Date: Fri, 6 Nov 2015 23:39:34 +0100 From: Hans Petter Selasky <hps@selasky.org> To: "Cui, Cheng" <Cheng.Cui@netapp.com> Cc: "svn-src-head@freebsd.org" <svn-src-head@freebsd.org> Subject: Re: question about trimning data "len" conditions in TSO in tcp_output.c Message-ID: <563D2C26.2070300@selasky.org> In-Reply-To: <E6DBCD15-A41C-49A4-88C5-FB79EC969716@netapp.com> References: <C44A2900-40E0-41EF-83B1-6DD4B31DABD5@netapp.com> <563D1892.3050406@selasky.org> <E6DBCD15-A41C-49A4-88C5-FB79EC969716@netapp.com>
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On 11/06/15 22:56, Cui, Cheng wrote: > On Nov 6, 2015, at 4:16 PM, Hans Petter Selasky <hps@selasky.org> 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
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