Re: Restricting payload size in do_tcp_sendpages.

[Eric Dumazet <eric.dumazet@gmail.com>]

Le jeudi 01 mars 2012 à 16:16 +0100, Ashwin Rao a écrit :
> Hi,
> 
> I would like to perform some experiments where I want TCP to send
> packets of size less than the MSS even more than an MSS worth of data
> is available in the TCP buffers.  I am performing these experiments to
> submit a patch that can avoid synchronization of TCP flows. I have
> modified the following code in the function do_tcp_sendpages in the
> file net/ipv4/tcp.c.  For testing purposes I have currently set the
> mss_now to half of the value that would have been typically used. I
> have even updated the size_goal which controls the maximum size that
> can be sent when offloading is enabled. To avoid coalescing smaller
> skbs I have even set the can_coalesce to 0. Despite these changes the
> frames sent over the Ethernet have the length of 1514 bytes. I would
> like to know which function is responsible for coalescing these
> packets despite forcing a small segment size.
> 
> *** linux-3.2.6/net/ipv4/tcp.c	
> --- linux-3.2.6-modified/net/ipv4/tcp.c
> *************** static ssize_t do_tcp_sendpages(struct s
> *** 774,779 ****
> --- 774,783 ----
>   	clear_bit(SOCK_ASYNC_NOSPACE, &sk->sk_socket->flags);
> 
>   	mss_now = tcp_send_mss(sk, &size_goal, flags);
> +     /* hack begin */
> +     mss_now = mss_now >> 1;
> +     size_goal = mss_now;
> +     /* hack end */
>   	copied = 0;
> 
>   	err = -EPIPE;
> *************** new_segment:
> *** 805,810 ****
> --- 809,817 ----
> 
>   		i = skb_shinfo(skb)->nr_frags;
>   		can_coalesce = skb_can_coalesce(skb, i, page, offset);
> +             /* hack begin */
> +             can_coalesce = 0;
> +             /* hack end */
>   		if (!can_coalesce && i >= MAX_SKB_FRAGS) {
>   			tcp_mark_push(tp, skb);
>   			goto new_segment;
> *************** wait_for_memory:
> *** 857,862 ****
> --- 864,873 ----
>   			goto do_error;
> 
>   		mss_now = tcp_send_mss(sk, &size_goal, flags);
> +             /* hack begin */
> +             mss_now = mss_now >> 1;
> +             size_goal = mss_now;
> +             /* hack end */
>   	}
> 
> 

You dont need to hack kernel

man 7 tcp

       TCP_MAXSEG
              The  maximum  segment  size  for  outgoing TCP packets.  If this
              option is set before connection establishment, it  also  changes
              the  MSS value announced to the other end in the initial packet.
              Values greater than the (eventual) interface MTU have no effect.
              TCP  will  also  impose  its minimum and maximum bounds over the
              value provided.