internet, and it changes every day or so...