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Operating Systems
Process Management
What is a Process?
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The basic unit of software that the operating system
deals with in scheduling the work done by the
processor.
Not the same as an application, since an application
can cause multiple processes to be executed.
Can be defined as software that performs some
action and can be controlled - by a user, by other
applications or by the operating system.
What Is A Process
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A process consists of the machine code image of the program
and a Process Control Block (PCB).
The PCB typically contains:
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An ID number that identifies the process
Pointers to the locations in the program and its data where
processing last occurred
Register contents
States of various flags and switches
Pointers to the upper and lower bounds of the memory required for
the process
A list of files opened by the process
The priority of the process
The status of all I/O devices needed by the process
Listing Processes in UNIX
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The ps command displays process
information in the following format:
PID
291
353
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TTY
01
01
TIME
00:02
00:00
COMMAND
sh
ps
PID: process id
TTY: id of terminal owning process
TIME: cumulative amount of CPU time used
COMMAND: command which generated the
process.
Listing Processes in Windows
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Right-click on task bar
Select Task Manager
Applications tab shows
running application
Processes tab shows
running processes and the
system resources each is
using.
Status Bar shows the
number of processes
running and the CPU
utilization.
Process Life-Cycle
I/O Completion
Process
Entry
Ready
Blocked
Running
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Termination
Process Life-Cycle
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READY
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BLOCKED
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process is waiting for an I/O operation to complete and
cannot utilise the CPU
many processes may be in a BLOCKED state at any instant
RUNNING
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process is able to use the CPU when it is assigned to it
many processes may be in a READY state at any instant
process is using the CPU
for a single-CPU computer only one process can be in a
RUNNING state at any instant
Process Scheduling
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The CPU can only do one thing at a time.
In order to give the appearance of lots of
things happening at the same time, the
operating system has to switch between
different processes thousands of times a
second.
This function is performed by the process
scheduler.
Objectives of Scheduling
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Equity
Maximize throughput
Be predictable
Balance resource use
Avoid indefinite postponement
Enforce priorities
Give preference to processes holding key resources
Degrade gracefully under heavy loads
Criteria for Scheduling
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Priority assigned to a process.
Class of process – batch, online or real-time.
Resource requirements.
CPU-bound or I/O bound.
Resources used to date.
Waiting time to date.
Scheduling Policies
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Preemptive
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Non-preemptive
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process runs until it terminates or incurs an I/O
wait.
Cooperative
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scheduler may suspend a process in order to
allow another process to run.
process periodically relinquishes control of the
CPU.
Scheduling Algorithms
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First-Come-First-Served (FCFS)
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Shortest Job First (SJF)
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the first process in the READY state gets the CPU
non-preemptive
long jobs favored over short ones
I/O bound processes will be idle for long periods.
the process with the shortest estimated run-time gets the
CPU
non-preemptive
long-running processes could be starved for the CPU
Scheduling Algorithms
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Shortest Remaining Time (SRT)
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Highest Response Ratio Next
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preemptive version of SJF
a process with a shorter estimated run-time than the current
process’ remaining run-time will get the CPU.
long-running processes could be starved for the CPU
non-preemptive
a process is assigned a priority based on the time it has
spent waiting and its estimated run-time
higher priority processes get the CPU first
Scheduling Algorithms
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Round Robin (RR)
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Multi-level Feedback Queues (MFQ)
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preemptive
processes get the CPU in FCFS sequence
once the process has run for a defined period (time quantum) it is
interrupted and the next process gets the CPU
no process can “hog” the CPU
maintains a number of separate queues, each of which represents
a different priority
processes in a lower priority queue do not get the CPU unless the
higher priority queues are empty
each queue uses RR, if a process uses up its time quantum is
moves to the end of the queue on the next lower level.
Scheduling in UNIX
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Uses the MFQ algorithm.
Processes are assigned an initial priority.
The priority is represented by an integer, the higher
the integer the lower the priority.
Priority is increased the longer the process waits.
Priority is reduced the longer the process runs.
A process’ initial priority can be adjusted using the
nice command.
Only a superuser can increase a process’ initial
priority.
Scheduling in Windows
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Windows 95 and 98 used a cooperative policy.
Windows NT (and later) use a preemptive policy with
a round-robin algorithm similar to UNIX.
Process priority is represented by six (6) states –
realtime, high, abovenormal, normal, belownormal,
low.
A process’ initial priority can be changed with the
START command.
Only a superuser can increase a process’ priority.
Process Thrashing
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Process switching happens without direct user
interference, and each process gets enough CPU
cycles to accomplish its task in a reasonable amount
of time.
If enough processes are started, and if the operating
system hasn't been carefully designed, the system
can begin to use the vast majority of its available
CPU cycles to swap between processes rather than
run processes.
This situation is known as thrashing.
Interrupt System
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In addition to executing processes, the CPU must
also respond to interrupts.
Interrupts are special signals sent by hardware or
software to the CPU.
Sometimes the operating system will schedule the
priority of processes so that interrupts are masked that is, the operating system will ignore the interrupts
from some sources so that a particular job can be
finished as quickly as possible.
Interrupt System
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There are some interrupts that are so important that
they can't be ignored. These non-maskable
interrupts (NMIs) must be dealt with immediately.
The importance of the interrupt system is that it
permits several programs and I/O activities to
proceed independently.
Types of Interrupts
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I/O – Generated by the controller of an I/O device, to
signal normal completion or the occurrence of an
error or failure condition.
Timer – Generated by the internal clock, used to
alert the operating system at predetermined
intervals.
Hardware Error – Generated by hardware faults.
Program – Generated by error conditions within user
programs.
Multi-CPU Systems
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In a system with two or more CPUs, the operating
system must divide the workload among the CPUs.
Asymmetric operating systems use one CPU for
their own needs and divide application processes
among the remaining CPUs.
Symmetric operating systems divide processes
among the various CPUs, balancing demand versus
CPU availability even when the operating system
itself is all that's running.