From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rick Jones Subject: Re: [PATCH] Add TCP_NO_DELAYED_ACK socket option Date: Wed, 26 Oct 2011 10:56:55 -0700 Message-ID: <4EA849E7.5030503@hp.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org To: Andy Lutomirski Return-path: Received: from g6t0186.atlanta.hp.com ([15.193.32.63]:42489 "EHLO g6t0186.atlanta.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932953Ab1JZR45 (ORCPT ); Wed, 26 Oct 2011 13:56:57 -0400 In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: On 10/25/2011 07:25 PM, Andy Lutomirski wrote: > When talking to an unfixable interactive peer that fails to set > TCP_NODELAY, disabling delayed ACKs can help mitigate the problem. > This is an evil thing to do, but if the entire network is private, > it's not that evil. > > This works around a problem with the remote *application*, so make > it a socket option instead of a sysctl or a per-route option. > > Signed-off-by: Andy Lutomirski > --- > > This patch is a bit embarrassing. We talk to remote applications over > TCP that are very much interactive but don't set TCP_NODELAY. These > applications apparently cannot be fixed. As a partial workaround, if we > ACK every incoming segment, then as long as they don't transmit two > segments per rtt, we do pretty well. Embarrassing/evil indeed - is it really something to go into the kernel? If the networks where this happens are indeed truly private, can they run a private kernel? Or use an LD_PRELOAD hack to wedge-in a setsockopt(TCP_NODELAY) call into the application? Or set something like tcp_naglim_def on the application system(s)? Or have the server application make a setsockopt(TCP_MAXSEG) call before listen() to a value one byte below that of what the application is sending? Is the application actually "virtuous" in sending logically associated data in one "send" call, and simply running afoul of Nagle+DelayedACK in having multiple distinct requests outstanding at once, or is it actually quite evil in that it is sending logically associated data in separate send calls? rick jones choir preaching follows: raj@tardy:~$ cat usenet_replies/nagle_algorithm > I'm not familiar with this issue, and I'm mostly ignorant about what > tcp does below the sockets interface. Can anybody briefly explain what > "nagle" is, and how and when to turn it off? Or point me to the > appropriate manual. In broad terms, whenever an application does a send() call, the logic of the Nagle algorithm is supposed to go something like this: 1) Is the quantity of data in this send, plus any queued, unsent data, greater than the MSS (Maximum Segment Size) for this connection? If yes, send the data in the user's send now (modulo any other constraints such as receiver's advertised window and the TCP congestion window). If no, go to 2. 2) Is the connection to the remote otherwise idle? That is, is there no unACKed data outstanding on the network. If yes, send the data in the user's send now. If no, queue the data and wait. Either the application will continue to call send() with enough data to get to a full MSS-worth of data, or the remote will ACK all the currently sent, unACKed data, or our retransmission timer will expire. Now, where applications run into trouble is when they have what might be described as "write, write, read" behaviour, where they present logically associated data to the transport in separate 'send' calls and those sends are typically less than the MSS for the connection. It isn't so much that they run afoul of Nagle as they run into issues with the interaction of Nagle and the other heuristics operating on the remote. In particular, the delayed ACK heuristics. When a receiving TCP is deciding whether or not to send an ACK back to the sender, in broad handwaving terms it goes through logic similar to this: a) is there data being sent back to the sender? if yes, piggy-back the ACK on the data segment. b) is there a window update being sent back to the sender? if yes, piggy-back the ACK on the window update. c) has the standalone ACK timer expired. Window updates are generally triggered by the following heuristics: i) would the window update be for a non-trivial fraction of the window - typically somewhere at or above 1/4 the window, that is, has the application "consumed" at least that much data? if yes, send a window update. if no, check ii. ii) would the window update be for, the application "consumed," at least 2*MSS worth of data? if yes, send a window update, if no wait. Now, going back to that write, write, read application, on the sending side, the first write will be transmitted by TCP via nagle rule 2 - the connection is otherwise idle. However, the second small send will be delayed as there is at that point unACKnowledged data outstanding on the connection. At the receiver, that small TCP segment will arrive and will be passed to the application. The application does not have the entire app-level message, so it will not send a reply (data to TCP) back. The typical TCP window is much much larger than the MSS, so no window update would be triggered by heuristic i. The data just arrived and consumed by the application is < 2*MSS, so no window update from heuristic ii. Since there is no window update, no ACK is sent by heuristic b. So, that leaves heuristic c - the standalone ACK timer. That ranges anywhere between 50 and 200 milliseconds depending on the TCP stack in use. If you've read this far :) now we can take a look at the effect of various things touted as "fixes" to applications experiencing this interaction. We take as our example a client-server application where both the client and the server are implemented with a write of a small application header, followed by application data. First, the "default" case which is with Nagle enabled (TCP_NODELAY _NOT_ set) and with standard ACK behaviour: Client Server Req Header -> <- Standalone ACK after Nms Req Data -> <- Possible standalone ACK <- Rsp Header Standalone ACK -> <- Rsp Data Possible standalone ACK -> For two "messages" we end-up with at least six segments on the wire. The possible standalone ACKs will depend on whether the server's response time, or client's think time is longer than the standalone ACK interval on their respective sides. Now, if TCP_NODELAY is set we see: Client Server Req Header -> Req Data -> <- Possible Standalone ACK after Nms <- Rsp Header <- Rsp Data Possible Standalone ACK -> In theory, we are down two four segments on the wire which seems good, but frankly we can do better. First though, consider what happens when someone disables delayed ACKs Client Server Req Header -> <- Immediate Standalone ACK Req Data -> <- Immediate Standalone ACK <- Rsp Header Immediate Standalone ACK -> <- Rsp Data Immediate Standalone ACK -> Now we definitly see 8 segments on the wire. It will also be that way if both TCP_NODELAY is set and delayed ACKs are disabled. How about if the application did the "right" think in the first place? That is sent the logically associated data at the same time: Client Server Request -> <- Possible Standalone ACK <- Response Possible Standalone ACK -> We are down to two segments on the wire. For "small" packets, the CPU cost is about the same regardless of data or ACK. This means that the application which is making the propper gathering send call will spend far fewer CPU cycles in the networking stack.