Process Description and Control

Process concepts Definitions

– replaces task, job– program in execution– entity that can be assigned to and executed on a processor

Process states– running– ready– blocked

The main responsibility of an OS is to control the execution of processes

OS objectives and processes Operating systems must interleave the execution of a number of processes

– to maximize processor use– while providing reasonable response rate

Operating systems must allocate resources to processes

– based on priorities– while avoiding deadlock

Operating systems may support– interprocess communications– user creation of processes

A two-state process model

Enter

Dispatch

Exit

Pause

Notrunning

Running

Process creation– new batch job– interactive login– OS created to provide a

service– spawned by existing

process

Process termination– normal completion– abnormal end

(abend) time related protection error program error data error I/O failure memory error

Process state transitions with suspend and resume

Running

Blocked

Ready

Suspended blocked

Suspended ready

I/O or event completion

Suspend

Resume

Suspend

Resume

Suspend

I/O or event completion

Dispatch

Timer-runout

I/O or event wait

ACT IVE

SWAPPED

Other related concepts Suspend and swapping

– not immediately available– may or may not be waiting for event or I/O

– blocked may be independent of suspend

– process was placed on suspend by an agent

OS and processes– the OS schedules and dispatches processes for execution by the processor, and allocates resources to processes

– OS is the entity that manages the use of system resources by processes

OS control structures What does the OS needs (a) to be able to control processes and (b) manage resources for them?

– memory tables– I/O tables– file tables– process tables

These tables are maintained by the OS

How does it know what and how to create?

– Initialization and configuration

Process control structures What is the physical manifestation of a process?

– a program or set of programs – a set of data locations for variables and constants

– memory to hold programs and data– stack for procedure calls, parameter passing

PCB - Process Control Block– entity that defines a process to the OS

– collection of process attributes: process identification, processor state and process control information.

OS core, kernel or supervisor Execution modes: system or user modes

OS core or kernel– Process management

process creation and terminationprocess scheduling and dispatching

process switchingprocess synchronization and support for inter-process communication

management of PCBs– Memory management– I/O management– Support functions

Is the Operating System a process?

Process switching

U1 . . . .

. . .

.U2 U3 Un O1 O2 O3 On

P1

P2

P3

. . .

.Kernel

Pn

Process switching

OS OS OS OS. . . .

P1

P2

P3

Pn

1. No,separate

2.No, subroutines

3. Yes

Processes and Threads Process is a unit of:

– resource ownership– dispatching

Threads– only an unit of dispatching– inside a process– saves time to create, switch and terminate

A good example: a file server process

– each request as a thread– the process spawn the threads