Process¶

约 1028 个字 26 行代码 11 张图片预计阅读时间 5 分钟

Concept¶

A process is one program being executing. (job 是历史遗留的称呼)

内存布局

Text 运行的代码
Data 全局变量、静态变量
heap 运行时动态申请
stack 调用函数的临时变量

一个 C 程序中各部分的内存布局

使用 size <executable_file_name> 命令查看，

text     data     bss     dec      hex       filename

1382     600       8    1990     7c6        test

State¶

Scheduler dispatch : OS 进行 CPU 调度

Process Control Block | PCB¶

上面提到单个进程在 memory 中的内存布局以及 state 的转换，但我们的操作系统同时会运行多个进程，这些进程不断地切换，所以我们需要 PCB 来保存一些管理资源的必要信息。

PCB 相关的维护代码在 kernel 部分，在中断时调用更新 PCB 的值

PCB 的一般结构

两个进程的示意图

通过 PCB 的切换来切换进程

Process Scheduling¶

In general, most processes can be described as either I/O bound or CPU bound. 按照花费时间的多寡分类

An I/O-bound process is one that spends more of its time doing I/O than it spends doing computations.

A CPU-bound process, in contrast, generates I/O requests infrequently, using more of its time doing computations.

Scheduling Queues¶

Job queue – set of all processes in the system
Ready queue – set of all processes residing in main memory, ready and waiting to execute
Device queues – set of processes waiting for an I/O device
I/O waiting queue
......

运行示意图

Process Scheduler¶

Long-term scheduler (or job scheduler) – selects which processes should be brought into memory (the ready queue) 选择哪一个程序进行运行
- 控制 degree of multiprogramming
Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU
Medium term 进行 swapping，可以将一个 process 从内存移动到 disk，降低 multiprogram 的程度，之后再存回来

Process Operation¶

Linux 可以使用 ps/pstree 命令查看当前进程

不同 process 之间使用 pid 区分
Systemd pid = 1

Creation¶

Fork 创建一个新的进程，对于新创建的子进程，其返回值为 0 returns the child’s pid to the parent, and 0 to the child.

From Linux fork Manual

The child process is an exact duplicate of the parent process except for the following points:

does not inherit its parent's memory locks

Return value

On success, the PID of the child process is returned in the parent, and 0 is returned in the child. On failure, -1 is returned in the parent, no child process is created, and err no is set appropriately.

example

How many processes does this C program create?

int main (int argc, char *arg[])

{

fork ();

if (fork ()) {

fork ();

}

fork (); 

}

The answer should be 12.

#todo:给出树形图

Exec 载入新的二进制程序，顶替之前的镜像。用在fork函数调用之后
wait 父进程阻塞，等待子进程

Resource sharing 的分类

子进程和父进程共享
子进程的资源严格限制在父进程的子集中
No resource sharing

父进程在创建子进程之后

两种可能性（都进入 ready queue，根据调度算法决定）

阻塞，等待 child process
并行执行

子进程

要么是父进程的 duplicate
要么 load 另一个程序

#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>
int value = 10;
int main()
{
    pid_t pid;
    pid = fork();
    value += 10;
    if (pid == 0)
    { /* child process */
        value += 5;
        printf("CHILD: value = %d", value); /* LINE C 25 */ 
    }
    else if (pid > 0)
    { /* parent process */
        wait(NULL);
        printf("PARENT: value = %d", value); /* LINE A 20 */
        exit(0);
    }
}

Termination¶

Exit : A process terminates when it finishes executing its final statement and asks the operating system to delete it by using the exit() system call. 运行结束 - 为 waiting 中的父进程返回 status value - Deallocated 所有分配的资源

Abort : Parent may terminate execution of children processes

Child has exceeded allocated resources
Task assigned to child is no longer required
Parent 存在
- cascading termination ：部分操作系统不允许没有父进程的子进程存在，在父进程中止时会级联中断子进程
- In some other operating systems, the child gets orphaned – and its parent becomes the “init” process (PID=1). 父进程不调用 wait 的情况下，子进程成为“孤儿”，这时部分操作系统会把父进程设置为 systemd，其周期性调用 wait 函数

Note

所有中断的进程都会变成 zombie process，只有在父进程调用 wait 之后，进程相关的 process table 的内容才会被释放

Interprocess Communication | IPC¶

Shared Memory 速度会更快，减少了对kernel代码的调用 (Message Passing requires the more time-consuming task of kernel intervention.)
Message Passing 实现起来更简单 useful for exchanging smaller amounts of data for no conflicts to be considered

Shared Memory¶

Note

Producer-Consumer Problem

unbounded-buffer places no practical limit on the size of the buffer. Consumer has to wait if no new item. buffer不会填满
bounded-buffer assumes that there is a fixed buffer size. Producer must wait if buffer full.

共享内存 —— sharing memory

Message Passing¶

需要 copy 多次内存/Buffer （在内存和内核之中）

Synchronization¶

TODO¶

看之后吧