From mboxrd@z Thu Jan 1 00:00:00 1970 From: "John Heffner" Subject: Re: RE: A Linux TCP SACK Question Date: Tue, 15 Apr 2008 10:23:56 -0700 Message-ID: <1e41a3230804151023s1658a83dye0a2e91d7a98c34@mail.gmail.com> References: <000301c89e81$80124570$6b5ee183@D2GT6T71> <1e41a3230804141748v2bf1f32bsc80307c9390c222@mail.gmail.com> <1e41a3230804151001gc9fa670p511b16bb01e0bc93@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: "Wenji Wu" , Netdev To: "=?ISO-8859-1?Q?Ilpo_J=E4rvinen?=" Return-path: Received: from el-out-1112.google.com ([209.85.162.176]:64176 "EHLO el-out-1112.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755987AbYDORYA convert rfc822-to-8bit (ORCPT ); Tue, 15 Apr 2008 13:24:00 -0400 Received: by el-out-1112.google.com with SMTP id n30so259105elf.21 for ; Tue, 15 Apr 2008 10:23:57 -0700 (PDT) In-Reply-To: Content-Disposition: inline Sender: netdev-owner@vger.kernel.org List-ID: On Tue, Apr 15, 2008 at 10:08 AM, Ilpo J=E4rvinen wrote: > On Tue, 15 Apr 2008, John Heffner wrote: > > We can see that in both cases you are getting throttled by > > tcp_moderate_cwnd (X_OtherReductionsCM). I'm not sure offhand why > > it's reaching this code - I would have thought that the high > > tp->reordering would prevent this. Ilpo, do you have any insights= ? > > What makes you think so? It's called from tcp_try_to_open as anyone = can > read from the source, basically when our state is CA_Disorder (some = very > small portion might happen in ca_recovery besides that). This is what X_OtherReductionsCM instruments, and that was the only thing holding back cwnd. I just looked at the source, and indeed it will be called on every ack when we are in the disorder state. Limiting cwnd to packets_in_flight() + 3 here is going to prevent cwnd from growing when the reordering is greater than 3. Making max_burst at least tp->reordering should help some, though I'm not sure it's the right thing to do. -John