Chapter 2 Processes and threads

15Biblioteka Transitions between these states
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进程基本状态的转换
时间片 用完
运行
进程 调度 I/O完成或等待 的事件完成
进程状态之 请求I/O或 等待某事件 间的转换：
就绪
阻塞

就绪→运行 运行→就绪 运行→阻塞 阻塞→就绪
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2.1.6 Implementation of processes
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2.1 Processes 2.1.1 The process model 2.1.2 process creation 2.1.3 process termination 2.1.4 Process hierarchies 2.1.5 Process state 2.1.6 Implementation of processes 2.1.7 Modeling multiprogramming
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example
How does UNIX initialize itself when it is started? A special process, called init, is present in the boot image. When it starts running, it reads a file telling how many terminals there are. Then it forks off one new process per terminal. These processes wait for someone to log in. If a login is successful, the login process executes a shell to accept commands. These commands may start up more processes, and so forth. Thus, all the processes in the whole system belong to a single tree, with init at the root.
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process creation In windows, in contrast, a single Win32 function call, CreatProcess, handles both process creation and loading the correct program into the new process.
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进程的定义
进程的参考定义有如下几种： 1、进程是程序的一次执行过程； 2、进程是一个可拥有资源的独立实体，同 时又是一个可以独立调度的基本单位。 3、进程是程序在一个数据集合上运行的过 程，它是系统进行资源分配和调度的一个 独立单位。
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进程与程序的区别
进程是一个动态的概念，而程序是一个静态的概 念 进程具有并发性 独立性：进程是能独立运行、独立分配资源和接 受调度的基本单位 异步性：进程各自独立的、按不可预知的速度向 前推进 一个进程可以包含多个程序；一个程序也可以对 应多个进程 进程＝程序＋数据＋PCB ，即进程是一个程序及 其数据在处理机上顺序地执行时所发生的活动 进程具有创建其他进程的功能
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Some of the fields of a typical PCB
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进程控制块(PCB) 进程＝程序＋数据＋PCB 当创建进程时，为之建立PCB ；当撤消进 程时， PCB也随之撤消。 因此， PCB是进程存在的唯一标志，是系 统感知进程的唯一实体。 PCB 中记录了OS所需的、用于描述进程的 当前情况以及控制进程运行的全部信息。 PCB应常驻内存。
To implement the process model, the OS maintains a table, called the process table, with one entry per process. some authors call these entries process control blocks (PCB, 进程控制块). PCB contains important information about the process’ state, including its program counter, stack pointer, memory allocation, the status of its open files, its accounting and scheduling information, and everything else about the process that must be saved when the process is switched from running to ready or blocked state.
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2.1.3 process termination Due to one of the following conditions, the process will terminate.
Normal exit (voluntary) Error exit (voluntary) Fatal error (involuntary) Killed by another process (involuntary)
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2.1.2 process creation In general-purpose systems, some way is needed to create and terminate processes as needed during operation. There are 4 principal events that cause processes to be created:
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2.1.5 Process state The three states of a process:
Running(运行状态): actually using the CPU at that instant Ready(就绪状态): runnable; temporarily stopped to let another process run Blocked(阻塞状态): unable to run until some external event happens
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2.1.4 Process hierarchies In some systems, when a process creates another process, the parent process and child process continue to be associated in certain ways. The child process can itself create more processes, forming a process hierarchy. In UNIX, a process and all of its children and further descendants together form a process group.
Principles of Operating System
Chapter 2 Processes and threads
Contents 2.1 Processes 2.2 Interprocess communication (IPC) 2.3 Classical IPC problems 2.4 Threads 2.5 Scheduling
System initialization Execution of a process creation system call by a running process A user request to create a new process Initiation of a batch job
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2.1.1 The process model All the runnable software on the computer, sometimes including the OS, is organized into a number of sequential processes, or just processes for short. A process is just an instance of an executing program, including the current values of the program counter, registers, and variables.
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Example
(a) Multiprogramming of four programs
(b)Conceptual model of four independent, sequential processes
(c) Only one program is active at once
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A process is an activity of some kind. It has a program, input, output, and a state. A single processor may be shared among several processes, with some scheduling algorithm being used to determine when to stop work on one process and service a different one.
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2.1.7 Modeling multiprogramming
When multiprogramming is used, the CPU utilization can be improved. A better model is to look at CPU usage from a probabilistic viewpoint. Suppose that a process spends a fraction p of its time waiting for I/O to complete. When n processes in memory at once, the probability that all n processes are waiting for I/O is pn. The CPU utilization is 1- pn.