Ch5 cpu-scheduling

Silberschatz, Galvin and Gagne ©2009
Operating System Concepts – 8th Edition,
Chapter 5: CPU Scheduling

5.2 Silberschatz, Galvin and Gagne ©2009
Operating System Concepts – 8th Edition
Chapter 5: CPU Scheduling
 Basic Concepts
 Scheduling Criteria
 Scheduling Algorithms
 Thread Scheduling
 Multiple-Processor Scheduling
 Operating Systems Examples
 Algorithm Evaluation

Objectives
 To introduce CPU scheduling, which is the basis for multiprogrammed
operating systems
 To describe various CPU-scheduling algorithms
 To discuss evaluation criteria for selecting a CPU-scheduling algorithm for a
particular system

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 burst distribution

Histogram of CPU-burst Times

Alternating Sequence of CPU And I/O Bursts

CPU Scheduler
 Selects from among the processes in memory that are ready to execute,
and allocates the CPU to one of them
 CPU scheduling decisions may take place when a process:
1. Switches from running to waiting state
2. Switches from running to ready state
3. Switches from waiting to ready
4. Terminates
 Scheduling under 1 and 4 is nonpreemptive
 All other scheduling is preemptive

Scheduling Criteria
 CPU utilization – keep the CPU as busy as possible
 Throughput – # of processes that complete their execution per
time unit
 Turnaround time – amount of time to execute a particular process
 Waiting time – amount of time a process has been waiting in the
ready queue
 Response time – amount of time it takes from when a request was
submitted until the first response is produced, not output (for time-
sharing environment)

Scheduling Algorithm Optimization Criteria
 Max CPU utilization
 Max throughput
 Min turnaround time
 Min waiting time
 Min response time

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

FCFS Scheduling (Cont)
Suppose that the processes arrive in the order
P2 , P3 , P1
 The Gantt chart for the schedule is:
 Waiting time for P1 = 6; P2 = 0; P3 = 3
 Average waiting time: (6 + 0 + 3)/3 = 3
 Much better than previous case
 Convoy effect short process behind long process
P1
P3
P2
6
3 30
0

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
 SJF is optimal – gives minimum average waiting time for a given set of
processes
 The difficulty is knowing the length of the next CPU request

Example of SJF
Process Arrival Time Burst Time
P1 0.0 6
P2 2.0 8
P3 4.0 7
P4 5.0 3
 SJF scheduling chart
 Average waiting time = (3 + 16 + 9 + 0) / 4 = 7
P4
P3
P1
3 16
0 9
P2
24

Determining Length of Next CPU Burst
 Can only estimate the length
 Can be done by using the length of previous CPU bursts, using exponential
averaging
:
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Prediction of the Length of the Next CPU Burst

Examples of Exponential Averaging
  =0
 n+1 = n
 Recent history does not count
  =1
 n+1 =  tn
 Only the actual last CPU burst counts
 If we expand the formula, we get:
n+1 =  tn+(1 - ) tn -1 + …
+(1 -  )j  tn -j + …
+(1 -  )n +1 0
 Since both  and (1 - ) are less than or equal to 1, each successive term
has less weight than its predecessor

Priority Scheduling
 A priority number (integer) is associated with each process
 The CPU is allocated to the process with the highest priority (smallest
integer  highest priority)
 Preemptive
 nonpreemptive
 SJF is a priority scheduling where 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.
 If there are n processes in the ready queue and the time
quantum is q, then each process gets 1/n of the CPU time in
chunks of at most q time units at once. No process waits more
than (n-1)q time units.
 Performance
 q large  FIFO
 q small  q must be large with respect to context switch,
otherwise overhead is too high

Example of RR with Time Quantum = 4
Process Burst Time
P1 24
P2 3
P3 3
 The Gantt chart is:
 Typically, higher average turnaround than SJF, but better response
P1 P2 P3 P1 P1 P1 P1 P1
0 4 7 10 14 18 22 26 30

Time Quantum and Context Switch Time

Turnaround Time Varies With The Time Quantum

Multilevel Queue
 Ready queue is partitioned into separate queues:
foreground (interactive)
background (batch)
 Each queue has its own scheduling algorithm
 foreground – RR
 background – FCFS
 Scheduling must be done between the queues
 Fixed priority scheduling; (i.e., serve all from foreground then from
background). Possibility of starvation.
 Time slice – each queue gets a certain amount of CPU time which it can
schedule amongst its processes; i.e., 80% to foreground in RR
 20% to background in FCFS

Multilevel Queue Scheduling

Multilevel Feedback Queue
 Problem with above approach
1)inflexible(but low scheduling overhead)
 A process can move between the various queues; aging can be
implemented this way
 Multilevel-feedback-queue scheduler defined by the following
parameters:
 number of queues
 scheduling algorithms for each queue
 method used to determine when to upgrade a process
 method used to determine when to demote a process
 method used to determine which queue a process will enter
when that process needs service

Example of Multilevel Feedback Queue
 Three queues:
 Q0 – RR with time quantum 8 milliseconds
 Q1 – RR time quantum 16 milliseconds
 Q2 – FCFS
 Scheduling
 A new job enters queue Q0 which is served FCFS. When it gains CPU,
job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is
moved to queue Q1.
 At Q1 job is again served FCFS and receives 16 additional milliseconds.
If it still does not complete, it is preempted and moved to queue Q2.

Multilevel Feedback Queues

Multiple-Processor Scheduling
 CPU scheduling more complex when multiple CPUs are
available
 Homogeneous processors within a multiprocessor
 Asymmetric multiprocessing – only one processor
accesses the system data structures, alleviating the need
for data sharing
 Symmetric multiprocessing (SMP) – each processor
is self-scheduling, all processes in common ready queue,
or each has its own private queue of ready processes

Real Time Scheduling
Hard Real time OS
Required resource reservation
Soft Real time OS (less restrictive)

Ch5 cpu-scheduling

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