Lecture 3 Processes and Threads Outline Process Concept

Lecture 3 Processes and Threads Outline: • Process Concept • Process Scheduling • Operations on Processes • Cooperating Processes • Threads 1

Process Concept (1) • An operating system executes a variety of programs: - batch systems - jobs - time-shared systems - user programs or tasks - here job and program used interchangeably • Process - a program in execution - The execution of a Process must progress in a sequential fashion 2

Process Concept (2) • When a program is loaded into the memory and it becomes a process, it can be divided into four sections ─ stack, heap, text and data. 3

Process Concept (3) • The following image shows a simplified layout of a process inside main memory: 4

Process Concept (4) 5

Processes The Process Model • Multiprogramming of four programs • Conceptual model of 4 independent, sequential processes • Only one program active at any instant 6

Process =? Program 7

Process =? Program • More to a process than just a program: - Program is just part of the process state - I run emacs on lectures. txt, you run it on homework. java – Same program, different processes • Less to a process than a program: - A program can invoke more than one process - cc starts up cpp, cc 1, cc 2, as, and ld 8

Process Creation Principal events that cause process creation: 1. System initialization. 2. Execution of a process creation system call by a running process. E. g. , data fetch over network 3. User request to create a new process. E. g, Type command 4. Initiation of a batch job. 9

Process Creation (2) • Resource sharing - Parent and children share all resources. - Children share subset of parent’s resources - prevents many processes from overloading the system. - Parent and children share no resources. • Execution - Parent and child execute concurrently. - Parent waits until child has terminated. • Address Space - Child process is duplicate of parent process. - Child process has a program loaded into it. 10

UNIX Process Creation • Fork system call creates new processes • execve system call is used after a fork to replace the processes memory space with a new program. Note: E. g. , when user type a command, sort, to the shell, the shell forks off a child process and the child executes sort. • On windows, Create. Process 11

Process Creation • Processes are created and deleted dynamically • Process which creates another process is called a parent process; the created process is called a child process. • Result is a tree of processes - e. g. UNIX - processes have dependencies and form a hierarchy. • Resources required when creating process - CPU time, files, memory, I/O devices etc. 12

What does it take to create a process? • Must construct new PCB - Inexpensive • Must set up new page tables for address space - More expensive • Copy data from parent process? (Unix fork() ) - Semantics of Unix fork() are that the child process gets a complete copy of the parent memory and I/O state - Originally very expensive - Much less expensive with “copy on write” • Copy I/O state (file handles, etc. ) - Medium expense 13

Process Termination Conditions which terminate processes 1. 2. 3. 4. Normal exit (voluntary) Fatal error (involuntary) Error exit (voluntary) Killed by another process (involuntary) Note: In UNIX this system call is kill, The corresponding WIN 32 function is Terminate. Process 14

Process Termination (2) • Most processes terminate because they have done their job. Example: • When a compiler has compiled the program given to it, the compiler executes a system call to tell the OS that it is finished. - this call is exit in UNIX and Exit. Process in Windows. 15

Process Hierarchies • Parent creates a child process, child processes can create its own process • Forms a hierarchy – UNIX calls this a "process group“ • Windows has no concept of process hierarchy – all processes are created equal 16

UNIX Process Hierarchy 17

Process Life Cycle • A process changes state as it executes. 18

Process State & Description • New: - The process is being created • Running: - Instructions are being executed • Waiting: - Waiting for some event to occur • Ready: - Waiting to be assigned to a processor • Terminated: - Process has finished execution 19

Process Control Block (1) Example 2: PCB 20

Example: PCB (Information and Description) • • • Process State: The current state of the process i. e. , whether it is ready, running, waiting, or whatever. Process privileges: This is required to allow/disallow access to system resources. Process ID: Unique identification for each of the process in the operating system. Pointer: A pointer to parent process. Program Counter: Counter is a pointer to the address of the next instruction to be executed for this process. CPU registers: Various CPU registers where process need to be stored for execution for running state. CPU Scheduling Information: Process priority and other scheduling information which is required to schedule the process. Memory management information: This includes the information of page table, memory limits, Segment table depending on memory used by the operating system. Accounting information: This includes the amount of CPU used for process execution, time limits, execution ID etc. I/O status information: This includes a list of I/O devices allocated to the process. 21

Process Scheduling • Process (PCB) moves from queue to queue • When does it move? Where? A scheduling decision 22

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. • Device Queues - set of processes waiting for an I/O device. • Process migration between the various queues. • Queue Structures - typically linked list, circular list etc. 23

Process Scheduling Queues 24

Context Switch (1) • A context switch is the mechanism to store and restore the state or context of a CPU in Process Control block so that a process execution can be resumed from the same point at a later time. • Using this technique, a context switcher enables multiple processes to share a single CPU. Context switching is an essential part of a multitasking operating system features. 25

Enabling Concurrency: Context Switch • Task that switches CPU from one process to another process - the CPU must save the PCB state of the old process and load the saved PCB state of the new process. • Context-switch time is overhead - System does no useful work while switching - Overhead sets minimum practical switching time; can become a bottleneck • Time for context switch is dependent on hardware support (1 - 1000 microseconds). 26

Context Switch (2) 27

CPU Switch From Process to Process • Code executed in kernel above is overhead - Overhead sets minimum practical switching time 28