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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進階的行程管理
|
205
這些年來，Linux 核心已獲得越來越好的即時支援，並且提供越來越低的延遲以及更一
致的抖動，而且不會影響系統的效能。這多半是因為改善延遲能夠幫助許多類型的應用
程式，例如桌面和 I/O 密集應用程式，並非只有即時應用程式。會有這樣的改善也可以
歸因於 Linux 在嵌入式和即時系統上的成功。
不幸的是，許多對 Linux 核心所進行的嵌入式和即時修改，僅以特製的 Linux 解決方案
存在著，並未納入主流的正式核心。這些修改中有些可以進一步減少延遲，甚至是提供
硬性即時的行為。接下來只會探討正式的核心介面，以及主流核心的行為。幸運的是，
大部分的即時修改會繼續利用 POSIX 介面。因此，隨後的討論還是跟修改過的系統
有關。
Linux 的排班方針與優先權值
Linux 排班器對於一個行程所採取的動作，取決於該行程的
排班方針
（
scheduling
policy
），也稱為
排班類型
（
scheduling class
）。除了預定的一般方針，Linux 還提供了
兩個即時排班方針。我們可以使用來自標頭檔
<sched.h>
的預處理器巨集（preprocessor
macro）來表示每個方針：這些巨集是
SCHED_FIFO
、
SCHED_RR
以及
SCHED_OTHER
。
每個行程都會具有一個與友善值無關的
靜態優先權值
（
static priority
）。對一般應用程
式而言，此優先權值往往是
0
。對即時行程而言，此優先權值的範圍可以從 1 到 99（包
含 1 和 99）。 Linux
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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.