1. SYNCHRONIZATION Module-4

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. Scheduler goals 1.Minimize the response time To provide suitable response time, preemption and priority must be implemented. 2. Share the processor among tasks fairly 3. Making efficient use of the processor 4. Increase the throughput

9. 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

10. 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.

11. 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

12. 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

13. 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

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

15. 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

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

17. 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

18. 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.

19. 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.

20. 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

21. 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

22. LISTING 6.2 LONG 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);}

23. 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

24. 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.

25. Local semaphore Local semaphores are an extremely efficient operating system mechanism It should be used to coordinate threads running at the server

26. Network semaphore network semaphores involve much more overhead and possibly network transmissions It should only be used to coordinate activity between many systems

27. 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

28. There are four main synchronization objects with WindowsNT: 1.Mutex 2.Semaphore 3.Event 4.Critical Section Each of these objects performs similar functions in synchronization.