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.

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今天跟一个网友讨论了这个话题，加上以前的笔记大概整理出了一个脉络，其间参考了不少资料，看了半天的外文，这里跟大家分享，省得大家看E文头晕了 呵呵

     嗯，嗯 今天没有白忙啊……  

 什么叫soft parse？ hard parse？

You Asked (Jump to Tom’s latest followup)

Hi Tom

Can you explain briefly the difference between soft and hard parse?

cheers

and we said…

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