4delaying execution方法(选自:FreeBSD Device Drivers):

Sleeping Sleeping is done when you must wait for something to occur before you can proceed.
Event Handlers Event handlers let you register one or more functions to be executed when an event occurs.
Callouts Callouts let you perform asynchronous code execution. Callouts are used to execute your functions at a specific time.
Taskqueues Taskqueues also let you perform asynchronous code execution. Taskqueues are used for deferred work.