From owner-freebsd-hackers Fri Nov 30 15: 1:10 2001 Delivered-To: freebsd-hackers@freebsd.org Received: from prism.flugsvamp.com (cb58709-a.mdsn1.wi.home.com [24.17.241.9]) by hub.freebsd.org (Postfix) with ESMTP id 61E7337B405; Fri, 30 Nov 2001 15:01:05 -0800 (PST) Received: (from jlemon@localhost) by prism.flugsvamp.com (8.11.0/8.11.0) id fAUMvlF92628; Fri, 30 Nov 2001 16:57:47 -0600 (CST) (envelope-from jlemon) Date: Fri, 30 Nov 2001 16:57:47 -0600 From: Jonathan Lemon To: Alfred Perlstein Cc: Matthew Dillon , Nate Williams , Alexander Haderer , jlemon@freebsd.org, freebsd-hackers@freebsd.org Subject: Re: Found the problem, w/patch (was Re: FreeBSD performing worse than Linux?) Message-ID: <20011130165747.I75389@prism.flugsvamp.com> References: <20011128153817.T61580@monorchid.lemis.com> <15364.38174.938500.946169@caddis.yogotech.com> <20011128104629.A43642@walton.maths.tcd.ie> <5.1.0.14.1.20011130181236.00a80160@postamt1.charite.de> <200111302047.fAUKlT811090@apollo.backplane.com> <200111302130.fAULUU324648@apollo.backplane.com> <15367.64883.390696.863120@caddis.yogotech.com> <200111302200.fAUM0hD27448@apollo.backplane.com> <20011130162832.N46769@elvis.mu.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Mailer: Mutt 1.0pre2i In-Reply-To: <20011130162832.N46769@elvis.mu.org> Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.ORG On Fri, Nov 30, 2001 at 04:28:32PM -0600, Alfred Perlstein wrote: > * Matthew Dillon [011130 16:02] wrote: > > > > Packet loss will screw up TCP performance no matter what you do. > > NewReno, assuming it is working properly, can improve performance > > for that case but it will not completely solve the problem (nothing will). > > Remember that our timers are only good to around 20ms by default, so > > even the best retransmission case is going to create a serious hicup. > > > > The question here is... is it actually packet loss that is creating > > this issue for you and John, or is it something else? The only way > > to tell for sure is to run tcpdump on BOTH the client and server > > and then observe whether packet loss is occuring by comparing the dumps. > > > > I would guess that turning off delayed-acks will improve performance > > in the face of packet loss, since a lost ack packet in that case will > > not be as big an issue. > > I have an odd theory that makes use of my waning remeberence of the > stack behavior, this may be totally off base but I'd appreciate it > if you guys would consider this scenerio if at all to put my mind > at ease. > > I seem to remeber several places in the stack that detect what looks > like a hiccup and immediately begin sending a sequence of ACKs in > order to trigger the other side's fast retrasmit code. One of the > things that I don't remember seeing is that state is persistant. There isn't anything in the receiver side that does this; ACKs are sent in response to incoming packets. However, state is maintained on the sender side as to whether we are performing fast retransmit or not. -- Jonathan To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message