use TCP_NODELAY on TCP sockets?

[Erin Shepherd]

send(..., MSG_MORE)
send(..., 0)

Is roughly equivalent to
setsockopt(..., TCP_CORK, 1)
send(..., 0)
send(..., 0)
setsockopt(..., TCP_CORK, 0)

In cases where you know that you're going to write more but you want to be able to discard the user space buffer, MSG_MORE is useful. If holding onto the buffers is easy so they can then be passed to sendmsg(), then you may as well do that. 

(And of course you can combine both techniques)

- Erin

On Thu, 16 May 2024, at 01:32, Job Snijders wrote:
> On Wed, May 15, 2024 at 09:09:47PM +0200, Erin Shepherd wrote:
> > It seems absent from the BSDs, but on Linux you can pass the MSG_MORE
> > flag to send() to override TCP_NODELAY for a specific write
> 
> Am I understanding correctly this is a variant on TCP_NOPUSH/TCP_CORK?
> "more data is coming, dont push the send button yet!"
> 
> In OpenBGPD, TCP_NODELAY is set on the socket (a socket option available
> on all platforms, I think?), and then all data is coalesced into
> sendmsg(), no need for 'corking'. From my limited testing it seems a
> full routing table should fit in ~ TCP 41,000 packets.
> 
> BIRD has a code path sk_sendmsg()->sendmsg() called from
> sk_maybe_write(); but based my limited testing I'm not sure this path is
> followed in all cases, because I see way more than 41K packets for a
> full table feed (with TCP_NODELAY enabled).
> 
> Perhaps there are two separate questions here:
> 
> - are BGP messages (slightly) delayed because of TCP_NODELAY not being
>   set? (I think yes)
> - are BGP messages as efficiently coalesced into as few TCP packets as
>   possible? (with TCP_NODELAY set, I am not sure)
> 
> Kind regards,
> 
> Job
> 
> ps. To clarify why I started this thread: last week I fell into the TCP
> subsystem rabbit hole: why are things the way they are? I started
> auditing various programs related to my $dayjob and thought it would be
> good to open a conversation with the BIRD developer community. My goal
> is not necessarily to get this patch 'as-is' merged, but to learn from
> and with friendly and respected BGP developers.
> 
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