SYNCHRONIZATIONModule-4

scheduling

• Scheduling is an operating system mechanism that arbitrate CPU resources between running

tasks . Different scheduling algorithms are there FCFS, SJF, priority based, pre-emptive, round

robin

Scheduling implementation

• Each platform provide native round robin scheduling mechanism

• Different O.S may offer different implementation of RR scheduling

• Preemptive O.s means, it allowed to interrupt an executing task

Preemptive and non-preemptive O.S• Preemptive means its allow an interruption of

current running task.• It replaces another ready to run task• In non-preemptive system, task execute up to

completion

Selecting which task to execute

• Scheduler also has the responsibility selecting next available task to execute

• In Strict round robin next available task can get from the run queue

• IBM OS/2, novell Unixware, windowsNT are all priority based , preemptive, timesliced O.S

• Novell Netware3 and4 are non preemptive, round robin scheduled O.S

Scheduler Responsibilities

• CPU resource distribution• Maintaining Task queues• Switching the task or allocating CPU• Optionally provide priorities and preemption• It should be efficient and robust

Scheduler goals

1. Minimize the response time To provide suitable response time,

preemption and priority must be implemented.2. Share the processor among tasks fairly3. Making efficient use of the processor4. Increase the throughput

Processing queues

• One of the most important job of scheduler is to maintain circular queue of available tasks

• Number of queues is dependent on Implementation • scheduler consist of three queues 1.run queue 2. Ready queue 3.Idle(wait) queue

• Scheduler maintain list of tasks which may be in one of three states

1.Ready to run tasks list of active awaiting process called run

queue. It is a round robin queue, managed using

FIFO list.

2. Task waiting for i/o operations

.i/o queues are used to maintain it .single task system cpu idle while i/o

operations . In multi tasking system, scheduler use the

processor when it wait for i/o

3.Idle task wait queue used for it . Idle task is a task which is not ready for

run and i/o operation. Suspend thread(), sleep() or delay() used to

place task on wait queue

Context switching

• Its an essential operation for an O.S scheduler

• Task switch define as the work required to switch the control from one task to another

• Amount of time required for switching is critical

TaskSwitch(){Disableinterrupts()SaveContext(currentTask)SelectedTask = SelectNextRunnable(Runlist)RemoveSelected(Runlist, selectedTask)RestoreContext (selectedTask);EnableInterrupts()StartExecution()}

• Accessing or updating shared data• Section of code that perform operation on the

shared data areas are known as critical section

task A critical section task B

CS require that only one task executing in them at a time

Critical section

One at a time

access

Critical section problem LONG STOCKquote;Main() {Begin(TaskA);Begin(TaskB);}TaskA() {………………stockQuote = 29.875;totalValue = numShares *stockQuote;………}TaskB() {………………stockQuote = 100.125;totalValue = numShares *stockQuote;………}

Mutual Exclusion

• One Task at a time access• mutually exclusive means they cant execute

same operation concurrently• Many ways to implement mutual exclusion locking variables disabling interrupt using semaphore

semaphores

• Synchronize access of shared data• Each O.S provide general semaphore operation• This variable is the controlling acces to the shared

resource and is maintained by incrementing or decrementing its value:

• A value less than 0 indicates that process are waiting for the shared resource or region.

• A value greater than 0 indicates that the resource is available.

P() and V() Operation on Semaphores

• Dijkstra suggested having two operations on the semaphore,p() and v() respectively .

• the p() operation decrements the semaphore value by 1, while v() increments the semaphore by 1.

• in Dijkstra’s orginal model, if the p() operation caused the Semaphore to go to 0,the process would spin –wait for the semaphore value to be incremented with another v() operation.

• This caused the requesting process to wait for completion of the region by another process.

• By extending the orginal model to allow the semaphore to become negative and thus queue up multiple waiting process, the semaphore becomes more useful

• If during p() operation the semaphore value becomes negative,the calling process is blocked.

• any semaphore with a value less than 0 signifies that process are waiting.

• If as a result of a p() operation the semaphore value is still > =0, the process is allowed access to the region

LISTING 6.2LONG stockQuote, totalValue, numShares;Semaphore mutex = 1;Main() {Begin(TaskA);Begin(TaskB);} TaskA() {………p(&mutex);stockQuote = 29.875;totalValue = numShares * stockQuote;V(&mutex);……….} TaskB() {………..p(&mutex);stockQuote = 100.125;totalValue = numShates * stockQuote;V(&mutex);}

Semaphore implementations

• Different types of implementations for different platform

• Each O.S provides basic semaphore operations for mutual exclusion

• some provide a richer set of functions for extended semaphore operations

Novell Netware

• Novell implements two classes of semaphore functions, local and network.

• Local semaphores are used for synchronization and mutual exclusion by programs running on NetWare servers as NLMs.

• Network semaphores are a mechanism by which a semaphore may be used by many different computers throughout a network.

• Local semaphore Local semaphores are an extremely

efficient operating system mechanismIt should be used to coordinate threads

running at the server

• Network semaphore network semaphores involve much more

overhead and possibly network transmissionsIt should only be used to coordinate activity

between many systems

Microsoft NT

• Windows NT has a very extensive set of synchronization options

• These options are provide by Microsoft in the form of synchronization objects

• and can be used to synchronize events between processes or threads

• There are four main synchronization objects with WindowsNT:

1.Mutex2.Semaphore3.Event4.Critical Section• Each of these objects performs similar

functions in synchronization.