mysql 6 网络层 IO 的设计

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- A new thread is created to serve this socket. (Actually, with a
thread cache, it might reuse an existing thtread.)
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In short: The above design does not scale.

- One (main) thread loops over a select/poll where one (or more) of
the sockets is the one for new connections.
- If a new connection was made, accept it and add the new socket to
the set of sockets.
- Input on the other sockets is read and the incomming messages
each assembled.
- Each complete request is passed on to a pthread condition
variable protected queue.

- A (fixed) pool of service threads waits for new requests on the
queue.
- When a new requests is received in the queue, one of the threads
will grab it and handle it, and then return to grab new requests
from the queue.

- It's possible to have more than one "main" thread doing the I/O,
if the I/O is believed to be a bottleneck in large, high-load
installations. However, this also causes some problems, with load
balancing among other things. A first step should be limited to
just having one main thread.