From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: [PATCH] tcp: remove 1ms offset in srtt computation Date: Thu, 06 Feb 2014 22:28:15 -0800 (PST) Message-ID: <20140206.222815.1199932808813246959.davem@davemloft.net> References: <1391731030.10160.36.camel@edumazet-glaptop2.roam.corp.google.com> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, ycheng@google.com, ncardwell@google.com, valas@google.com To: eric.dumazet@gmail.com Return-path: Received: from shards.monkeyblade.net ([149.20.54.216]:40344 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750707AbaBGG2T (ORCPT ); Fri, 7 Feb 2014 01:28:19 -0500 In-Reply-To: <1391731030.10160.36.camel@edumazet-glaptop2.roam.corp.google.com> Sender: netdev-owner@vger.kernel.org List-ID: From: Eric Dumazet Date: Thu, 06 Feb 2014 15:57:10 -0800 > From: Eric Dumazet > > TCP pacing depends on an accurate srtt estimation. > > Current srtt estimation is using jiffie resolution, > and has an artificial offset of at least 1 ms, which can produce > slowdowns when FQ/pacing is used, especially in DC world, > where typical rtt is below 1 ms. > > We are planning a switch to usec resolution for linux-3.15, > but in the meantime, this patch removes the 1 ms offset. > > All we need is to have tp->srtt minimal value of 1 to differentiate > the case of srtt being initialized or not, not 8. > > The problematic behavior was observed on a 40Gbit testbed, > where 32 concurrent netperf were reaching 12Gbps of aggregate > speed, instead of line speed. > > This patch also has the effect of reporting more accurate srtt and send > rates to iproute2 ss command as in : > > $ ss -i dst cca2 > Netid State Recv-Q Send-Q Local Address:Port > Peer Address:Port > tcp ESTAB 0 0 10.244.129.1:56984 > 10.244.129.2:12865 > cubic wscale:6,6 rto:200 rtt:0.25/0.25 ato:40 mss:1448 cwnd:10 send > 463.4Mbps rcv_rtt:1 rcv_space:29200 > tcp ESTAB 0 390960 10.244.129.1:60247 > 10.244.129.2:50204 > cubic wscale:6,6 rto:200 rtt:0.875/0.75 mss:1448 cwnd:73 ssthresh:51 > send 966.4Mbps unacked:73 retrans:0/121 rcv_space:29200 > > Reported-by: Vytautas Valancius > Signed-off-by: Eric Dumazet Applied.