Operating System Study Notes Department of Computer science and Engineering Prepared by TKG, SM and MS

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1 Operating System Study Notes Department of Computer science and Engineering Prepared by TKG, SM and MS Chapter1: Introduction of Operating System An operating system acts as an intermediary between the user of a computer and the computer hardware. An Operating System (OS) is software that manages the computer hardware. Hardware: It provides the basic computing resources for the system. It consists of CPU, memory and the input/output (I/O) devices. Application Programs: Define the ways in which these resources are used to solve user s computing problems. e.g., word processors, spreadsheets, compilers and web browsers. Components of a Computer System Process Management: The operating system manages many kinds of activities ranging from user programs to system programs like printer spooler, name servers, file server etc. Main-Memory Management: Primary-Memory or Main-Memory is a large array of words or bytes. Each word or byte has its own address. Main-memory provides storage that can be access directly by the CPU. That is to say for a program to be executed, it must in the main memory. File Management: A file is a collected of related information defined by its creator. Computer can store files on the disk (secondary storage), which provide long term storage. Some examples of storage media are magnetic tape, magnetic disk and optical disk. Each of these media has its own properties like speed, capacity, data transfer rate and access methods. I/O System Management: I/O subsystem hides the peculiarities of specific hardware devices from the user. Only the device driver knows the peculiarities of the specific device to which it is assigned. Secondary-Storage Management: Secondary storage consists of tapes, disks, and other media designed to hold information that will eventually be accessed in primary storage

2 (primary, secondary, cache) is ordinarily divided into bytes or words consisting of a fixed number of bytes. Each location in storage has an address; the set of all addresses available to a program is called an address space. Protection System: Protection refers to mechanism for controlling the access of programs, processes, or users to the resources defined by computer systems. Networking: generalizes network access Command-Interpreter System: interface between the user and the OS. Functions of Operating System Operating system provides many functions for ensuring the efficient operation of the system itself. Some functions are listed below Resource Allocation: Allocation of resources to the various processes is managed by operating system. Accounting: Operating system may also be used to keep track of the various computer resources and how much and which users are using these resources. Protection: Protection ensures that all access to the system resources is controlled. When several processes execute concurrently, it should not be possible for one process to interface with the other or with the operating system itself. Operating System Services Many services are provided by OS to the user s programs. Program Execution The operating system helps to load a program into memory and run it. I/O Operations Each running program may request for I/O operation and for efficiency and protection the users cannot control I/O devices directly. Thus, the operating system must provide some means to do I/O operations. File System Manipulation Files are the most important part which is needed by programs to read and write the files and files may also be created and deleted by names or by the programs. The operating system is responsible for the file management.

3 Communications Many times, one process needs to exchange information with another process, this exchange of information can takes place between the processes executing on the same computer or the exchange of information may occur between the processes executing on the different computer systems, tied together by a computer network. All these things are taken care by operating system. Error Detection It is necessary that the operating system must be aware of possible errors and should take the appropriate action to ensure correct and consistent computing. Types of Operating Systems The Operating system can perform a Single Operation and also Multiple Operations at a Time. So there are many types of Operating systems those are organized by using their Working Techniques. 1. Serial Processing: The Serial Processing Operating Systems are those which Performs all the instructions into a Sequence Manner or the Instructions those are given by the user will be executed by using the FIFO Manner means First in First Out.Mainly the Punch Cards are used for this. In this all the Jobs are firstly Prepared and Stored on the Card and after that card will be entered in the System and after that all the Instructions will be executed one by One. But the Main Problem is that a user doesn t interact with the System while he is working on the System, means the user can not be able to enter the data for Execution. 2. Batch Processing: The Batch Processing is same as the Serial Processing Technique. But in the Batch Processing Similar Types of jobs are Firstly Prepared and they are Stored on the Card and that card will be Submit to the System for the Processing. The Main Problem is that the Jobs those are prepared for Execution must be the Same Type and if a job requires for any type of Input then this will not be Possible for the user. The Batch Contains the Jobs and all those jobs will be executed without the user Intervention. 3. Multi-Programming: Execute Multiple Programs on the System at a Time and in the Multi-programming the CPU will never get idle, because with the help of Multi-Programming we can Execute Many Programs on the System and When we are Working with the Program then we can also Submit the Second or Another Program for Running and the CPU will then

4 Execute the Second Program after the completion of the First Program. And in this we can also specify our Input means a user can also interact with the System. 4. Real Time System: In this Response Time is already fixed. Means time to Display the Results after Possessing has fixed by the Processor or CPU. Real Time System is used at those Places in which we Requires higher and Timely Response. o o Hard Real Time System: In the Hard Real Time System, Time is fixed and we can t Change any Moments of the Time of Processing. Means CPU will Process the data as we Enters the Data. Soft Real Time System: In the Soft Real Time System, some Moments can be Change. Means after giving the Command to the CPU, CPU Performs the Operation after a Microsecond. 5. Distributed Operating System: Distributed Means Data is Stored and Processed on Multiple Locations. When a Data is stored on to the Multiple Computers, those are placed in Different Locations. Distributed means In the Network, Network Collections of Computers are connected with Each other. Then if we want to Take Some Data From other Computer, Then we uses the Distributed Processing System. And we can also Insert and Remove the Data from out Location to another Location. In this Data is shared between many users. And we can also Access all the Input and Output Devices are also accessed by Multiple Users. 6. Multiprocessing: In the Multi Processing there are two or More CPU in a Single Operating System if one CPU will fail, then other CPU is used for providing backup to the first CPU. With the help of Multi-processing, we can Execute Many Jobs at a Time. All the Operations are divided into the Number of CPU s. if first CPU Completed his Work before the Second CPU, then the Work of Second CPU will be divided into the First and Second. 7. Parallel operating systems: These are used to interface multiple networked computers to complete tasks in parallel. Parallel operating systems are able to use software to manage all of the different resources of the computers running in parallel, such as memory, caches, storage space, and processing power. A parallel operating system works by dividing sets of calculations into smaller parts and distributing them between the machines on a network.

5 Question and answer section: 1) To schedule the processes, they are considered. a) Infinitely long b) periodic c) heavy weight d) light weight Answer: b 2) If the period of a process is p, then the rate of the task is : a) p^2 b) 2*p c) 1/p d) p Answer: c 3) The scheduler admits a process using : a) two phase locking protocol b) admission control algorithm c) busy wait polling d) None of these Answer: c 4) The scheduling algorithm schedules periodic tasks using a static priority policy with preemption. a) Earliest deadline first b) rate monotonic c) first cum first served d) priority Answer: b 5) Rate monotonic scheduling assumes that the : a) processing time of a periodic process is same for each CPU burst b) processing time of a periodic process is different for each CPU burst

6 c) periods of all processes is the same d) None of these Answer: a 6) In rate monotonic scheduling, a process with a shorter period is assigned: a) a higher priority b) a lower priority c) None of these Answer: a 7) There are two processes P1 and P2, whose periods are 50 and 100 respectively. P1 is assigned higher priority than P2. The processing times are t1 = 20 for P1 and t2 = 35 for P2. Is it possible to schedule these tasks so that each meets its deadline using Rate monotonic scheduling? a) Yes b) No c) maybe d) none of these Answer: a 8) If a set of processes cannot be scheduled by rate monotonic scheduling algorithm, then : a) they can be scheduled by EDF algorithm b) they cannot be scheduled by EDF algorithm c) they cannot be scheduled by any other algorithm d) None of these Answer: c 9) A process P1 has a period of 50 and a CPU burst of t1 = 25, P2 has a period of 80 and a CPU burst of 35. The total CPU utilization is: a) 0.90 b) 0.74 c) 0.94

7 d) 0.80 View Answer Answer: c 10) Can the processes in the previous question be scheduled without missing the deadlines? a) Yes b) No c) maybe Answer: b Chapter2: CPU Scheduling All of the processes which are ready to execute and are placed in main memory then selection of one of those processes is known as scheduling, and after selection that process gets the control of CPU. Scheduling Criteria: The criteria for comparing CPU scheduling algorithms include the following CPU Utilization: Means keeping the CPU as busy as possible. Throughput: It is nothing but the measure of work i.e., the number of processes that are completed per time unit. Turnaround Time: The interval from the time of submission of a process to the time of completion. It is the sum of the periods spent waiting to get into memory, waiting in the ready queue, executing on the CPU and doing I/O. Waiting Time: The sum of the periods spent waiting in the ready queue. Response Time: The time from the submission of a request until the first response is produced. First Come First Served (FCFS) Scheduling 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 FIFO queue. When a process enters the ready queue, its PCB (Process Control Block) is linked onto the tail of the queue.

8 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. FCFS scheduling is non-preemptive, very simple, can be implemented with a FIFO queue, Not a good choice when there are variable burst times (CPU bound and I/O bound), drawback: causes short processes to wait for longer ones. Shortest Job First (SJF) Scheduling When the CPU is available, it is assigned to the process that has the smallest next CPU burst. If the two processes have the same length or amount of next CPU burst, FCFS scheduling is used to break the tie.

9 A more appropriate term for this scheduling method would be the shortest next CPU burst algorithm because scheduling depends on the length of the next CPU burst of a process rather than its total length. SJF works based on burst times (not total job time), difficult to get this information, must approximate it for short term scheduling, used frequently for long term scheduling, pre-emptive (SJF) or non-pre-emptive (SRTF), Special case of priority scheduling. Preemptive and Non-preemptive Algorithm The SJF algorithm can either be pre-emptive or non-pre-emptive. The choice arises when a new process arrives at the ready queue while a previous process is still executing. The next CPU burst of the newly arrived process may be shorter than what is left of the currently executing process. A pre-emptive SJF algorithm will pre-empt the currently executing process, whereas a, non-preemptive algorithm will allow the currently running process to finish its CPU burst. Pre-emptive SJF scheduling, is sometimes called shortest-remaining-time-first-scheduling,

10 Process Table Waiting time for P 1 = 10 1 = 9 Waiting time for P 2 = 1 1 = 0 Waiting time for P 3 = 17 2 = 15 Waiting time for P 4 = 5 3 Average waiting time Priority Scheduling A priority is associated with each process and the CPU is allocated to the process with the highest priority, Equal priority processes are scheduled in FCFS order. We can be provided that low numbers represent high priority or low numbers represent low priority, According to the question, we need to assume anyone of the above. Priority scheduling can be either pre-emptive or non-pre-emptive. A pre-emptive priority scheduling algorithm will pre-empt the CPU, if the priority of the newly arrived process is higher than the priority of the currently running process. A non-pre-emptive priority scheduling algorithm will simply put the new process at the head of the ready queue. A major problem with priority scheduling algorithm is indefinite blocking or starvation.

11 Process Table Waiting time for P 1 = 6 Waiting time for P 2 = 0 Waiting time for P 3 = 16 Waiting time for P 4 = 18 Waiting time for P 5 = 1 Average waiting time Round Robin (RR) Scheduling The RR scheduling algorithm is designed especially for time sharing systems. It is similar to FCFS scheduling but pre-emption is added to switch between processes. A small unit of time called a time quantum or time slice is defined. If time quantum is too large, this just becomes FCFS, since context-switching costs time, the rule of thumb is 80% of CPU bursts should be shorter than the time quantum. Process Table Let s take time quantum = 4 ms. Then the resulting RR schedule is as Follows

12 P 1 waits for the 6 ms (10 4), P 2 waits for 4 ms and P 3 waits for 7 ms. Thus, Average waiting time Multilevel Queue Scheduling: This scheduling algorithm has been created for saturations in which processes are easily classified into different groups A multilevel queue scheduling algorithm partitions the ready queue into several separate queues. The processes are permanently assigned to one queue, generally based on some property of the process, such as memory size, process priority or process type. Multilevel Feedback Queue Scheduling This scheduling algorithm allows a process to move between queues. The idea is to separate processes according to the characteristics of their CPU bursts. If a process uses too much CPU time, it will be moved to a lower priority queue. Similarly, a process that waits too long in a lower priority queue may be moved to a higher priority queue. This form of aging prevents starvation. CPU Overhead: First In First Out: Low

13 Shortest job first: Medium Priority based Scheduling: Medium Multilevel Queue Scheduling: High Throughput: First In First Out: Low Shortest job first: High Priority based Scheduling: Low Multilevel Queue Scheduling: Medium Turn Around Time: First In First Out: High Shortest job first: Medium Priority based Scheduling: Medium Multilevel Queue Scheduling: Medium Response Time: First In First Out: Low Shortest job first: Medium Priority based Scheduling: High Multilevel Queue Scheduling: Medium Question answer section: 1) Which of the following do not belong to queues for processes? a) Job Queue b) PCB queue c) Device Queue d) Ready Queue Answer: b

14 2) When the process issues an I/O request : a) It is placed in an I/O queue b) It is placed in a waiting queue c) It is placed in the ready queue d) It is placed in the Job queue Answer: a 3) When a process terminates : (Choose Two) a) It is removed from all queues b) It is removed from all, but the job queue c) Its process control block is de-allocated d) Its process control block is never de-allocated View Answer Answer: a and c 4) What is a long-term scheduler? a) It selects which process has to be brought into the ready queue b) It selects which process has to be executed next and allocates CPU c) It selects which process to remove from memory by swapping d) None of these Answer: a 5) If all processes I/O bound, the ready queue will almost always be, and the Short term Scheduler will have a to do. a) Full, little b) full, lot c) empty, little d) empty, lot Answer: c 6) What is a medium-term scheduler? a) It selects which process has to be brought into the ready queue b) It selects which process has to be executed next and allocates CPU

15 c) It selects which process to remove from memory by swapping d) None of these Answer: c 7) What is a short-term scheduler? a) It selects which process has to be brought into the ready queue b) It selects which process has to be executed next and allocates CPU c) It selects which process to remove from memory by swapping d) None of these Answer: b 8) The primary distinction between the short term scheduler and the long term scheduler is: a) The length of their queues b) The type of processes they schedule c) The frequency of their execution d) None of these Answer: c 9) The only state transition that is initiated by the user process itself is : a) block b) wakeup c) dispatch d) None of these Answer: a 10) In a time-sharing operating system, when the time slot given to a process is completed, the process goes from the running state to the : a) Blocked state b) Ready state c) Suspended state d) Terminated state

16 Answer: b 11) In a multi-programming environment : a) the processor executes more than one process at a time b) the programs are developed by more than one person c) more than one process resides in the memory d) a single user can execute many programs at the same time Answer: c 12) Suppose that a process is in Blocked state waiting for some I/O service. When the service is completed, it goes to the : a) Running state b) Ready state c) Suspended state d) Terminated state Answer: b 13) The context of a process in the PCB of a process does not contain : a) the value of the CPU registers b) the process state c) memory-management information d) context switch time Answer: d 14) Which of the following need not necessarily be saved on a context switch between processes? (GATE CS 2000) a) General purpose registers b) Translation look-aside buffer c) Program counter d) All of these Answer: b 14) Which of the following does not interrupt a running process? (GATE CS 2001) a) A device

17 b) Timer c) Scheduler process d) Power failure Answer: c 15) Several processes access and manipulate the same data concurrently and the outcome of the execution depends on the particular order in which the access takes place, is called a(n). a) Shared Memory Segments b) Entry Section c) Race condition d) Process Synchronization Answer: c 16) Which of the following state transitions is not possible? a) Blocked to running b) ready to running c) blocked to ready d) running to blocked Answer: a Chapter3: Processes Process: A process can be defined in any of the following ways A process is a program in execution. It is an asynchronous activity. It is the entity to which processors are assigned. It is the dispatch-able unit. It is the unit of work in a system.

18 A process is more than the program code. It also includes the current activity as represented by following: Current value of Program Counter (PC) Contents of the processors registers Value of the variables The process stack which contains temporary data such as subroutine parameter, return address, and temporary variables. A data section that contains global variables. Process in Memory: Each process is represented in the as by a Process Control Block (PCB) also called a task control block. PCB: A process in an operating system is represented by a data structure known as a process control block (PCB) or process descriptor. The PCB contains important information about the specific process including The current state of the process i.e., whether it is ready, running, waiting, or whatever. Unique identification of the process in order to track which is which information. A pointer to parent process. Similarly, a pointer to child process (if it exists). The priority of process (a part of CPU scheduling information). Pointers to locate memory of processes. A register save area.

19 The processor it is running on. Process State Model Process state: The process state consist of everything necessary to resume the process execution if it is somehow put aside temporarily. The process state consists of at least following: Code for the program. Program s static data. Program s dynamic data. Program s procedure call stack. Contents of general purpose register. Contents of program counter (PC) Contents of program status word (PSW). Operating Systems resource in use. A process goes through a series of discrete process states. New State: The process being created. Running State: A process is said to be running if it has the CPU, that is, process actually using the CPU at that particular instant. Blocked (or waiting) State: A process is said to be blocked if it is waiting for some event to happen such that as an I/O completion before it can proceed. Note that a process is unable to run until some external event happens. Ready State: A process is said to be ready if it use a CPU if one were available. A ready state process is ready to run but temporarily stopped running to let another process run.

20 Terminated state: The process has finished execution. Schedulers: A process migrates among various scheduling queues throughout its lifetime. The OS must select for scheduling purposes, processes from those queues in some fashion. The selection process is carried out by the appropriate scheduler. Long Term Scheduler: A long term scheduler or job scheduler selects processes from job pool (mass storage device, where processes are kept for later execution) and loads them into memory for execution. The long term scheduler controls the degree of multiprogramming (the number of processes in memory). Short Term Scheduler: A short term scheduler or CPU scheduler selects from the main memory among the processes that are ready to execute and allocates the CPU to one of them. Medium Term Scheduler: The medium term scheduler available in all systems which is responsible for the swapping in and out operations which means loading the process into, main memory from secondary memory (swap in) and take out the process from main memory and store it into the secondary memory (swap out).

21 Dispatcher: It is the module that gives control of the CPU to the process selected by the short term scheduler. Functions of Dispatcher: Switching context, Switching to user mode, and Jumping to the proper location in the user program to restart that program. Question answer section: 1. Which process can be affected by other processes executing in the system? a) Cooperating process b) child process c) parent process

22 d) init process Answer: a 2. When several processes access the same data concurrently and the outcome of the execution depends on the particular order in which the access takes place, is called a) dynamic condition b) race condition c) essential condition d) critical condition Answer: b 3. If a process is executing in its critical section, then no other processes can be executing in their critical section. This condition is called a) mutual exclusion b) critical exclusion c) synchronous exclusion d) asynchronous exclusion View Answer Answer: a 4. Which one of the following is a synchronization tool? a) Thread b) pipe c) semaphore d) socket Answer: c 5. A semaphore is a shared integer variable a) that cannot drop below zero b) that cannot be more than zero c) that cannot drop below one d) that cannot be more than one Answer: a

23 6. Mutual exclusion can be provided by the a) mutex locks b) binary semaphores c) both (a) and (b) d) none of the mentioned Answer: c Explanation: Binary Semaphores are known as mutex locks. 7. When high priority task is indirectly preempted by medium priority task effectively inverting the relative priority of the two tasks, the scenario is called a) priority inversion b) priority removal c) priority exchange d) priority modification Answer: a 8. Process synchronization can be done on a) hardware level b) software level c) both (a) and (b) d) none of the mentioned Answer: c 9. A monitor is a module that encapsulates a) shared data structures b) procedures that operate on shared data structure c) synchronization between concurrent procedure invocation d) all of the mentioned Answer: d 10. To enable a process to wait within the monitor, a) a condition variable must be declared as condition b) condition variables must be used as Boolean objects c) semaphore must be used

24 d) all of the mentioned Answer: a Chapter4: Inter-Process Communication Processes executing concurrently in the operating system may be either independent or cooperating processes. A process is independent, if it can t affect or be affected by the other processes executing in the system. Any process that shares data with other processes is a cooperating process. Advantages of process cooperation are information sharing, computation speed up, modularity and convenience to work on many tasks at the same time. Cooperating processes require an Inter-Process Communication (IPC) mechanism that will allow them to exchange data and information. There are two fundamental models of IPC: o Shared memory: In the shared memory model, a region of memory that is shared by cooperating process is established. Process can then exchange information by reading and writing data to the shared region. o Message passing: In the message passing model, communication takes place by means of messages exchanged between the cooperating processes. Question and Answer section: 1) Inter process communication : a) allows processes to communicate and synchronize their actions when using the same address space. b) Allows processes to communicate and synchronize their actions without using the same address space.

25 c) Allows the processes to only synchronize their actions without communication. d) None of these View Answer Answer: b 2) Message passing system allows processes to : a) communicate with one another without resorting to shared data. b) Communicate with one another by resorting to shared data. c) Share data d) name the recipient or sender of the message View Answer Answer: a 3) An IPC facility provides at least two operations : (choose two) a) write message b) delete message c) send message d) receive message Answer: c and d 4) Messages sent by a process : a) have to be of a fixed size b) have to be a variable size c) can be fixed or variable sized d) None of these Answer: c 5) The link between two processes P and Q to send and receive messages is called : a) communication link b) message-passing link c) synchronization link d) All of these Answer: a

26 6) Which of the following are TRUE for direct communication :(choose two) a) A communication link can be associated with N number of process(n = max. number of processes supported by system) b) A communication link can be associated with exactly two processes c) Exactly N/2 links exist between each pair of processes(n = max. number of processes supported by system) d) Exactly one link exists between each pair of processes Answer: b and d 7) In indirect communication between processes P and Q : a) there is another process R to handle and pass on the messages between P and Q b) there is another machine between the two processes to help communication c) there is a mailbox to help communication between P and Q d) None of these Answer: c 8) In the non blocking send : a) the sending process keeps sending until the message is received b) the sending process sends the message and resumes operation c) the sending process keeps sending until it receives a message d) None of these Answer: b 9) In the Zero capacity queue : (choose two) a) the queue has zero capacity b) the sender blocks until the receiver receives the message c) the sender keeps sending and the messages don t wait in the queue d) the queue can store at least one message Answer: a and b 10) The Zero Capacity queue : a) is referred to as a message system with buffering

27 b) is referred to as a message system with no buffering c) is referred to as a link d) None of these Answer: b 11) Bounded capacity and Unbounded capacity queues are referred to as: a) Programmed buffering b) Automatic buffering c) User defined buffering d) No buffering Answer: b Chapter5: Concurrency and Process Synchronization & Deadlock Process Synchronization means sharing system resources by processes in a way that, Concurrent access to shared data is handled thereby minimizing the chance of inconsistent data. Maintaining data consistency demands mechanisms to ensure synchronized execution of cooperating processes. Process Synchronization was introduced to handle problems that arose while multiple process executions. Some of the problems are discussed below. Critical Section Problem A Critical Section is a code segment that accesses shared variables and has to be executed as an atomic action. It means that in a group of cooperating processes, at a given point of time, only one process must be executing its critical section. If any other process also wants to execute its critical section, it must wait until the first one finishes.

28 Solution to Critical Section Problem A solution to the critical section problem must satisfy the following three conditions : 1. Mutual Exclusion Out of a group of cooperating processes, only one process can be in its critical section at a given point of time. 2. Progress If no process is in its critical section, and if one or more threads want to execute their critical section then any one of these threads must be allowed to get into its critical section. 3. Bounded Waiting After a process makes a request for getting into its critical section, there is a limit for how many other processes can get into their critical section, before this process's request is

29 granted. So after the limit is reached, system must grant the process permission to get into its critical section. Synchronization Hardware Many systems provide hardware support for critical section code. The critical section problem could be solved easily in a single-processor environment if we could disallow interrupts to occur while a shared variable or resource is being modified. In this manner, we could be sure that the current sequence of instructions would be allowed to execute in order without pre-emption. Unfortunately, this solution is not feasible in a multiprocessor environment. Disabling interrupt on a multiprocessor environment can be time consuming as the message is passed to all the processors. This message transmission lag, delays entry of threads into critical section and the system efficiency decreases. Mutex Locks As the synchronization hardware solution is not easy to implement fro everyone, a strict software approach called Mutex Locks was introduced. In this approach, in the entry section of code, a LOCK is acquired over the critical resources modified and used inside critical section, and in the exit section that LOCK is released. As the resource is locked while a process executes its critical section hence no other process can access it. Semaphores In 1965, Dijkstra proposed a new and very significant technique for managing concurrent processes by using the value of a simple integer variable to synchronize the progress of interacting processes. This integer variable is called semaphore. So it is basically a

30 synchronizing tool and is accessed only through two low standard atomic operations, wait and signal designated by P() and V() respectively. The classical definition of wait and signal are : Wait: decrement the value of its argument S as soon as it would become non-negative. Signal: increment the value of its argument, S as an individual operation. Properties of Semaphores 1. Simple 2. Works with many processes 3. Can have many different critical sections with different semaphores 4. Each critical section has unique access semaphores 5. Can permit multiple processes into the critical section at once, if desirable. Types of Semaphores Semaphores are mainly of two types: 1. Binary Semaphore It is a special form of semaphore used for implementing mutual exclusion, hence it is often called Mutex. A binary semaphore is initialized to 1 and only takes the value 0 and 1 during execution of a program. 2. Counting Semaphores These are used to implement bounded concurrency.

31 Limitations of Semaphores 1. Priority Inversion is a big limitation os semaphores. 2. Their use is not enforced, but is by convention only. 3. With improper use, a process may block indefinitely. Such a situation is called Deadlock. We will be studying deadlocks in details in coming lessons. Chapter6: Deadlock What is a Deadlock? Deadlocks are a set of blocked processes each holding a resource and waiting to acquire a resource held by another process.

32 How to avoid Deadlocks Deadlocks can be avoided by avoiding at least one of the four conditions, because all this four conditions are required simultaneously to cause deadlock. 1. Mutual Exclusion Resources shared such as read-only files do not lead to deadlocks but resources, such as printers and tape drives, requires exclusive access by a single process. 2. Hold and Wait In this condition processes must be prevented from holding one or more resources while simultaneously waiting for one or more others. 3. No Preemption Preemption of process resource allocations can avoid the condition of deadlocks, where ever possible. 4. Circular Wait Circular wait can be avoided if we number all resources, and require that processes request resources only in strictly increasing(or decreasing) order. Handling Deadlock The above points focus on preventing deadlocks. But what to do once a deadlock has occurred. Following three strategies can be used to remove deadlock after its occurrence. 1. Preemption

33 We can take a resource from one process and give it to other. This will resolve the deadlock situation, but sometimes it does causes problems. 2. Rollback In situations where deadlock is a real possibility, the system can periodically make a record of the state of each process and when deadlock occurs, roll everything back to the last checkpoint, and restart, but allocating resources differently so that deadlock does not occur. 3. Kill one or more processes Previous Year Solved Questions with Explanation: GATE A critical section is a program segment (a) which should run in a certain specified amount of time (b) which avoids deadlocks (c) where shared resources are accessed (d) which must be enclosed by a pair of semaphore operations, P and V Ans: option (c) GATE A solution to the Dining Philosophers Problem which avoids deadlock is (a) ensure that all philosophers pick up the left fork before the right fork (b) ensure that all philosophers pick up the right fork before the left fork (c) ensure that one particular philosopher picks up the left fork before the right fork, and that all other philosophers pick up the right fork before the left fork (d) None of the above Ans: option (c) GATE Consider the methods used by processes P1 and P2 for accessing their critical sections whenever needed, as given below. The initial values of shared boolean variables S1 and S2 are randomly assigned.

34 Method Used by P1 while (S1 == S2) ; Critica1 Section S1 = S2; Method Used by P2 while (S1!= S2) ; Critica1 Section S2 = not (S1); Which one of the following statements describes the properties achieved? (a) Mutual exclusion but not progress (b) Progress but not mutual exclusion (c) Neither mutual exclusion nor progress (d) Both mutual exclusion and progress Ans: option (a) Explanation: Principle of Mutual Exclusion: No two processes may be simultaneously present in the critical section at the same time. That is, if one process is present in the critical section other should not be allowed. P1 can enter critical section only if S1 is not equal to S2, and P2 can enter critical section only if S1 is equal to S2. Therefore Mutual Exclusion is satisfied. Progress: no process running outside the critical section should block the other interested process from entering critical section whenever critical section is free. Suppose P1 after executing critical section again want to execute the critical section and P2 dont want to enter the critical section, then in that case P1 has to unnecesarily wait for P2. Hence progress is not satisfied. GATE A counting semaphore was initialized to 10. Then 6 P (wait) operations and 4V (signal) operations were completed on this semaphore. The resulting value of the semaphore is

35 (a) 0 (b) 8 (c) 10 (d) 12 Ans: option(b) Explanation: 6P => decrements the semaphore 6 times. Hence, the value becomes 4. 4V => increments the semaphore 4 times. Hence, the value becomes 8. Note: The positive value of counting semaphore indicates that those many down (P) operations can be carried out successfully. The negative value of counting semaphore indicates that the number of blocked processes. GATE Let m[0] m[4] be mutex (binary semaphores) and P[0]. P[4] be processes. Suppose each process P[i] executes the following: wait (m[i]);wait (m[(i+1) mode 4]);... release (m[i]); release (m[(i+1)mod 4]); This could cause (a) Thrashing (c) Starvation, but not deadlock (b) Deadlock (d) None of the above Ans: option (b) Explanation: Deadlock occurs, if each process gets preempted after executing wait(m[i]); GATE The enter_cs() and leave_cs() functions to implement critical section of a process are realized using test-and-set instruction as follows: void enter_cs(x) { while test-and-set(x) ; } void leave_cs(x) {

36 X = 0; } In the above solution, X is a memory location associated with the CS and is initialized to 0. Now consider the following statements: I. The above solution to CS problem is deadlock-free II. The solution is starvation free. III. The processes enter CS in FIFO order. IV More than one process can enter CS at the same time. Which of the above statements is TRUE? (a) I only (b) I and II (c) II and III (d) IV only Ans: option (a) Explanation: The test-and-set instruction is an instruction used to write to a memory location and return its old value as a single atomic (i.e., non-interruptible) operation. Since it is an atomic instruction it guarantees mutual exclusion. Reference: GATE The following program consists of 3 concurrent processes and 3 binary semaphores. The semaphores are initialized as S0=1, S1=0, S2=0. Process P0 Process P1 Process P2 while (true) { wait (S0); print (0); release (S1); release (S2); } wait (S1); Release (S0); wait (S2); How many times will process P0 print '0'? (a) At least twice (b) Exactly twice (c) Exactly thrice (d) Exactly once Ans: option (a) Explanation: release (S0);

37 P0 will execute first because only S0=1. Hence it will print 0 (for the first time). Also P0 releases S1 and S2. Since S1=1 and S2=1, therefore P1 or P2, any one of them can be executed. Let us assume that P1 executes and releases S0 (Now value of S0 = 1). Note that P1 process is completed. Now S0=1 and S2=1, hence either P0 can execute or P2 can execute. Let us check both the conditions:- 1. Let us assume that P2 executes, and releases S0 and completes its execution. Now P0 executes; S0=0 and prints 0 (i.e. second 0). And then releases S1 and S2. But note that P1 and P2 processes have already finished their execution. Again if P0 tries to execute it goes into sleep condition because S0=0. Therefore, minimum number of times '0' gets printed is Now, let us assume that P0 executes. Hence S0=0, (due to wait(s0)), and it will print 0 (second 0) and releases S1 and S2. Now only P2 can execute, because P1 has already completed its execution and P0 cannot execute because S0 = 0. Now P2 executes and releases S0 (i.e. S0=1) and finishes its execution. Now P0 starts its execution and again prints 0 (thrid 0) and releases S1 and S2 (Note that now S0=0). P1 and P2 has already completed its execution therefore again P1 takes its turn, but since S0=0, it goes into sleep condition. And the processes P1 and P2 which could wakeup P0 has already finished their execution.therefore, maximum number of times '0' gets printed is 2. GATE Fetch_And_Add(X,i) is an atomic Read-Modify-Write instruction that reads the value of memory location X, increments it by the value i, and returns the old value of X. It is used in the pseudocode shown below to implement a busy-wait lock. L is an unsigned integer shared variable initialized to 0. The value of 0 corresponds to lock being available, while any non-zero value corresponds to the lock being not available. AcquireLock(L){ while (Fetch_And_Add(L,1)) L = 1; } ReleaseLock(L){

38 L = 0; } This implementation (a) fails as L can overflow (b) fails as L can take on a non-zero value when the lock is actually available (c) works correctly but may starve some processes (d) works correctly without starvation Ans: option (b) Explanation: Assume that L=0, that means the lock is now available. A process P1 wants to acquire the lock by executing the Acquire Lock () function. We can see that the while loop fails because the Fetch_And_Add instruction returns the previous value of L, which was 0. (Note that after the execution of the atomic instruction, the present value of L is now 1). Since the returned value was 0, the while loop fails and P1 process comes out of the while loop and hence acquires the lock. Now scheduler schedules another process P2 and context switching takes place. P2 tries to acquire the lock by executing the Acquire Lock () function. But it goes on executing the while loop infinitely (because the atomic instruction always returns a non-zero value and the while loop condition is always true). After some time, scheduler again schedules P1. We assume that P2 was stopped soon after the Fetch_And_Add() instruction returned the value. Hence L has some non-zero value. Now P1 releases the lock L. That means now L=0. Assume that again context switch takes place and P2 arrives. We have assumed that P2 was switched out when it executed the Fetch_And_Add instruction and a non-zero value has been returned. Since the returned value was non-zero the condition becomes true and the statement L=1 is executed. Hence now L=1. Again the while loop is executed, Fetch_And_Add instruction will return value 1 and hence again the while loop enters

39 into infinite loop. Now none of the process is able to acquire the lock. Therefore the above implementation fails. GATE Two processes, P1 and P2, need to access a critical section of code. Consider the following synchronization construct used by the processes: /* P1 */ /* P2 */ while (true) { while (true) { wants1 = true; wants2 = true; while (wants2 == true); while (wants1==true); /* Critical /* Critical Section */ Section */ wants1=false; wants2 = false; } } /* Remainder section */ /* Remainder section */ Here, wants1 and wants2 are shared variables, which are initialized to false. Which one of the following statements is TRUE about the above construct? (a) It does not ensure mutual exclusion. (b) It does not ensure bounded waiting. (c) It requires that processes enter the critical section in strict alternation. (d) It does not prevent deadlocks, but ensures mutual exclusion. Ans: option (d) Explanation: The code ensures the condition of mutual exclusion: Assume P1 is initiated. It sets wants1=true. Now since wants2 = false, P1 exists from its while loop and enters its critical section. Now suppose context switch takes place and P2 gets executed. Now it sets wants2=true, and now enters the while loop and remains busy till P1 comes out of the critical section and sets wants1=false, because wants1=true ( as set by P1). So we can see that the mutual exclusion condition is satisfied.

40 The code does not prevent deadlock: Assume that P1 starts its execution. It sets wants1=true and then gets preempted. Now P2 starts its execution. P2 sets wants2=true and suddenly gets preempted. Now P1 starts execution; it enters the while loop and finds that wants2=true and remains busy in the while loop. Now P1 gets preempted. P2 enters into execution; it enters the while loop and finds that wants1=true remains busy in the while loop. Hence both P1 and P2 remains busy forever. GATE The atomic fetch-and-set x, y instruction unconditionally sets the memory location x to 1 and fetches the old value of x in y without allowing any intervening access to the memory location x. consider the following implementation of P and V functions on a binary semaphore. void P (binary_semaphore *s) { unsigned y; unsigned *x = &(s->value); do { fetch-and-set x, y; } while (y); } void V (binary_semaphore *s) { S->value = 0; } Which one of the following is true? (a) The implementation may not work if context switching is disabled in P. (b) Instead of using fetch-and-set, a pair of normal load/store can be used (c) The implementation of V is wrong (d) The code does not implement a binary semaphore

41 Ans: option (a) Explanation: Assume that P1 process enters the P function. Initially if the value of semaphore is 1, according to the definition of the fetch-and-set instruction, the value of y will be 1 when the fetch-and-set instruction is executed. Hence the while loop goes on executing until other processes execute the V function. But if context switching is disabled in P while loop will keep on executing and other processes are not allowed to execute. GATE-2006 Linked Answer Questions for 11 & 12 Barrier is a synchronization construct where a set of processes synchronizes globally i.e. each process in the set arrives at the barrier and waits for all others to arrive and then all processes leave the barrier. Let the number of processes in the set be three and S be a binary semaphore with the usual P and V functions. Consider the following C implementation of a barrier with line numbers shown on left. void barrier (void) { 1: P(S); 2: process_arrived++; 3: V(S); 4: while (process_arrived!=3); 5: P(S); 6: process_left++; 7: if (process_left==3) { 8: process_arrived = 0; 9: process_left = 0; 10: } 11: V(S); }

42 The variables process_arrived and process_left are shared among all processes and are initialized to zero. In a concurrent program all the three processes call the barrier function when they need to synchronize globally. 11. The above implementation of barrier is incorrect. Which one of the following is true? (a) The barrier implementation is wrong due to the use of binary semaphore S (b) The barrier implementation may lead to a deadlock if two barrier in invocations are used in immediate succession. (c) Lines 6 to 10 need not be inside a critical section (d) The barrier implementation is correct if there are only two processes instead of three. Ans: option (b) Explanation: It is mentioned that there are 3 processes. Let it be P1, P2 & P3. Every process arrives at the barrier that is it completes its first section of the code. Hence process_arrived = 3. Now assume that P1 continues its execution and leaves the barrier and makes process_left=1. Now assume that P1 again invokes the barrier function immediately. At this stage, process_arrived =4. P1 now waits. Soon P2 leaves the barrier, which makes process_left=2. Now P3 leaves the barrier which makes process_left=3. "If" condition is true and now process_arrived and process_left is reset to 0. We know that P1 is waiting. At this stage assume that P2 invokes the barrier function, hence process_arrived=1 and it waits. P3 invokes the barrier function, hence process_arrived=2. All process have reached the barrier but since the process_arrived=2, all processes keeps on waiting. Hence deadlock. 12. Which one of the following rectifies the problem in the implementation? (a) Lines 6 to 10 are simply replaced by process_arrived-- (b) At the beginning of the barrier the first process to enter the barrier waits until process_arrived becomes zero before proceeding to execute P(S).

43 (c) Context switch is disabled at the beginning of the barrier and re-enabled at the end. (d) The variable process_left is made private instead of shared. Ans: option (b) Additional Questions 1. Which process can be affected by other processes executing in the system? a) cooperating process b) child process c) parent process d) init process Answer: a 2. When several processes access the same data concurrently and the outcome of the execution depends on the particular order in which the access takes place, is called a) dynamic condition b) race condition c) essential condition d) critical condition Answer: b 3. If a process is executing in its critical section, then no other processes can be executing in their critical section. This condition is called a) mutual exclusion b) critical exclusion c) synchronous exclusion d) asynchronous exclusion Answer: a

44 4. Which one of the following is a synchronization tool? a) thread b) pipe c) semaphore d) socket Answer: c 5. A semaphore is a shared integer variable a) that cannot drop below zero b) that cannot be more than zero c) that cannot drop below one d) that cannot be more than one Answer: a 6. Mutual exclusion can be provided by the a) mutex locks b) binary semaphores c) both (a) and (b) d) none of the mentioned Answer: c Explanation: Binary Semaphores are known as mutex locks. 7. When high priority task is indirectly preempted by medium priority task effectively inverting the relative priority of the two tasks, the scenario is called a) priority inversion b) priority removal c) priority exchange d) priority modification Answer: a 8. Process synchronization can be done on a) hardware level b) software level

45 c) both (a) and (b) d) none of the mentioned View Answer Answer: c 9. A monitor is a module that encapsulates a) shared data structures b) procedures that operate on shared data structure c) synchronization between concurrent procedure invocation d) all of the mentioned Answer: d 10. To enable a process to wait within the monitor, a) a condition variable must be declared as condition b) condition variables must be used as boolean objects c) semaphore must be used d) all of the mentioned View Answer Answer: a 11. In order to allow only one process to enter its critical section, binary semaphore are initialized to: A. 0 B. 1 C. 2 D. 3 Ans: B. 12. A deadlock in an operating system is A. desirable process B. undesirable process C. definite waiting process D. all of the above Ans: B. 13. Three concurrent processes X, Y, and Z execute three different code segments that access andupdate certain shared variables. Process X executes the P operation (i.e., wait) on semaphores a, band c; process Y executes the P operation on semaphores b, c and d; process Z executes the Poperation on semaphores c, d, and a before entering the respective code segments. Aftercompleting the execution of its code segment, each process invokes the V operation (i.e.,

46 signal)on its three semaphores. All semaphores are binary semaphores initialized to one. Which one ofthe following represents a deadlock free order of invoking the P operations by the processes? A. X :P (a )P B.P C. Y :P (b )P C.P D. Z :P (c )P D.P A. B. X :P (b )P A.P C. Y :P (b )P C.P D. Z :P (a )P C.P D. C. X :P (b )P A.P C. Y :P (c )P B.P D. Z :P (a )P C.P D. D. X :P (a )P B.P C. Y :P (c )P B.P D. Z :P (c )P D.P A. Ans: B. 14. A semaphore count of negative n means(s = -n) that the queue contains. Waiting processes: A. n+1 B. n C. n-1 D. 0 Ans: A. 15.Which of the following approaches do not require knowledge of the system state? A.deadlock detection. B.deadlock prevention. C.deadlock avoidance. D.none of the above. Ans: D. Chapter7: Threads Thread is an execution unit which consists of its own program counter, a stack, and a set of registers. Threads are also known as Lightweight processes. Threads are popular way to improve application through parallelism. The CPU switches rapidly back and forth among the threads giving illusion that the threads are running in parallel. As each thread has its own independent resource for process execution, multpile processes can be executed parallely by increasing number of threads.

47 Types of Thread There are two types of threads: User Threads Kernel Threads User threads are above the kernel and without kernel support. These are the threads that application programmers use in their programs. Kernel threads are supported within the kernel of the OS itself. All modern OSs support kernel level threads, allowing the kernel to perform multiple simultaneous tasks and/or to service multiple kernel system calls simultaneously. Multithreading Models The user threads must be mapped to kernel threads, by one of the following strategies. Many-To-One Model One-To-One Model Many-To-Many Model