There's still the problem of sending to multiple destinations: OK sendmmsg() can send multiple datagrams, but for a given socket. When you have small windows (thank you cubic), you'll just send a few datagrams this way and don't save much.
> There's still the problem of sending to multiple destinations: OK sendmmsg() can send multiple datagrams, but for a given socket.
Hmm? sendmsg takes the destination address in the `struct msghdr` structure, and sendmmsg takes an array of those structures.
At the same time, the discussion of efficiency is about UDP vs. TCP. TCP writes are per socket, to the connected peer, and so UDP has the upper hand here. The concerns were about how TCP allows giving a large buffer to the kernel in a single write that then gets sliced into smaller packets automatically, vs. having to slice it in userspace and call send more, which sendmmsg solves.
(You can of course do single-syscall or even zero-syscall "send to many" with io_uring for any socket type, but that's a different discussion.)
> > There's still the problem of sending to multiple destinations: OK sendmmsg() can send multiple datagrams, but for a given socket.
> Hmm? sendmsg takes the destination address in the `struct msghdr` structure, and sendmmsg takes an array of those structures.
But that's still pointless on a connected socket. And if you're not using connected sockets, you're performing destination lookups for each and every datagram you're trying to send. It also means you're running with small buffers by default (the 212kB default buffers per socket are shared with all your destinations, no longer per destination). Thus normally you want to use connected socket when dealing with UDP in environments having performance requirements.
That should really be in the `SEE ALSO` of `man 3 sendmsg`...