4种delaying 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. |