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12 Solution: Waiting time for P1 = 0ms Waiting time for P2 = 6ms Waiting time for P3 = 14ms Waiting time for P4 = 21ms Average waiting time = 41/4 = 10.25ms Average Turn around time=( )/4=16.5ms In above example note that we have assumed that all the process has arrived at the same point of time. WT for P1=0,P2=6,P3=14 and P4=21. Hence AWT=41/4=10.25 The A.T.A.T =A.W.T+ A.Exec.Time= ( )/4=16.5 A.R.T= ( )/4=

13 AWT=(0+(10-2)+(14))/3=7.33 A.T.A.T= /3= =14 A.R.T=(0+8+14)/3=

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16 Average Waiting time: Waiting time for P1= Quantum Allocated to P2,P3,P4 in First Round=11 Waiting time for P2= Quantum Allocated to P1,P3,P4 in First Round=13 Waiting time for P3= Quantum Allocated to P1,P2,P4 in First Round and Quantum Allocated to P1 &P2 =17 Waiting time for P4= Quantum Allocated to P1,P2,P3 in First Round=12 Average waiting time=( )/4=13.25 TAT=AWT+ Total Exec time of all processes. 16

17 AWT=((0+4)+0+14)/3=6 A.T.A.T=6+20/3=6+6.67=12.67 A.R.T=(0+0+14)/3=

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20 The three queues are implemented as below A process may be assigned to one of the above three queue based on some characteristics of the process. E.g system process, interactive process or Batch process. User can also specify the characteristics. The processes from the highest priority queue are serviced until that queue becomes empty. The scheduling policy is priority based pre-emptive. When the first queue (highest-priority) becomes empty, the next queue may be serviced using RR policy. When both the above queues are empty, the third (lowest-priority) queue is serviced using FCFS. A lower priority process may be preempted when a higher-priority process arrives in one of the upper-level queues.

21 Aging can be achieved by moving the process from one queue to other. 21

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23 A new process enters queue Q 0. In Q0 the algorithm is RR with time quantum of 6 ms. When the CPU will be allocated to Process it will be served for 6 ms. If process will not be finished in 6 milliseconds it will be preempted and process will move to queue Q 1. In Q 1 process will be served again using RR algorithm, with a time quantum of 12 ms. If still, it will not complete, it will be preempted and moved to queue Q 2. In Q2 process will be served using FCFS algorithm. 23

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26 For example in editor when you are typing, spell checking can also be done concurrently, so two threads are executing concurrently. Recall that process in execution is called process and process structure is divided into code region, data region,stack region & PC. The resources that are allocated to the parent thread is used by the child threads. Context switch is also easy because threads belong to the same process. At least one thread should be created in order to execute a program. Windows OS uses this thread concept. The data and code region are the same for all the threads, but have different stack region (to hold local data) and PC. 26

27 On creation of a new thread of a process, it share the resources with other threads of the same process. Threads also share data section, code section and other common resources allocated to the process. 27

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30 - Because of the mechanical nature of I/O device, these devices are very slow as compare to CPU. So I/O can be performed concurrently with other work to improve the performance of system. Ex 1: Building a house. Building an entire house requires many tasks to be done. The tasks can be done one after the other but it may require a lot of time. So as stated earlier to minimize latency and to maximize parallelism some tasks can be carried out in parallel. Like if painter is painting the walls of the house the plasterer is doing his job in some other room and at the same time the plumber is taking care of his work in the bathroom. So if the tasks are carried out concurrently it will reduce the time of work. 30

31 Processes can be cooperative or independent processes. -Independent processes are those which do not effect any other process and are not effected by any other process. -Cooperative processes are those which can effect other processes and can be effected by other processes. IPC techniques : Message passing Synchronization Shared memory Remote procedure calls (RPC)

32 Processes can be cooperative or independent processes. -Independent processes are those which do not effect any other process and are not effected by any other process. -Cooperative processes are those which can effect other processes and can be effected by other processes. 32

33 Example In the producer-consumer problem, assume p1 is producer and p2 is consumer. When producer is writing into a memory location, the consumer has to mutually exclude from consuming the data in the same location and vice versa. Example: Swipe machine in Infosys. When one person is swiping, others have to wait (mutually exclude) for their turn. Here the shared resource is swipe machine and the employees are the processes.

34 Critical Section: The area of the process which is sensitive to IPC complications A classical Critical Section Example is Railway reservation system where several booking clerks can book the seats. Assume there is only 1 seat left in Rajdhani Express from Bangalore to Delhi. Now at the same time two customers Amit and Satyendra arrive in different booking counters to book the seats. Booking clerks of both the booking counter checks the seat availability at the same time from the shared database. Clerk s process will read the copy of the available seats in to their local memory for the inspection. Both clerks have fetch 1 seat is available. Satyendra s clerk is little fast in booking and reserved the seat first, and update the database. Amit s clerk also retain the copy of the original data in memory(local), i.e 1 seat is available. Amit s clerk will also book the ticket and updated data is written in shared database. The result is both Amit and Satyendra believe that they are booked on the train, but database would show only Amit has booked the seat.

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38 Guarding this railway track is an example of semaphore. A train before entering the track should check the semaphore state, if it permits (green, signal) train can enter to this track. Once the train enters the track, the semaphore changes state (red, wait) of semaphore to stop other trains from entering the track. After leaving this track train must again change the state (green, signal) of semaphore to allow another train to enter to this track. 38

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41 Example: A video streaming on the web. Producer is server which is streaming video Consumer is video player in user s machine Buffer is the temporary memory in user s machine When buffering, the video player needs to wait to play ( consume). Once the buffering is over, the player can start playing (consuming). When you have paused, the player is not consuming. Hence the producer (server) can t produce (stream) when the buffer is full.

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44 Producer : Produce item Produce data item wait (space) In case buffer is full, wait for space signal wait(free) if buffer is in use by consumer, then wait for free signal add item in buffer Everything is fine, put item in next buffer slot signal (free) Signal that buffer is free now signal(data) Signal that data has been put in buffer Consumer : Wait(data) Wait for data in buffer(data should e there to consume) wait(free) If buffer is in use by producer, then wait for free signal get data from buffer Everything is fine, get data from buffer signal(free) Signal that buffer is free now Signal(space) Signal that space is there in buffer(buffer is not full now) Use/process the data This is application specific processing of the data. 44

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