The document discusses processes and process scheduling in operating systems. It defines a process as a program in execution that contains a program counter, stack, and data section. Processes can be in various states like new, ready, running, waiting, and terminated. A process control block contains information about each process like its state, program counter, memory allocation, and more. Scheduling aims to optimize CPU utilization, throughput, turnaround time, waiting time, and response time using algorithms like first come first serve, shortest job first, priority, and round robin scheduling.
3. 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
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4. 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
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5. 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.
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7. 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
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8. 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.
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11. 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
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12. 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.
13. 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.
14. 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.
15. 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
16. 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.
17. CPU scheduling decisions
new
new admitted
interrupt
I/O or
event
completion
Scheduler
dispatch I/O or
event wait
exit
ready
running
terminated
waiting
18. 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.
19. 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.
20. Optimization Criteria
Max CPU Utilization
Max Throughput
Min Turnaround time
Min Waiting time
Min response time
21. 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.
22. 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
23. 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
Gantt Chart for Schedule
24. 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.
25. 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
Gantt Chart for Schedule
P4
127
Average waiting time =
(0+6+3+7)/4 = 4
26. 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
Gantt Chart for Schedule
P4
115
Average waiting time =
(9+1+0+2)/4 = 3
P2 P1
2 4
27. 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
28. 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.
29. 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
30. Round Robin Example
Time Quantum = 20
Process Burst Time
P1 53
P2 17
P3 68
P4 24
0
P1 P4P3
Gantt Chart for Schedule
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