Process Description and Control

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Process Description and Control

Chapter 3

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Requirements of an Operating System

•  Interleave the execution of multiple processes to maximize processor utilization while providing reasonable response time

•  Allocate resources to processes •  Support interprocess communication and

user creation of processes

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Manage Execution of Applications

•  Resources made available to multiple applications

•  Processor is switched among multiple application

•  The processor and I/O devices can be used efficiently

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Process •  A program in execution •  An instance of a program running on a

computer •  The entity that can be assigned to and

executed on a processor •  A unit of activity characterized by the

execution of a sequence of instructions, a current state, and an associated set of system instructions

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Process Elements •  Identifier •  State •  Priority •  Program counter •  Memory pointers •  Context data •  I/O status information •  Accounting information

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Process Control Block

•  Contains the process elements •  Created and manage by the operating

system •  Allows support for multiple processes

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Process Control Block Identifier

Figure 3.1 Simplified Process Control Block

State

Priority

Program counter

Memory pointers

Context data

I/O statusinformation

Accountinginformation

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Trace of Process

•  Sequence of instruction that execute for a process

•  Dispatcher switches the processor from one process to another

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Example Execution Main MemoryAddress

Dispatcher

Process A

Process B

Process C

Program Counter0

100

5000

8000

8000

12000

Figure 3.2 Snapshot of Example Execution (Figure 3.4)at Instruction Cycle 13

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Trace of Processes

Process B is waiting for I/O

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Two-State Process Model •  Process may be in one of two states

–  Running –  Not-running

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Not-Running Process in a Queue

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Process Creation

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Process Termination

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Process Termination

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Processes

•  Not-running –  ready to execute

•  Blocked – waiting for I/O

•  Dispatcher cannot just select the process that has been in the queue the longest because it may be blocked

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A Five-State Model

•  Running •  Ready •  Blocked •  New •  Exit

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Five-State Process Model

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Let’s look at the execution again. What are the state transitions of the processes?

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Process States

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Using Two Queues

Is the Blocked Queue a FIFO queue? What is the problem with the Blocked Queue?

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Multiple Blocked Queues

Event 1 Wait

Event 2 Wait

Event n Wait

DispatchReleaseReady Queue

AdmitProcessor

Timeout

Event 1 QueueEvent 1Occurs

Event 2Occurs

Event nOccurs

Event 2 Queue

Event n Queue

(b) Multiple blocked queues

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Suspended Processes •  Processor is faster than I/O so all

processes could be waiting for I/O •  Swap these processes to disk to free up

more memory •  Blocked state becomes suspend state

when swapped to disk •  Two new states

– Blocked/Suspend – Ready/Suspend

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One Suspend State

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Two Suspend States

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Reasons for Process Suspension