Subject: Re: SCTP data chunk bundling when SCTP_NODELAY is set

From: Vlad Yasevich <[email-masked]>
Date: Thu, 19 Jun 2014 09:27:22 -0400

On 06/18/2014 12:38 PM, David Laight wrote:
> From: David Laight
>> From: Vlad Yasevich
>> ...
>>>>> I suppose we could implement SCTP_CORK to do the right thing.
>>>>>
>>>>> I thought is possibly utilizing something like sendmmsg() and passing
>>>>> an extra flag to let it be know that this is a multi-message send
>>>>> that should be queued up by sctp..
>>>>
>>>> It would be as easy to expose the extra flag to the 'application'
>>>> allowing it to use sendmsg() or sendmmsg().
>>>> While sendmmsg() saves a system call, it is fairly horrid to use.
>>>> (and I'm sending from a kernel driver so don't care about the
>>>> system call cost!)
>>>>
>>>> Possibly MSG_MORE with Nagle disabled could invoke the Nagle send
>>>> delay - but you'd need to know whether any chunks in the queue
>>>> had MSG_MORE clear.
>>>
>>> That's why doing this with cork would be simpler.  The ULP can just
>>> queue up a bunch of small data and if we pass nagle checks, it will be
>>> flushed.  If not, uncork will flush it.
>>
>> I think you need only care about the 'MSG_MORE' flag of the last data chunk.
>> Any earlier data (with MSG_MORE clear) will usually have been sent (unless
>> prevented by Nagle or flow control), so you probably wouldn't be able to
>> send it regardless of the state of MSG_MORE on a chunk being queued.
>> There is also the expectation that another send without MSG_MORE will
>> happen almost immediately.
>>
>> So MSG_MORE could have the same effect as corking the socket.
>> Although you'd need separate bits - but uncork could clear both.
>>
>> What I would like to implement (from M3UA) is to hold data for a maximum
>> of (say) 5ms awaiting M3UA data chunks. To do this properly requires
>> knowledge of the actual ethernet packet boundaries.
>>
>> The problem is there are (at least) three cases:
>> 1) This data should be sent as soon as possible.
>> 2) Send this data some time soonish.
>> 3) I've got another data block I'm going to give you after this one.
>>
>>> I could work up a patch for you if you want.
>>
>> I was thinking I might try to write one.
> 
> Actually this might work for what I'm trying to do.
> (untested).
> 
> diff --git a/net/sctp/output.c b/net/sctp/output.c
> index 0f4d15f..51030bc 100644
> --- a/net/sctp/output.c
> +++ b/net/sctp/output.c
> @@ -691,7 +691,7 @@ static sctp_xmit_t sctp_packet_can_append_data(struct sctp_packet *packet,
>  	 * if any previously transmitted data on the connection remains
>  	 * unacknowledged.
>  	 */
> -	if (!sctp_sk(asoc->base.sk)->nodelay && sctp_packet_empty(packet) &&
> +	if (sctp_sk(asoc->base.sk)->nodelay != 1 && sctp_packet_empty(packet) &&
>  	    inflight && sctp_state(asoc, ESTABLISHED)) {
>  		unsigned int max = transport->pathmtu - packet->overhead;
>  		unsigned int len = chunk->skb->len + q->out_qlen;
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index fee06b9..084b957 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -1928,7 +1928,10 @@ static int sctp_sendmsg(struct kiocb *iocb, struct sock *sk,
>  	}
>  
>  	/* Break the message into multiple chunks of maximum size. */
> +	if (msg->msg_flags & MSG_MORE)
> +		sp->nodelay |= 2;
>  	datamsg = sctp_datamsg_from_user(asoc, sinfo, msg, msg_len);
> +	sp->nodelay &= 1;

I think you reset it too early.  You have to reset after the call to
sctp_primitive_SEND().  This way, you queue up the data and go through
the state machine with nodelay != 1, thus triggering the updated code
on output.

>  	if (IS_ERR(datamsg)) {
>  		err = PTR_ERR(datamsg);
>  		goto out_free;
> 
> Ideally MSG_MORE should delay sending even if 'inflight' is false.
> But that would require 'flush on timeout'.

You can use a lack of MSG_MORE to be an indication of a flush.  Thus
MSG_MORE would always queue up data until MSG_MORE is 0, at which point
flush should happen.

> I'd prefer that, and with a configurable timeout.
> But I can implement the timeout in the 'application'.
> 
> Given the way Nagle is implemented in sctp, I could keep flipping
> it on and off - but that probably has undocumented behaviour
> (ie it might suddenly change).

With the above MSG_MORE, I think you can just turn off nagle once and
use MSG_MORE and when you drain your application queue, clear MSG_MORE
on the last write.

-vlad

> 	David