CPU Scheduling
Basic Concepts
Chapter 5: CPU Scheduling
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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
CPU-I/O Burst Cycle
• Process execution consists of a cycle
of CPU execution and I/O wait
• Processes alternate between these
two states
• Each scheduling decision is about
which job to give to the CPU for use
by its next CPU burst
• With time slicing, job may be forced
to give up CPU before finishing
current CPU burst
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
• Managing resource (e.g., CPU)
• n processes
– A schedule is a listing
– Number of possible schedules if no interdependencies?
• Environment
– Number of processors involved
– Whether preemption is allowed (priorities or time slicing)
– Whether a process once active can become blocked (due to I/O, etc…)
Scheduling
• First the easy case
– One processor, no preemption, no blocking
– n/1 static scheduling problem
• n processes
• Know arrival times, processing times
Scheduling
• Given the following information about process Pi
– Arrival Time ai
– Execution Time ei
– Due Time di
• We can obtain
– Completion (finish) Time fi
– Turn-around Time ti = (fi – ai)
– Waiting Time wi = (ti – ei)
– Lateness Li = (fi – di)
– Tardiness Ti = max(0, Li)
Example
Process
pi
P1
P2
P3
P4
P5
P6
15
25
5
20
25
30
P4
ti
wi
P1
65
50
P2
120
95
P3
50
45
P4
20
0
P5
45
20
P6
95
65
0
P5
20
P3
45
50
P1
P6
65
P2
95
120
Scheduling Criteria
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CPU utilization
– keep the CPU as busy as possible
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Throughput
– # of processes that complete their execution per time unit
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Turnaround time
– amount of time to execute a particular process
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Waiting time
– amount of time a process has been waiting in the ready queue
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Response time
– amount of time it takes from when a request was submitted until the first response is
produced
How good is a schedule? Performance Measures
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Max CPU utilization
Max throughput
Min turnaround time
Min waiting time
Min response time
First-Come, First-Served (FCFS) Scheduling
• First-Come, First-Served (FCFS)
– Also “First In, First Out” (FIFO) or “Run until done”
• In early systems, FCFS meant one program scheduled
until done (including I/O)
• Now, means keep CPU until process blocks
First-Come, First-Served (FCFS) Scheduling
Process
pi
P1
P2
P3
24
3
3
• Suppose that the processes arrive in the order: P1 , P2 , P3 .
Draw the Gantt Chart for the schedule and compute the
average waiting time: 3
P2
0
P3
3
P1
6
30
First-Come, First-Served (FCFS) Scheduling
Process
pi
P1
P2
P3
24
3
3
• Suppose that the processes arrive in the order: P2, P3, P1 .
Draw the Gantt Chart for the schedule and compute the
average waiting time: 17
P1
0
P2
24
P3
27
30
First-Come, First-Served (FCFS) Scheduling
• There is a convoy effect as all the other processes wait for the
one big process to get off the CPU
– Average waiting time for FCFS is often quite long
– What about average turnaround time?
• Pros and cons
– Simple (+)
– Short jobs get stuck behind long ones (-)
• Analogy: getting milk at a supermarket and waiting to pay, always stuck
behind carts full of items. Upside: get to read about space aliens!
Example of (Shortest Job First)
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
P4
0
P3
P1
3
9
P2
16
24
• Average waiting time = (3 + 16 + 9 + 0) / 4 = 7
SJF Scheduling example
Process
pi
P1
P2
P3
P4
P5
P6
15
25
5
20
18
30
• Sometimes called Shortest Processing Time (SPT) First
• Draw the Gantt Chart for a SPT schedule and compute the
average waiting time and average turnaround time (assume
all processes arrive at time 0)
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