UNIT-2 Process Management

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PROCESS MANAGEMENT

By : Deepti Bhagwani

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PROCESSES

Process Concept

Process Scheduling Operations on Processes

Cooperating Processes

Threads

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PROCESS CONCEPT

An operating system executes a variety of programs:Batch system – jobsTime-shared systems – user programs or tasks

Textbook uses the terms job and process almostinterchangeably.

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

A process includes:program counterstack

data section

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PROCESS STATE

As a process executes, it changes statenew: The process is being created.

running: Instructions are being executed.

waiting: The process is waiting for some event tooccur.

ready: The process is waiting to be assigned to a

processorterminated: The process has finished execution.

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DIAGRAM OF PROCESS STATE

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PROCESS CONTROL BLOCK (PCB)

Information associated with each process.Process IDProcess state

Program counterCPU registersCPU scheduling information

Memory-management informationAccounting informationI/O status information

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PROCESS CONTROL BLOCK (PCB)

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CPU SWITCH FROM PROCESS TO PROCESS

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PROCESS SCHEDULING QUEUES

Job queue – set of all processes in the system.

Ready queue – set of all processes residing inmain memory, ready and waiting to execute.

Device queues – set of processes waiting for anI/O device.

Processes migrate between the variousqueues.

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READY QUEUE AND VARIOUS I/O DEVICE QUEUES

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REPRESENTATION OF PROCESS SCHEDULING

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SCHEDULERS

Long-term scheduler (or job scheduler) –selects which processes should be brought into

the ready queue.

Short-term scheduler (or CPU scheduler) –selects which process should be executed nextand allocates CPU.

Medium Term Scheduling

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MEDIUM TERM SCHEDULING

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SCHEDULERS (CONT.)

Short-term scheduler is invoked very frequently(milliseconds) Þ (must be fast).

Long-term scheduler is invoked very infrequently(seconds, minutes) Þ(may be slow).The long-term scheduler controls the degree of multiprogramming.

Processes can be described as either:I/O- bound process – spends more time doing I/O thancomputations, many short CPU bursts.CPU-bound process – spends more time doing computations; few very long CPU bursts.

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CONTEXT SWITCH

When CPU switches to another process, thesystem must save the state of the old processand load the saved state for the new process.

Context-switch time is overhead; the systemdoes no useful work while switching.

Time dependent on hardware support.

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OPERATIONS ON PROCESS

Process Creation

Process Termination

Run a processSuspend a process

Get process information

Set process information

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PROCESS CREATION

Parent process creates children processes, which,in turn create other processes, forming a tree of processes.

Resource sharing Parent and children share all resources.Children share subset of parent’s resources.Parent and child share no resources.

ExecutionParent and children execute concurrently.Parent waits until children terminate.

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PROCESS CREATION (CONT.)

Address spaceChild duplicate of parent.

Child has a program loaded into it.

UNIX examplesfork system call creates new process

fork returns 0 to child , process id of child for parent

exec system call used after a fork to replace theprocess’ memory space with a new program.

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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 operating system.

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.Cascading termination.

In Unix, if parent exits children are assigned init as parent

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COOPERATING PROCESSES

Independent process cannot affect or beaffected by the execution of another process.Cooperating process can affect or be affected

by the execution of another processAdvantages of process cooperation

Information sharing

Computation speed-upModularityConvenience

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PRODUCER - CONSUMER PROBLEM

Paradigm for cooperating processes, producerprocess produces information that is consumedby a consumer process.

unbounded-buffer places no practical limit on thesize of the buffer.

bounded-buffer assumes that there is a fixed buffer

size.

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THREADS

A thread is a single sequential flow of controlwithin a program.

A thread is similar to a program : a single thread

also has a beginning , an end, a sequence , and atany given time during the runtime of the thread,

there is a single point of execution.

A thread itself is not a program – it cannot run on its own – but run within a program.