From owner-svn-src-head@freebsd.org Fri Nov 6 22:37:55 2015 Return-Path: Delivered-To: svn-src-head@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id BB5BBA28DB4 for ; Fri, 6 Nov 2015 22:37:55 +0000 (UTC) (envelope-from hps@selasky.org) Received: from mail.turbocat.net (heidi.turbocat.net [88.198.202.214]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 7E8D41C0F for ; Fri, 6 Nov 2015 22:37:55 +0000 (UTC) (envelope-from hps@selasky.org) Received: from laptop015.home.selasky.org (unknown [62.141.129.119]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.turbocat.net (Postfix) with ESMTPSA id 2DEAF1FE023; Fri, 6 Nov 2015 23:37:52 +0100 (CET) Subject: Re: question about trimning data "len" conditions in TSO in tcp_output.c To: "Cui, Cheng" References: <563D1892.3050406@selasky.org> Cc: "svn-src-head@freebsd.org" From: Hans Petter Selasky Message-ID: <563D2C26.2070300@selasky.org> Date: Fri, 6 Nov 2015 23:39:34 +0100 User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit X-BeenThere: svn-src-head@freebsd.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: SVN commit messages for the src tree for head/-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 06 Nov 2015 22:37:55 -0000 On 11/06/15 22:56, Cui, Cheng wrote: > On Nov 6, 2015, at 4:16 PM, Hans Petter Selasky wrote: > >> On 11/06/15 21:46, Cui, Cheng wrote: >>> Hello Hans, >>> >>> Sorry if my previous email does not reach you because of a bad subject. >>> >>> This is Cheng Cui. I am reading the CURRENT FreeBSD code in tcp_output.c, and find this question regarding your change in revision 271946. >>> https://svnweb.freebsd.org/base/head/sys/netinet/tcp_output.c?r1=271946&r2=271945&pathrev=271946 >>> >>> trim data "len" under TSO: >>> >>> 885 /* >>> 886 * Prevent the last segment from being >>> 887 * fractional unless the send sockbuf can be >>> 888 * emptied: >>> 889 */ >>> 890 max_len = (tp->t_maxopd - optlen); >>> 891 if ((off + len) < sbavail(&so->so_snd)) { <== >>> 892 moff = len % max_len; >>> 893 if (moff != 0) { >>> 894 len -= moff; >>> 895 sendalot = 1; >>> 896 } >>> 897 } >>> >>> Is there a specific reason that it should skip trimming the data "len" under the condition of "(off + len) == sbavail(&so->so_snd)" in TSO? >>> Because I am wondering if we can trim the data "len" directly without checking the "(off + len)" condition. >> >> Hi Cheng, >> >> I believe the reason is to avoid looping one more time outputting a single packet containing the remainder of the available data, with regard to max_len. > How did you envision the removal of this check would influence the generated packet sequence? >> >> --HPS >> > Hi Hans, > > I may be wrong but my assumption is that the remainder of the available data may be larger than one single packet. > > Suppose max_len==1500, sb_acc==3001, off==2, and (off+len)==3001. In this case, the current code will not trim the "len" > and let it go directly to the NIC. I think it skips the Nagle's algorithm. As len==2999, the last packet is 1499, > it is supposed to be held until all outstanding data are ACKed, but it has been sent out. Hi Cheng, That is correct. Nagle's algorithm is not active when "(off+len) == sb_acc". Anyhow, the check for "(off+len) == sb_acc" does not go away. It has to be put before sendalot = 1 to avoid sending the so-called "small packet" in the next iteration. Possibly you will need to add a check for TCP nodelay being active, which disable Nagle's algorithm. Have you done any tests removing this check? --HPS