question about trimning data "len" conditions in TSO in tcp_output.c
Hans Petter Selasky
hps at selasky.org
Fri Nov 6 22:37:55 UTC 2015
On 11/06/15 22:56, Cui, Cheng wrote:
> On Nov 6, 2015, at 4:16 PM, Hans Petter Selasky <hps at 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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