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Chapter 4: Processes

引入进程–>描述CPU的活动–>研究CPU的活动–>提高CPU的利用率

What is a Process–>程序的一次执行过程

Process – a program in execution; process execution must progress in sequential fashion.

A process includes:

◆program counter 程序执行到哪一步了

◆contents of the processor’s registers状态

◆stack 局部变量

◆data section全局变量

Process in Memory 进程空间

text：程序（二进制）

heap：动态分配的内存

Process State （进程的生命周期）

Process Control Block (PCB)

PCB放在内存的内核段中

PCB存储的地方：内存 内核段

Information associated with each process：

Process state

Program counter 程序运行到哪 （context）

CPU registers （context）

CPU scheduling information

Memory-management information 程序访问的相关地址

Accounting information 审计CPU时间

I/O status information

Process 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.所有能在CPu上执行的进程，被调度器调度的进程

Device queues – set of processes waiting for an I/O device.处于waiting状态的 进程

Process migration between the various queues.

Schedulers

Long-term scheduler (or job scheduler) – selects which processes should be loaded into memory for execution. 决定进入队列的进程 (may be slow)The long-term scheduler controls the degree of multiprogramming.

Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU. 决定CPU执行的进程 (must be fast).

Processes can be described as either:

• I/O-bound process
• CPU-bound process

Context Switch

What is a process context?

The context of a process includes the values of CPU registers, the process state, the program counter, and other memory/file management information.

After the CPU scheduler selects a process (from the ready queue) and before allocates CPU to it, the CPU scheduler must :

• save the context of the currently running process
• put it into a queue
• load the context of the selected process
• let it run

Operations on Processes

Process Creation

Parent process create children processes, which, in turn create other processes, forming a tree of processes.

• Resource sharing
• Execution
• Address space 子进程和父进程的地址空间不一样

#include 
   
    #include 
    
     #include 
     
      int main(int argc,char *argv[]){
   	int i,id1,id2;	for(i=1;i<2;i++)	{
   		id1=fork();		id2=fork();		if(id1==0||id2==0)			fork();	}	printf(" I am %d\n",getpid());}
     
    
   

I am 3696 I am 3697 I am 3698 I am 3699 I am 3701 I am 3702 I am 3700

Process Termination

Process executes last statement and asks the operating system to delete it (exit).

• Output data from child to parent (via wait).
• Process’ resources are deallocated by OS.

Parent may terminate execution of children processes (abort).

• Child has exceeded allocated resources.
• Task assigned to child is no longer required.
• Parent is exiting. (Operating system does not allow child to continue if its parent terminates)

Cooperating Processes

Advantages of process cooperation :

◆Information sharing

◆Computation speed-up

◆Modularity

◆Convenience

Producer-Consumer Problem

Context

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