更多關于軟件時鐘處理

　　在前一篇文章中，我研究了軟件中處理時鐘的基本知識。在本文中，我們將繼續(xù)討論全球時鐘恢復、分布式時鐘同步和事件同步。

　　全局時鐘恢復

　　假設一個復雜系統(tǒng)中有幾個代理。每一個都有自己的參考時鐘，給人一種時間感。一個代理的時間概念是從啟動以來有多少個參考時鐘滴答通過的。

　　為了同步操作和共享時鐘，代理需要有一個共同的、全局的時間感。這是很容易的總線系統(tǒng)，如USB或火線，因為巴士往往攜帶自己的時鐘。然而，對于分布式的、基于包的互連，情況變得更加困難。IEEE標準1588、1588V2和802.1as和IETF的NTP標準設計的專門針對分布式全局時鐘恢復問題。

　　The first task of the system is to elect one of the agents as the master. It is this agent‘s clock that will become the common global clock. IEEE 1588 and variants use a peer-to-peer leadership election protocol that tries to elect the highest quality clock amongst the agents. It is guaranteed to elect one， and only one， master.

　　Once the master is elected， each agent needs to know the following information：

　　Is my clock going slower/faster than the global master clock？ If so， by how much？

　　What is the current time now according to the master clock？

　　The fundamental method used is to regularly transmit synchronization messages from the master to the other agents. The message contains the global time that the message was sent. The following diagram shows this happening：