TSO and FreeBSD vs Linux

[Luigi Rizzo]

On Tue, Aug 13, 2013 at 7:37 PM, Navdeep Parhar <np at freebsd.org> wrote:

> On 08/13/13 10:29, Julian Elischer wrote:
> ..
> >
> > Has anyone done any work on aggregating ACKs, or delaying responding to
> > them?
>
> If LRO is enabled on the FreeBSD receiver, ACKs are already aggregated
> (a duplicate ACK will result in an immediate flush though.)  See
> tcp_lro_rx.
>

>From what I have heard (no direct experience though), when TSO is enabled
linux may decide to hold a transmission in the hope of getting more acks
in the future hence a larger segment sent in one shot.

I am not sure i find similar code in FreeBSD; there is something mentioned
in tcp_output() but then the check only seems to be for t_maxseg

        /*
         * Sender silly window avoidance.   We transmit under the following
         * conditions when len is non-zero:
         *
         *      - We have a full segment (or more with TSO)
         *      - This is the last buffer in a write()/send() and we are
         *        either idle or running NODELAY
         *      - we've timed out (e.g. persist timer)
         *      - we have more then 1/2 the maximum send window's worth of
         *        data (receiver may be limited the window size)
         *      - we need to retransmit
         */
        if (len) {
                if (len >= tp->t_maxseg)
                        goto send;
                /*

and the t_maxseg seems to be capped to the mss.

This could be implemented in tcp_output(), i suppose.

cheers
luigi


> Regards,
> Navdeep
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