Date: Fri, 6 Nov 2015 22:16:02 +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: <563D1892.3050406@selasky.org> In-Reply-To: <C44A2900-40E0-41EF-83B1-6DD4B31DABD5@netapp.com> References: <C44A2900-40E0-41EF-83B1-6DD4B31DABD5@netapp.com>
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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 > > Thanks, > --Cheng Cui > NetApp Scale Out Networking > >
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