From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from alpha.coverfire.com (dsiemon-2-pt.tunnel.tserv21.tor1.ipv6.he.net [IPv6:2001:470:1c:44e::2]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by huchra.bufferbloat.net (Postfix) with ESMTPS id 0F6A9201B88; Tue, 4 Dec 2012 19:41:17 -0800 (PST) Received: from [192.168.88.98] (titan.home [69.41.199.68]) (authenticated bits=0) by alpha.coverfire.com (8.14.5/8.14.5) with ESMTP id qB53fEQA001661 (version=TLSv1/SSLv3 cipher=DHE-RSA-CAMELLIA256-SHA bits=256 verify=NO); Tue, 4 Dec 2012 22:41:14 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=coverfire.com; s=alpha2011102501; t=1354678875; bh=Mz7q+PlTN7bzKGvLSejFn59q1LZcQ/Ea26H1xhYj/fc=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=OBJm1DZKq/pgAPKfzbavgJbM5SFrWOj+gMoiItHT22EMvlOA3k9/NdLeXol5p27zo xodk2+Ga9MLx7DKsKL7AjZdl3Oywj05Y1/pQzINqHGpk9QejCHajnFIONT+PGW04eZ CBKBm2LaGQUpN1nWUxffWIAVO5HqUpULy73m9qqY= Message-ID: <1354678874.29387.12.camel@ganymede.home> From: Dan Siemon To: Alex Burr Date: Tue, 04 Dec 2012 22:41:14 -0500 In-Reply-To: <1354613026.72238.YahooMailNeo@web126202.mail.ne1.yahoo.com> References: <20121123221842.GD2829@linux.vnet.ibm.com> <20121128172058.GB2474@linux.vnet.ibm.com> <20121202230635.GA16359@linux.vnet.ibm.com> <87obib5qf8.fsf@toke.dk> <1354550303.24281.103.camel@shinybook.infradead.org> <1354590837.29387.9.camel@ganymede.home> <1354613026.72238.YahooMailNeo@web126202.mail.ne1.yahoo.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.4.4 (3.4.4-2.fc17) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.73 on 69.41.199.58 X-Spam-Status: No, score=-3.0 required=5.0 tests=ALL_TRUSTED,BAYES_00, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU autolearn=ham version=3.3.2 X-Spam-Checker-Version: SpamAssassin 3.3.2 (2011-06-06) on alpha.coverfire.com Cc: "codel@lists.bufferbloat.net" , "cerowrt-devel@lists.bufferbloat.net" , bloat Subject: Re: [Codel] [Bloat] [Cerowrt-devel] FQ_Codel lwn draft article review X-BeenThere: codel@lists.bufferbloat.net X-Mailman-Version: 2.1.13 Precedence: list List-Id: CoDel AQM discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 05 Dec 2012 03:41:18 -0000 On Tue, 2012-12-04 at 01:23 -0800, Alex Burr wrote: > [Oops, intended to CC the list.] > > An extra 15-20ms of latency, which is what you're seeing there, > shouldn't be caused by packet overhead in the actual VDSL modem part of > the device. Worst case the implementation of the PTM-TC layer (the > equivalent of ATM) might round the packet up to a multiple of 64 bytes > (although it's not supposed to), but unless your line rate is 25kbps, > that should not cause 20ms of latency. > > VDSL2 operates at 4Khz or > 8Khz symbol rate, and while it contains and interleaving or > retransmission layer which can add up to 64ms in its most high-latency > configuration, those layers do not operate at the level of packets and > I'm pretty sure that that latency cannot be affected by packet size in a > conformant implementation. > This latency is being caused by something else. > > The > best way to figure out the per-packet overhead in your VDSL2 modem is > probably to count the number of packets that get through, not measure > latency. Thanks for the info. I guess I'll have to keep digging to figure out where the latency comes from. I did a couple more experiments which appear to confirm the large amount of per-packet overhead: http://www.coverfire.com/archives/2012/12/04/per-packet-overhead-on-vdsl2-2/