Subject: Re: [PATCH net-next v2 3/3] net: sctp: Add partial support for MSG_MORE on SCTP

From: Vlad Yasevich <[email-masked]>
Date: Tue, 15 Jul 2014 11:24:29 -0400

On 07/15/2014 10:33 AM, David Laight wrote:
> From: Vlad Yasevich 
>> On 07/14/2014 12:27 PM, David Laight wrote:
>>> From: Vlad Yasevich
>>> ...
>>>>> +	/* Setting MSG_MORE currently has the same effect as enabling Nagle.
>>>>> +	 * This means that the user can't force bundling of the first two data
>>>>> +	 * chunks.  It does mean that all the data chunks will be sent
>>>>> +	 * without an extra timer.
>>>>> +	 * It is enough to save the last value since any data sent with
>>>>> +	 * MSG_MORE clear will already have been sent (subject to flow control).
>>>>> +	 */
>>>>> +	if (msg->msg_flags & MSG_MORE)
>>>>> +		sp->tx_delay |= SCTP_F_TX_MSG_MORE;
>>>>> +	else
>>>>> +		sp->tx_delay &= ~SCTP_F_TX_MSG_MORE;
>>>>> +
>>>>
>>>> This is ok for 1-1 sockets, but it doesn't really work for 1-many sockets.  If one of
>>>> the associations uses MSG_MORE while another does not, we'll see some interesting
>>>> side-effects on the wire.
>>>
> ...
>>> I don't think this is a problem.
>>
>> Not, it is not a _problem_, but it does make MSG_MORE rather useless
>> in some situations.  Waiting for an ACK across low-latency links
>> is rare, but in a high-latency scenarios where you want to utilize the
>> bandwidth better with bundling, you may not see the gains you expect.
>>
>> Since MSG_MORE is association, it should be handled as such and an
>> a change on one association should not effect the others.
> 
> I think the comments already say that it is only a partial implementation.
> (If you send 2 chunks on an idle connection, they get sent separately.)
> Perhaps I'll add a note about possibly 'odd' effects for 1-many sockets
> with multi-threaded apps.
> 
> It helps a lot for my M3UA traffic.
> I can get the same effect on an old kernel by repeatedly changing SCTP_NODELAY,
> but that does rather rely on the way Nagle is implemented.

You can fix this by having an sp->tx_delay value and a assoc->tx_delay value
and simple check (sp->tx_delay | assoc->tx_delay).  MSG_MORE would only set
the assoc->tx_delay while SCTP_NODELAY would effect the socket.

This way, when one association uses MSG_MORE, it will not effect other associations
on the same socket that don't use it.

-vlad

> 	David