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OPERATING SYSTEMS(CS C372 & IS C362)

LECTURE 6: PROCESS

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Process concept In multi programming / multi tasking environment

number of processes can reside in the system

At any instance of time processes can be in differentstate such as Ready, Blocked, Running etc. and

processes move from one state to another state

depending upon certain conditions.

Processes use available system resources

OS is responsible for managing all the processes in

the system

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Process Concept Program has just code NOT Live entity

Process with data, work-space Stack

Heap

Open files File descriptors

Other primitives

ipc, memory,

Execution snapshots Register content

Program counter General purpose registers

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Process in Memory

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Linux process run-time memory

Fig courtesy: Computer Systems: a programmers perspective9 August 2011 6Biju K Raveendran@BITS Pilani.

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Example ExecutionMain MemoryAddressDispatcher

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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A simple scenario: 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

What processes are in queue?

Process timed outProcess gone for I/OBoth are 2 different scenarios

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Process Not-running

Ready to run given chance

Blocked/waiting for event

Why not three states then?

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Three state model

Ready Running

Blocked

Admit

Time out

Dispatch

Release

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Three state model: Queuing Diagram

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Multiple Blocked Queues

Event 1 Wait

Event 2 Wait

Event nWait

Dispatch

ReleaseReady QueueAdmit

Processor

Timeout

Event 1 Queue

Event 1

Occurs

Event 2

Occurs

Event n

Occurs

Event 2 Queue

Event nQueue

(b) Multiple blocked queues

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Why do we need New state New job submission OS creates data structure for keeping track of the

process context

OS tries to load the process.

Load a new process only if

System have enough memory to hold the process

System have enough resources to executeprocesses in efficient manner

To efficiently execute processes, system mayput a maximum limit on processes in readyqueue

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Five State Model As a process executes, it changes state

New

The process is being created Running

Instructions are being executed

Waiting The process is waiting for some event to occur

Ready

The process is waiting to be assigned to a processor Terminated

The process has finished execution

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Diagram of Process State

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Process Scheduling Queues

Job queue set of all processes in the

system

Ready queue set of all processes residingin main memory, ready and waiting to execute

Device queues set of processes waiting for

an I/O device

Processes migrate among the various queues

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Ready Queue And Various I/O Device Queues

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Representation of Process Scheduling

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