From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: [PATCH v3 net-next 2/2] tcp: TCP_NOTSENT_LOWAT socket option Date: Wed, 24 Jul 2013 17:55:22 -0700 (PDT) Message-ID: <20130724.175522.1183139427742327918.davem@davemloft.net> References: <1374550027.4990.141.camel@edumazet-glaptop> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: rick.jones2@hp.com, netdev@vger.kernel.org, ycheng@google.com, ncardwell@google.com, mtk.manpages@gmail.com To: eric.dumazet@gmail.com Return-path: Received: from shards.monkeyblade.net ([149.20.54.216]:37134 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754523Ab3GYAzX (ORCPT ); Wed, 24 Jul 2013 20:55:23 -0400 In-Reply-To: <1374550027.4990.141.camel@edumazet-glaptop> Sender: netdev-owner@vger.kernel.org List-ID: From: Eric Dumazet Date: Mon, 22 Jul 2013 20:27:07 -0700 > From: Eric Dumazet > > Idea of this patch is to add optional limitation of number of > unsent bytes in TCP sockets, to reduce usage of kernel memory. > > TCP receiver might announce a big window, and TCP sender autotuning > might allow a large amount of bytes in write queue, but this has little > performance impact if a large part of this buffering is wasted : > > Write queue needs to be large only to deal with large BDP, not > necessarily to cope with scheduling delays (incoming ACKS make room > for the application to queue more bytes) > > For most workloads, using a value of 128 KB or less is OK to give > applications enough time to react to POLLOUT events in time > (or being awaken in a blocking sendmsg()) > > This patch adds two ways to set the limit : > > 1) Per socket option TCP_NOTSENT_LOWAT > > 2) A sysctl (/proc/sys/net/ipv4/tcp_notsent_lowat) for sockets > not using TCP_NOTSENT_LOWAT socket option (or setting a zero value) > Default value being UINT_MAX (0xFFFFFFFF), meaning this has no effect. > > > This changes poll()/select()/epoll() to report POLLOUT > only if number of unsent bytes is below tp->nosent_lowat > > Note this might increase number of sendmsg()/sendfile() calls > when using non blocking sockets, > and increase number of context switches for blocking sockets. > > Note this is not related to SO_SNDLOWAT (as SO_SNDLOWAT is > defined as : > Specify the minimum number of bytes in the buffer until > the socket layer will pass the data to the protocol) > > Tested: ... > Signed-off-by: Eric Dumazet Applied