TCP I O multiplexing

Given command GET foo bar, suppose we need to retrieve key "foo" from memcached server A and key "bar" from memcached server B. The simplest way is to retrieve them one by one:

send "GET foo\r\n" to mc A
recv from mc A
send "GET bar\r\n" to mc B
recv from mc B

This's the blocking I/O model.

When recv from mc A is blocked by mc A but it's ready to send to mc B, we have to wait here and doing nothing. The solution is to use I/O multiplex:

events = [
    when mc A is ready to send,
    when mc B is ready to send
]
not_done = 2

while not_done:
    captured_events = poll(events, timeout)
    for ev in captured_events:
        if ev.mc is ready to send:
            all_sent = ev.mc.send(...)
            if all_sent:
                events.remove(when ev.mc is ready to send)
                events.append(when ev.mc is ready to recv)
        if ev.mc is ready to recv:
            all_recved = ev.mc.recv(...)
            if all_recved:
                events.remove(when ev.mc is ready to recv)
                not_done -= 1
    

This's the I/O multiplexing model.

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The I/O multiplexing model is used in libmc. You can find the related code here

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