Zero-copy network transmission with io_uring

It's not the time it blocks that's gonna be the problem, but the impact on buffering.

Waiting until step 4 effectively removes local kernel buffering -- when one send completes there must be no further data available to send, thus outgoing packets will pause. And, like TCP_NODELAY, would cause extra partially filled packets to be sent at the tail of each send. If you're trying to send a stream of data as fast as the network allowed, this will all be counterproductive, unless all the data is provided in a single send. Sometimes that may be possible, but it seems like a very limited use case.

And, if your primary goal is not scalability or cpu, but rather memory usage, it seems like it'd be far simpler and just as effective to reduce SO_SNDBUF and use a regular blocking send loop.