I have a code that tries to be smart and prepare data to be chunked efficiently before sending, so I was happy to read about:

https://docs.python.org/3/library/asyncio-protocol.html#asyncio.WriteTransport.writelines

Only to see that it simply does:

self.write(b''.join(lines))

So I've attempted to write an version that uses sendmsg (scatter I/O) instead (will be attached in PR).

What I've learnt is:
1. It's hard to benchmark (If someone has an good example on checking if it's worth it, feel free to add such).
2. sendmsg has an OS limit on how many items can be done in one call. If the user does not call writer.drain() it might have too many items in the buffer, in that case I concat them (might be an expensive operation ? but that should not be th enormal case).
3. socket.socket.sendmsg can accept any bytes like iterable, but os.writev can only accept sequences, is that a bug ?
4. This is for the selector stream socket for now.

BTW, if wanted a much more simpler PR can be made, where writelines simply calls sendmsg on the input if no buffer exists, and if not only then concats and acts like the current code base.

A duplicate of #47010