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8/14/2019 UNIT-2 Process Management
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PROCESS MANAGEMENT
By : Deepti Bhagwani
8/14/2019 UNIT-2 Process Management
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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.
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