OS Review Chapter1: Introduction
OS Review Chapter 2: Computer-System Structures
OS Review Chapter 3: Operating-System Structures
OS Review Chapter 4: Process
OS Review Chapter 5: Thread

Chapter 4: Processes

Introduce process-> describe CPU activity-> study CPU activity-> improve CPU utilization

What is a Process-> One execution process of the program

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

A process includes:

◆ program counterWhere did the program go?

◆ contents of the processor’s registersState

◆ stackLocal variables

◆ data sectionGlobal Variables

Process in Memory

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text: Program (binary)

heap: Dynamically allocated memory

Process State (process life cycle)

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Process Control Block (PCB)

PCB storage place:内存内核段

Information associated with each process：

Process state

Program counter where to run the program (context)

CPU registers （context）

CPU scheduling information

Memory-management information related addresses accessed by the program

Accounting information audit CPU time

I/O status information

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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. All processes that can be executed on CPu are processes scheduled by the scheduler

Device queues – set of processes waiting for an I / O device. Processes in the waiting state

Process migration between the various queues.

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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
The address space of the child process and the parent process are different

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#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

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)