Operating System Scheduling

Operating System
(Processes)
Lecture -2
By:
Aman Gupta
Assistant Professor
Operating systems - 2 1

Outline
 Process Concept
 Process Scheduling
 Operations on Processes
2Operating systems - 2

Process Concept
 An operating system executes a variety of
programs
 batch systems - jobs
 time-shared systems - user programs or tasks
 job and program used interchangeably
 Process - a program in execution
 process execution proceeds in a sequential fashion
 A process contains
 program counter, stack and data section

Process State
 A process changes state as it executes.
new
new admitted
interrupt
I/O or
event
completion
Scheduler
dispatch I/O or
event wait
exit
ready
running
terminated
waiting

Process States
 New - The process is being created.
 Running - Instructions are being executed.
 Waiting - Waiting for some event to occur.
 Ready - Waiting to be assigned to a
processor.
 Terminated - Process has finished execution.

Process Control Block

Process Control Block
 Contains information associated with each
process
 Process State - e.g. new, ready, running etc.
 Program Counter - address of next instruction to be
executed
 CPU registers - general purpose registers, stack pointer
etc.
 CPU scheduling information - process priority, pointer
 Memory Management information - base/limit information
 Accounting information - time limits, process number
 I/O Status information - list of I/O devices allocated

Process Scheduling Queues
 Job Queue - set of all processes in the system
 Ready Queue - set of all processes residing in main
memory, ready and waiting to execute.
 Device Queues - set of processes waiting for an I/O
device.
 Process migration between the various queues.
 Queue Structures - typically linked list, circular list
etc.

Process Queues
Device
Queue
Ready
Queue

Process Scheduling Queues

Schedulers
 Long-term scheduler (or job scheduler) -
 selects which processes should be brought into the ready
queue.
 invoked very infrequently (seconds, minutes); may be slow.
 controls the degree of multiprogramming
 Short term scheduler (or CPU scheduler) -
 selects which process should execute next and allocates
CPU.
 invoked very frequently (milliseconds) - must be very fast
 Medium Term Scheduler
 swaps out process temporarily
 balances load for better throughput

Scheduling Objectives
 Enforcement of fairness
 in allocating resources to processes
 Enforcement of priorities
 Make best use of available system resources
 Give preference to processes holding key
resources.
 Give preference to processes exhibiting good
behavior.
 Degrade gracefully under heavy loads.

Program Behavior Issues
 I/O boundedness
 short burst of CPU before blocking for I/O
 CPU boundedness
 extensive use of CPU before blocking for I/O
 Urgency and Priorities
 Frequency of preemption
 Process execution time
 Time sharing
 amount of execution time process has already received.

Basic Concepts
 Maximum CPU utilization obtained with
multiprogramming.
 CPU-I/O Burst Cycle
 Process execution consists of a cycle of CPU execution
and I/O wait.

CPU Scheduler
 Selects from among the processes in
memory that are ready to execute, and
allocates the CPU to one of them.
 Non-preemptive Scheduling
 Once CPU has been allocated to a process, the process
keeps the CPU until
 Process exits OR
 Process switches to waiting state
 Preemptive Scheduling
 Process can be interrupted and must release the CPU.
 Need to coordinate access to shared data

CPU Scheduling Decisions
 CPU scheduling decisions may take place
when a process:
 switches from running state to waiting state
 switches from running state to ready state
 switches from waiting to ready
 terminates
 Scheduling under 1 and 4 is non-preemptive.
 All other scheduling is preemptive.

CPU scheduling decisions
new
new admitted
interrupt
I/O or
event
completion
Scheduler
dispatch I/O or
event wait
exit
ready
running
terminated
waiting

Scheduling Criteria
 CPU Utilization
 Keep the CPU and other resources as busy as possible
 Throughput
 # of processes that complete their execution per time
unit.
 Turnaround time
 amount of time to execute a particular process from its
entry time.

Scheduling Criteria (cont.)
 Waiting time
 amount of time a process has been waiting in the ready
queue.
 Response Time (in a time-sharing
environment)
 amount of time it takes from when a request was
submitted until the first response is produced, NOT
output.

Optimization Criteria
 Max CPU Utilization
 Max Throughput
 Min Turnaround time
 Min Waiting time
 Min response time

First Come First Serve (FCFS)
Scheduling
 Policy: Process that requests the CPU FIRST
is allocated the CPU FIRST.
 FCFS is a non-preemptive algorithm.
 Implementation - using FIFO queues
 incoming process is added to the tail of the queue.
 Process selected for execution is taken from head of
queue.
 Performance metric - Average waiting time in
queue.
 Gantt Charts are used to visualize schedules.

First-Come, First-Served(FCFS)
Scheduling
 Example
Process Burst Time
P1 24
P2 3
P3 3
 Suppose the arrival
order for the processes
is
 P1, P2, P3
 Waiting time
 P1 = 0;
 P2 = 24;
 P3 = 27;
 Average waiting time
 (0+24+27)/3 = 17
0 24 27 30
P1 P2 P3
Gantt Chart for Schedule

FCFS Scheduling (cont.)
 Example
Process Burst Time
P1 24
P2 3
P3 3
 Suppose the arrival order
for the processes is
 P2, P3, P1
 Waiting time
 P1 = 6; P2 = 0; P3 = 3;
 Average waiting time
 (6+0+3)/3 = 3 , better..
 Convoy Effect:
 short process behind long
process, e.g. 1 CPU bound
process, many I/O bound
processes.
0 3 6 30
P1P2 P3

Shortest-Job-First(SJF) Scheduling
 Associate with each process the length of its next
CPU burst. Use these lengths to schedule the
process with the shortest time.
 Two Schemes:
 Scheme 1: Non-preemptive
 Once CPU is given to the process it cannot be preempted
until it completes its CPU burst.
 Scheme 2: Preemptive
 If a new CPU process arrives with CPU burst length less
than remaining time of current executing process, preempt.
Also called Shortest-Remaining-Time-First (SRTF).
 SJF is optimal - gives minimum average waiting time for
a given set of processes.

Non-Preemptive SJF Scheduling
 Example
Process Arrival TimeBurst Time
P1 0 7
P2 2 4
P3 4 1
P4 5 4
0 8 16
P1 P2P3
P4
127
Average waiting time =
(0+6+3+7)/4 = 4

Preemptive SJF Scheduling(SRTF)
 Example
Process Arrival TimeBurst Time
P1 0 7
P2 2 4
P3 4 1
P4 5 4
0 7 16
P1 P2P3
P4
115
Average waiting time =
(9+1+0+2)/4 = 3
P2 P1
2 4

Priority Scheduling
 A priority value (integer) is associated with
each process. Can be based on
 Cost to user
 Importance to user
 Aging
 %CPU time used in last X hours.
 CPU is allocated to process with the highest
priority.
 Preemptive
 Nonpreemptive

Priority Scheduling (cont.)
 SJN is a priority scheme where the priority is
the predicted next CPU burst time.
 Problem
 Starvation!! - Low priority processes may never execute.
 Solution
 Aging - as time progresses increase the priority of the
process.

Round Robin (RR)
 Each process gets a small unit of CPU time
 Time quantum usually 10-100 milliseconds.
 After this time has elapsed, the process is preempted and
added to the end of the ready queue.
 n processes, time quantum = q
 Each process gets 1/n CPU time in chunks of at most q
time units at a time.
 No process waits more than (n-1)q time units.
 Performance
 Time slice q too large - FIFO behavior
 Time slice q too small - Overhead of context switch is
too expensive.
 Heuristic - 70-80% of jobs block within timeslice

Round Robin Example
 Time Quantum = 20
Process Burst Time
P1 53
P2 17
P3 68
P4 24
0
P1 P4P3
P1P2
20
P3 P3 P3P4 P1
37 57 77 97 117 121 134 154 162
Typically, higher average turnaround time than SRTF, but better response

Operating System Scheduling

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