From mboxrd@z Thu Jan 1 00:00:00 1970 From: "David S. Miller" Subject: Re: Possible BUG in IPv4 TCP window handling, all recent 2.4.x/2.6.x kernels Date: Thu, 01 Sep 2005 23:28:23 -0700 (PDT) Message-ID: <20050901.232823.123760177.davem@davemloft.net> References: <20050901.154300.118239765.davem@davemloft.net> <2d02c76a84655d212634a91002b3eccd@psc.edu> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: lists@limebrokerage.com, linux-net@vger.kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org Return-path: To: jheffner@psc.edu In-Reply-To: <2d02c76a84655d212634a91002b3eccd@psc.edu> Sender: linux-net-owner@vger.kernel.org List-Id: netdev.vger.kernel.org From: John Heffner Date: Thu, 1 Sep 2005 22:51:48 -0400 > I have an idea why this is going on. Packets are pre-allocated by the > driver to be a max packet size, so when you send small packets, it > wastes a lot of memory. Currently Linux uses the packets at the > beginning of a connection to make a guess at how best to advertise its > window so as not to overflow the socket's memory bounds. Since you > start out with big segments then go to small ones, this is defeating > that mechanism. It's actually documented in the comments in > tcp_input.c. :) > > * The scheme does not work when sender sends good segments opening > * window and then starts to feed us spagetti. But it should work > * in common situations. Otherwise, we have to rely on queue collapsing. That's a strong possibility, good catch John. Although, I'm still not ruling out some box in the middle even though I consider it less likely than your theory. So you're suggesting that tcp_prune_queue() should do the: if (atomic_read(&sk->sk_rmem_alloc) >= sk->sk_rcvbuf) tcp_clamp_window(sk, tp); check after attempting to collapse the queue. But, that window clamping should fix the problem, as we recalculate the window to advertise.