December 2013
Intermediate to advanced
496 pages
8h 57m
Chinese
Content preview from LINUX系統程式設計 第二版
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第六章
單處理器（single-processor）機器上，
多任務
（
multitasking
）作業系統可以交錯執
行一個以上的行程，這會給人一個錯覺，彷彿有多個行程同時運行一般。多處理器
（multiprocessor）機器上，多任務作業系統允許多個行程實際地在不同的處理器上同時
運行。非多任務的作業系統，例如 DOS，同一時間只能執行一個應用程式。
多任務作業系統又分成兩類：
協同式
（
cooperative
）以及
先佔式
（
preemptive
）。 Linux
所實作的是先佔式多任務機制，在此多任務機制中，排班器會決定何時可以中止一個行
程的運行，好讓不同的行程得以繼續運行。我們會把「暫停一個運行中的行程而讓另一
個行程繼續運行」的行為稱為搶先佔用或簡稱
先佔
（
preemption
）。在排班器先佔一個行
程之前，該行程所能運行的時間長短，稱為處理器的
時段
（
timeslice
）；之所以稱為時
段，是因為排班器會把處理器的每個時間片段（slice）分配給每個可運行的行程。
反過來說，在協同多任務機制中，一個行程不會中止運行，除非該行程自願這麼做。我
們把「一個行程自願暫停自己」的行為，稱為
讓出
（
yielding
）CPU。理想上，行程會經
常讓出 CPU，但是作業系統並不會強迫行程這麼做。一個粗略或不完整的程式可能會執
行比較久的時間，使得多任務機制失去效用，或甚至是拖累整個系統。由於此做法有缺
點，所以現代的作業系統幾乎普遍地採用先佔式多任務的做法；Linux 也不例外。
這些年來 Linux 的行程排班器已有所改變。當前的行程 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.