context switches
在英文原版书上经常看到这个名词，特别找到解释来解释一下。
An operating system running a program runs in either user mode or operating system mode. Switching between user and operating system mode is a context switch. For example, a program that makes a system call while in user mode makes a context switch. Context switches can hinder performance because, in this example, the context of the user program must be stored while also transferring the context of the operating system kernel into memory. Performance can be even more adversely affected when multiple system calls compete for operating system resources.
