Process ID and special processes

最新推荐文章于 2021-11-30 14:46:12 发布

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最新推荐文章于 2021-11-30 14:46:12 发布

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分类专栏： Memo 文章标签： system unix initialization 存储系统 delay user

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本文介绍了进程ID的特点，包括其唯一性和复用机制，并详细解释了操作系统中几个关键的特殊进程，如调度进程(swapper)、初始化进程(init)和页守护进程(pagedaemon)的功能与重要性。

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Process ID and special processes

(From 8.2. Process Identifiers)

Process ID

1. Keywords: unique, reuse

2. "unique”: "Because the process ID is the only well-known identifier of a process that is always unique, it is often used as a piece of other identifiers, to guarantee uniqueness."

3. “reuse”: Instant reuse? no. "Most UNIX systems implement algorithms to delay reuse so that newly created processes are assigned IDs different from those used by processes that terminated recently. This prevents a new process from being mistaken for the previous process to have used the same ID."

Special processes

1. NOTE: the details differ from implementation to implementation.

2. ALL:

0 swapper (scheduler process) system process

1 init process user process

2 pagedaemon kernel process (NOTE: on some virtual memory implementations of the UNIX System)

About swapper:

"No program on disk corresponds to this process, which is part of the kernel and is known as a system process."

About init process:

" Invoked by the kernel at the end of the bootstrap procedure. The program file for this process was /etc/init in older versions of the UNIX System and is /sbin/init in newer versions. This process is responsible for bringing up a UNIX system after the kernel has been bootstrapped. init usually reads the system-dependent initialization files (the /etc/rc* files or /etc/inittab and the files in /etc/init.d)and brings the system to a certain state, such as multiuser. The init process never dies. It is a normal user process, not a system process within the kernel, like the swapper, although it does run with superuser privileges. Later in this chapter, we'll see how init becomes the parent process of any orphaned child process."

About pagedaemon:

"This process is responsible for supporting the paging of the virtual memory system."

支持虚存存储系统的请页操作

3. What is bootstrap?

bootstrap是皮鞋后部的一条小带子或一个小环，它可以使你方便地把鞋子穿起来。在计算机中，是指使用一个很小的程序将某个特定的程序（通常是指操作系统）载入计算机中。在生活中指的是，引导在付出一点努力后就能取得更大且更有意义的成果这个过程中起着杠杆作用。还有一句很通俗的表述，“用你的引导性思维使自己前进”，意思是说在一个低的人生起点阶段要想尽办法使自己取得成功。

4. Kernel processes provide operating system services, for example, pagedaemon.

Figure 1.1

Figure 1.2