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Subject: Re: question about trimning data "len" conditions in TSO in
 tcp_output.c
To: "Cui, Cheng" <Cheng.Cui@netapp.com>
References: <C44A2900-40E0-41EF-83B1-6DD4B31DABD5@netapp.com>
Cc: "svn-src-head@freebsd.org" <svn-src-head@freebsd.org>
From: Hans Petter Selasky <hps@selasky.org>
Message-ID: <563D1892.3050406@selasky.org>
Date: Fri, 6 Nov 2015 22:16:02 +0100
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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
>
>