CPU scheduling are using in operating systems.ppt

5.2 Silberschatz, Galvin and Gagne ©2005
Operating System Concepts
Chapter 5: CPU Scheduling
 Basic Concepts
 Scheduling Criteria
 Scheduling Algorithms
 Multiple-Processor Scheduling
 Real-Time Scheduling
 Thread Scheduling
 Operating Systems Examples
 Java Thread Scheduling
 Algorithm Evaluation

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

CPU–I/O Burst Cycle
 The success of CPU scheduling depends on an observed
property of processes:
 process execution consists of a cycle of CPU execution
and I/O wait. Processes alternate between these two
states. Process execution begins with a CPU burst. That is
followed by an I/O burst, which is followed by another
CPU burst, then another I/O burst, and so on. Eventually,
the final CPU burst ends with a system request to terminate
execution

Alternating Sequence of CPU And I/O Bursts

 The durations of CPU bursts have been measured extensively.
Although they vary greatly from process to process and from
computer to computer, they tend to have a frequency curve similar
to that shown in Figure on next slide.
 The curve is generally characterized as exponential or
hyperexponential, with a large number of short CPU bursts and a
small number of long CPU bursts.
 An I/O-bound program typically has many short CPU bursts. A
CPU-bound program might have a few long CPU bursts.
 This distribution can be important in the selection of an appropriate
CPU-scheduling algorithm

Histogram of CPU-burst Times

CPU Scheduler
 Whenever the CPU becomes idle, the operating
system must select one of the processes in the
ready queue to be executed. The selection
process is carried out by the short-term
scheduler, or CPU scheduler.

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. The CPU-scheduling algorithm does not affect the amount of time
during which a process executes or does I/O. It affects only the amount of
time that a process spends 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). In an interactive system, turnaround time may not
be the best criterion. Often, a process can produce some output fairly
early and can continue computing new results while previous results are
being output to the user. Response time, is the time it takes to start
responding, not the time it takes to output the response

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

First-Come, First-Served (FCFS) Scheduling
 By far the simplest CPU-scheduling algorithm is the first-come, first-
served(FCFS) scheduling algorithm. With this scheme, the
process that requests the CPU first is allocated the CPU first.
 The implementation of the FCFS policy is easily managed with a
FIFO queue. When a process enters the ready queue, its PCB is
linked onto the tail of the queue. When the CPU is free, it is allocated
to the process at the head of the queue. The running process is then
removed from the queue

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

FCFS Scheduling (Cont.)
 Note also that the FCFS scheduling algorithm is
nonpreemptive. Once the CPU has been allocated
to a process, that process keeps the CPU until it
releases the CPU, either by terminating or by
requesting I/O.
 The FCFS algorithm is thus particularly
troublesome for time-sharing systems, where it is
important that each user get a share of the CPU at
regular intervals. It would be disastrous to allow
one process to keep the CPU for an extended
period.

Shortest-Job-First (SJR) Scheduling
 Associate with each process the length of its next CPU burst. Use
these lengths to schedule the process with the shortest time
 When the CPU is available, it is assigned to the process that has
the smallest next CPU burst.
 If the next CPU bursts of two processes are the same, FCFS
scheduling is used to break the tie

Shortest-Job-First (SJF) Scheduling
 Two schemes:
 nonpreemptive – once CPU given to the process it cannot be
preempted until completes its CPU burst
 preemptive – if a new process arrives with CPU burst length
less than remaining time of current executing process,
preempt. This scheme is know as the
Shortest-Remaining-Time-First (SRTF)
 SJF is optimal – gives minimum average waiting time for a given
set of processes

Process Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
 SJF (non-preemptive)
 Average waiting time = (0 + 6 + 3 + 7)/4 = 4
Example of Non-Preemptive SJF
P1 P3 P2
7
3 16
0
P4
8 12

Example of Preemptive SJF
Process Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
 SJF (preemptive)
 Average waiting time = (9 + 1 + 0 +2)/4 = 3
P1 P3
P2
4
2 11
0
P4
5 7
P2 P1
16

Determining Length of Next CPU Burst
 With short-term scheduling, there is no way to know the
length of the next CPU burst.
 One approach to this problem is to try to approximate SJF
scheduling. We may not know the length of the next CPU
burst, but we may be able to predict its value.
 We expect that the next CPU burst will be similar in length to
the previous ones. By computing an approximation of the
length of the next CPU burst, we can pick the process with
the shortest predicted CPU burst.
 The next CPU burst is generally predicted as an
exponential average of the measured lengths of previous
CPU bursts

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). Equal-priority processes are
scheduled in FCFS order.
 Preemptive
 nonpreemptive
 When a process arrives at the ready queue, its priority is compared
with the priority of the currently running process. A preemptive
priority scheduling algorithm will preempt the CPU if the priority of
the newly arrived process is higher than the priority of the currently
running process. A nonpreemptive priority scheduling algorithm will
simply put the new process at the head of the ready queue

Priority Scheduling
 SJF is a priority scheduling where priority is the predicted next CPU
burst time
 Problem  Starvation – low priority processes may never execute
(Rumor has it that when they shut down the IBM 7094 at MIT in
1973, they found a low-priority process that had been submitted in
1967 and had not yet been run.)
 Solution  Aging – as time progresses increase the priority of the
process.
• Aging involves gradually increasing the priority of processes that wait
in the system for a long time. For example, if priorities range from 127
(low) to 0 (high), we could increase the priority of a waiting process by
1 every 15minutes. Eventually, even a process with an initial priority of
127 would have the highest priority in the system and would be
executed. In fact, it would take no more than 32 hours for a priority-127
process to age to a priority-0 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.
 To implement RR scheduling, we again treat the ready queue as a FIFO
queue of processes. New processes are added to the tail of the ready
queue. The CPU scheduler picks the first process from the ready queue,
sets a timer to interrupt after 1 time quantum, and dispatches the
process.
 One of two things will then happen. The process may have a CPU burst
of less than 1 time quantum. In this case, the process itself will release
the CPU voluntarily. The scheduler will then proceed to the next process
in the ready queue. If the CPU burst of the currently running process is
longer than 1 time quantum, the timer will go off and will cause an
interrupt to the operating system. A context switch will be executed, and
the process will be put at the tail of the ready queue. The CPU scheduler
will then select the next process in the ready queue

Round Robin (RR)
 Performance
 q large  FCFS
 q small  q must be large with respect to context switch,
otherwise overhead is too high

Example of RR with Time Quantum = 20
Process Burst Time
P1 53
P2 17
P3 68
P4 24
 The Gantt chart is:
 Typically, higher average turnaround than SJF, but better response
P1 P2 P3 P4 P1 P3 P4 P1 P3 P3
0 20 37 57 77 97 117 121 134 154 162

Time Quantum and Context Switch Time
Assume, for example, that we have only one process of 10
time units. If the quantum is 12 time units, the process finishes in
less than 1time quantum, with no overhead. If the quantum is 6
time units, however, the process requires 2 quanta, resulting in a
context switch. If the time quantum is1 time unit, then nine context
switches will occur, slowing the execution of the process
accordingly

Turnaround Time Varies With The Time Quantum

Multilevel Queue
 Ready queue is partitioned into separate queues:
foreground (interactive)
background
 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
 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

CPU scheduling are using in operating systems.ppt

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