Window scheduling algorithm

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““Windows Scheduling Algorithm””
ADVANCE OPERATING SYSTEM

Basic ConceptsBasic Concepts
 Maximum CPU utilization is obtained with multiprogramming
◦ Several processes are kept in memory at one time
◦ Every time a running process has to wait, another process can take over use of
the CPU
 Scheduling of the CPU is fundamental to operating system design
 Process execution consists of a cycle of a CPU time burst and an I/O time burst
(i.e. wait)
◦ Processes alternate between these two states (i.e., CPU burst and I/O burst)
◦ Eventually, the final CPU burst ends with a system request to terminate
execution

Alternating Sequence of CPUAlternating Sequence of CPU
And I/O BurstsAnd I/O Bursts

CPU SchedulerCPU Scheduler
 The CPU scheduler selects from among the processes in memory that are ready to
execute and allocates the CPU to one of them
 CPU scheduling is affected by the following set of circumstances:
(N) A process switches from running to waiting state
(P) A process switches from running to ready state
(P) A process switches from waiting to ready state
(N) A processes switches from running to terminated state
 Circumstances 1 and 4 are non-preemptive; they offer no schedule choice
 Circumstances 2 and 3 are pre-emptive; they can be scheduled

Scheduling CriteriaScheduling Criteria
Different CPU scheduling algorithms have different properties
 In choosing which algorithm to use, the properties of the various algorithms should
be considered
 Criteria for comparing CPU scheduling algorithms may include the following
◦ CPU utilization – percent of time that the CPU is busy executing a process
◦ Throughput – number of processes that are completed per time unit
◦ Response time – amount of time it takes from when a request was submitted
until the first response occurs (but not the time it takes to output the entire
response)
◦ Waiting time – the amount of time before a process starts after first entering the
ready queue (or the sum of the amount of time a process has spent waiting in the
ready queue)
◦ Turnaround time – amount of time to execute a particular process from the time
of submission through the time of completion

Optimization CriteriaOptimization Criteria
 It is desirable to
◦ Maximize CPU utilization
◦ Maximize throughput
◦ Minimize turnaround time
◦ Minimize start time
◦ Minimize waiting time
◦ Minimize response time
 In most cases, we strive to optimize the average measure of each metric
 In other cases, it is more important to optimize the minimum or maximum values
rather than the average

 First Come, First Served (FCFS)
 Shortest Job First (SJF)
 Priority
 Round Robin (RR)
SINGLE PROCESSOR SCHEDULINGSINGLE PROCESSOR SCHEDULING
ALGORITHMSALGORITHMS

ProcessBurst Time
P1 24
P2 3
P3 3
 With FCFS, the process that requests the CPU first is allocated the CPU first
 Case #1: 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
 Average turn-around time: (24 + 27 + 30)/3 = 27
FIRST-COME, FIRST-SERVED (FCFS)FIRST-COME, FIRST-SERVED (FCFS)
SCHEDULINGSCHEDULING
P1 P2 P3
24 27 300

FCFS Scheduling (Cont.)FCFS Scheduling (Cont.)
 Case #2: Suppose that the processes arrive in the order: P2 , P3 , P1
 The Gantt chart for the schedule is:
P1P3P2
63 300
Waiting time for P1 = 6; P2 = 0; P3 = 3
Average waiting time: (6 + 0 + 3)/3 = 3 (Much better than Case #1)
Average turn-around time: (3 + 6 + 30)/3 = 13

FCFS Scheduling (Cont.)FCFS Scheduling (Cont.)
 Case #1 is an example of the convoy effect; all the other processes wait for one
long-running process to finish using the CPU
◦ This problem results in lower CPU and device utilization;
 Case #2 shows that higher utilization might be possible if the short processes
were allowed to run first
 The FCFS scheduling algorithm is non-preemptive
◦ Once the CPU has been allocated to a process, that process keeps the CPU until
it releases it either by terminating or by requesting I/O
◦ It is a troublesome algorithm for time-sharing systems

Priority SchedulingPriority Scheduling
 The SJF algorithm is a special case of the general priority scheduling algorithm
 A priority number (integer) is associated with each process
 The CPU is allocated to the process with the highest priority (smallest integer =
highest priority)
 Priority scheduling can be either preemptive or non-preemptive
◦ A preemptive approach will preempt the CPU if the priority of the newly-arrived
process is higher than the priority of the currently running process
◦ A non-preemptive approach will simply put the new process (with the highest
priority) at the head of the ready queue
 SJF is a priority scheduling algorithm where priority is the predicted next CPU
burst time
 The main problem with priority scheduling is starvation, that is, low priority
processes may never execute
 A solution is aging; as time progresses, the priority of a process in the ready queue
is increased

ROUND ROBIN (RR) SCHEDULINGROUND ROBIN (RR) SCHEDULING
•In the round robin algorithm, each process gets a small unit of CPU time (a 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 of the round robin algorithm
•q large  FCFS
•q small  q must be greater than the context switch time; otherwise, the overhead
is too high
•One rule of thumb is that 80% of the CPU bursts should be shorter than the time
quantum

Process Burst Time
P1 53
P2 17
P3 68
P4 24
 The Gantt chart is:
EXAMPLE OF RR WITH TIMEEXAMPLE OF RR WITH TIME
QUANTUM = 20QUANTUM = 20
P1 P2 P3 P4 P1 P3 P4 P1 P3 P3
0 20 37 57 77 97 117 121 134 154 162

Example of RR with TimeExample of RR with Time
Quantum = 20Quantum = 20
 Typically, higher average turnaround than SJF, but better response time
 Average waiting time
= ( [(0 – 0) + (77 - 20) + (121 – 97)] + (20 – 0) + [(37 – 0) + (97 - 57) +
(134 – 117)] + [(57 – 0) + (117 – 77)] ) / 4
= (0 + 57 + 24) + 20 + (37 + 40 + 17) + (57 + 40) ) / 4
= (81 + 20 + 94 + 97)/4
= 292 / 4 = 73
 Average turn-around time = 134 + 37 + 162 + 121) / 4 = 113.5

 In multi-level feedback queue scheduling, a process can move between the various
queues; aging can be implemented this way
 A multilevel-feedback-queue scheduler is defined by the following parameters:
◦ Number of queues
◦ Scheduling algorithms for each queue
◦ Method used to determine when to promote 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
MULTILEVEL FEEDBACK QUEUEMULTILEVEL FEEDBACK QUEUE
SCHEDULINGSCHEDULING

EXAMPLE OF MULTILEVEL FEEDBACKEXAMPLE OF MULTILEVEL FEEDBACK
QUEUEQUEUE
 Scheduling
◦ A new job enters queue Q0 (RR) and is placed at the end. When it gains the
CPU, the job receives 8 milliseconds. If it does not finish in 8 milliseconds, the
job is moved to the end of queue Q1.
◦ A Q1 (RR) job receives 16 milliseconds. If it still does not complete, it is
preempted and moved to queue Q2 (FCFS).

WINDOWS XP SCHEDULINGWINDOWS XP SCHEDULING
 Windows XP schedules threads using a priority-based, preemptive scheduling
algorithm
 The scheduler ensures that the highest priority thread will always run
 The portion of the Windows kernel that handles scheduling is called the
dispatcher
 A thread selected to run by the dispatcher will run until it is preempted by a
higher-priority thread, until it terminates, until its time quantum ends, or until it
calls a blocking system call such as I/O
 If a higher-priority real-time thread becomes ready while a lower-priority thread
is running, lower-priority thread will be preempted
◦ This preemption gives a real-time thread preferential access to the CPU when
the thread needs such access

 The dispatcher uses a 32-level priority scheme to determine the order of thread
execution
 Priorities are divided into two classes
◦ The variable class contains threads having priorities 1 to 15
◦ The real-time class contains threads with priorities ranging from 16 to 31
◦ There is also a thread running at priority 0 that is used for memory management
 The dispatcher uses a queue for each scheduling priority and traverses the set of
queues from highest to lowest until it finds a thread that is ready to run
 If no ready thread is found, the dispatcher will execute a special thread called the
idle thread
WINDOWS XP SCHEDULINGWINDOWS XP SCHEDULING

Windows XP SchedulingWindows XP Scheduling
 There is a relationship between the numeric priorities of the Windows XP kernel
and the Win32 API
 The Windows Win32 API identifies six priority classes to which a process can
belong as shown below
◦ Real-time priority class
◦ High priority class
◦ Above normal priority class
◦ Normal priority class
◦ Below normal priority class
◦ Low priority class
 Priorities in all classes except the read-time priority class are variable
◦ This means that the priority of a thread in one of these classes can change

 Within each of the priority classes is a relative priority as shown below
 The priority of each thread is based on the priority class it belongs to and its relative
priority within that class

 The initial priority of a thread is typically the base priority of the process that the
thread belongs to
 When a thread’s time quantum runs out, that thread is interrupted
 If the thread is in the variable-priority class, its priority is lowered
◦ However, the priority is never lowered below the base priority
 Lowering the thread’s priority tends to limit the CPU consumption of compute-
bound threads

When a variable-priority thread is released from a wait operation, the
dispatcher boosts the priority
The amount of boost depends on what the thread was waiting for
A thread that was waiting for keyboard I/O would get a large increase
A thread that was waiting for a disk operation would get a moderate
increase
This strategy tends to give good response time to interactive threads that are
using the mouse and windows
It also enables I/O-bound threads to keep the I/O devices busy while
permitting compute-bound threads to use spare CPU cycles in the background
This strategy is used by several time-sharing operating systems, including
UNIX
In addition, the window with which the user is currently interacting receives a
priority boost to enhance its response time

 When a user is running an interactive program, the system needs to provide
especially good performance for that process
 Therefore, Windows XP has a special scheduling rule for processes in the normal
priority class
 Windows XP distinguishes between the foreground process that is currently
selected on the screen and the background processes that are not currently selected
 When a process moves in the foreground, Windows XP increases the scheduling
quantum by some factor – typically by 3
 This increase gives the foreground process three times longer to run before a time-
sharing preemption occurs

Window scheduling algorithm

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