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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140
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第四章
此外，目錄
/sys/block/[device]/queue/iosched
中所包含的檔案，讓管理者得以取得以及設
定與 I/O 排班器有關的可調整值。至於要操作哪些選項，取決於當前所使用的 I/O 排班
器。欲變更任何設定值，必須具備 root 的特權。
一個好的程式設計者，並不需要瞭解底層的 I/O 子系統。儘管如此，對這個子系統有所
認識，將有利於程式的優化。
優化 I/O 效能
因為相對於系統中其他元件而言，磁碟 I/O 的速度是如此緩慢，然而 I/O 卻是現代電腦
技術一個重要的面向，所以 I/O 效能的最大化至關重要。
儘量減少 I/O 操作（將許多規模較小的操作，凝聚成少數規模較大的操作）、進行對齊
區塊大小的 I/O 或是採取用戶緩衝機制（見第 3 章）是系統程式設計者之工具箱中重要
的工具。同樣的，進階 I/O 技術，例如向量式 I/O、特定位置的 I/O（見第 2 章）、非同
步 I/O，也是進行系統程式設計時，必須加以考量的重要模式。
然而，I/O 密集的關鍵應用程式，則可以利用額外的技巧來最大化效能。儘管 Linux 核
心，正如先前所提到的，會利用進階的 I/O 排班器來儘量減少可怕的磁碟查找操作，不
過用戶空間應用程式也可以朝向相同的目標，使用類似的方式，以進一步改善效能。
用戶空間中的 I/O 排班
I/O 密集應用程式會發出大量的 I/O 要求，而且需要花時間排序及合併其未決之 I/O 要
求，進行跟 Linux I/O 排班器一樣的工作。
8
如果你知道 I/O ...
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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.