更多關(guān)于軟件時(shí)鐘處理

　　更多關(guān)于軟件時(shí)鐘處理

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

　　全局時(shí)鐘恢復(fù)

　　假設(shè)一個(gè)復(fù)雜系統(tǒng)中有幾個(gè)代理。每一個(gè)都有自己的參考時(shí)鐘，給人一種時(shí)間感。一個(gè)代理的時(shí)間概念是從啟動(dòng)以來(lái)有多少個(gè)參考時(shí)鐘滴答通過(guò)的。

　　為了同步操作和共享時(shí)鐘，代理需要有一個(gè)共同的、全局的時(shí)間感。這是很容易的總線系統(tǒng)，如USB或火線，因?yàn)榘褪客鶖y帶自己的時(shí)鐘。然而，對(duì)于分布式的、基于包的互連，情況變得更加困難。IEEE標(biāo)準(zhǔn)1588、1588V2和802.1as和IETF的NTP標(biāo)準(zhǔn)設(shè)計(jì)的專門針對(duì)分布式全局時(shí)鐘恢復(fù)問(wèn)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：