Zero-copy network transmission with io_uring

Zero-copy network transmission with io_uring

You've explained why it's inadequate in terms of CPU time given a large number of clients, but not in terms of memory usage, nor in terms of small servers handling a few tens of clients at peak; different optimization targets for different needs.

For the general case, io_uring and async is the "best" option, but it brings in a lot of complexity managing the state machines in user code rather than simply relying on thread per client. Zero-copy reduces memory demand as compared to current send syscalls, and having a way to do simple buffer management would be useful for the subset of systems that don't actually care about CPU load that much, don't have many clients at a time to multiplex (hence not many threads), but do want a simple "one thread per client" model that avoids cross-thread synchronisation fun.

Not everything is a Google/Facebook/Netflix level problem, and on embedded systems I've worked on, a zero-copy blocking until ACK send would have made the code smaller and simpler; we emulated it in userspace via mutexes, but that's not exactly a high performance option.

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