netdev - Re: [PATCH v2 net-next 0/3] net: sctp: Add partial MSG

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Message-Id: <20140710.161427.343089945230334314.davem@davemloft.net>
Date:	Thu, 10 Jul 2014 16:14:27 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	David.Laight@...LAB.COM
Cc:	netdev@...r.kernel.org, linux-sctp@...r.kernel.org,
	vyasevich@...il.com, nhorman@...driver.com
Subject: Re: [PATCH v2 net-next 0/3] net: sctp: Add partial MSG_MORE
 support to SCTP

From: David Laight <David.Laight@...LAB.COM>
Date: Wed, 9 Jul 2014 08:29:11 +0000

> If an application has disabled Nagle then it is almost impossible
> to get more than one DATA chunk into an ethernet packet even if
> the application has more than one data chunk ready to transmit.
> 
> This could be fixed by adding an SCTP_CORK socket option - but
> using that requires a lot of system calls.
> An alternative is to honour MSG_MORE - using it to mean that
> another chunk will be sent soon.
> (There isn't much point using MSG_MORE to allow a chunk be extended,
> sendv() can be used for fragmented data.)
> 
> This is a partial implementation and takes a couple of shortcuts:
> 1) We only worry about whether MSG_MORE was set on the last send.
>    Data sent (by the application) with MSG_MORE unset will only be
>    unsent for flow control reasons.
>    So if the last send had MSG_MORE set, and an ack opens the window
>    then the unsent data won't be sent immediately.
> 
> 2) If the application doesn't do a send with MSG_MORE unset, then
>    buffered data shouldn't be buffered forever.
>    Rather than using a timer (as TCP does - which ought to be configurable
>    on a per-socket basis) we use the same rules as Nagle and ensure
>    that there is always some data outstanding.
>    This does mean that the first data chunk on an idle connection
>    is send in its own packet even if MSG_MORE is set.

I need some SCTP experts to review this series, thanks.
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