2 Process and Thread_java20231123-3.pptx

It's a java based presentation regarding the process and threats

1. Operating system BIT253CO By: Er Suvash Chandra Gautam (PhD Scholar) December , 2023

2. Processes and Threads

3. Introduction A process is an instance of a program that is being executed.  When we run a program, it does not execute directly. It takes some time to follow all the steps required to execute the program, and following these execution steps is known as a process. A process can create other processes to perform multiple tasks at a time; the created processes are known as  clone or child process , and the main process is known as the  parent process . Each process contains its own memory space and does not share it with the other processes. It is known as the active entity. A typical process remains in the below form in memory. 12/31/2023 Process and Thread 3

4. Process Process memory  is divided into four sections for efficient working : The  Text section  is made up of the compiled program code, read in from non-volatile storage when the program is launched. The  Data section  is made up of the global and static variables, allocated and initialized prior to executing the main. The  Heap  is used for the dynamic memory allocation and is managed via calls to new, delete, malloc, free, etc. The  Stack  is used for local variables. Space on the stack is reserved for local variables when they are declared. 12/31/2023 Process and Thread 4

5. Process  A process is an 'active' entity as opposed to the program which is considered to be a 'passive' entity. Attributes held by the process include hardware state, memory, CPU, etc. 12/31/2023 Process and Thread 5

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7. Process State NEW : A new process is being created. READY : A process is ready and waiting to be allocated to a processor. RUNNING : The program is being executed. WAITING : Waiting for some event to happen or occur. TERMINATED : Execution finished. 12/31/2023 Process and Thread 7

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9. Features of Process Each time we create a process, we need to make a separate system call for each process to the OS. The  fork () function creates the process. Each process exists within its own address or memory space. Each process is independent and treated as an isolated process by the OS. Processes need IPC (Inter-process Communication) in order to communicate with each other. A proper synchronization between processes is not required. 12/31/2023 Process and Thread 9

10. Process Model A process is a program under execution that consists of a number of elements including, program code and a set of data. To execute a program, a process has to be created for that program. Here the process may or may not run but if it is in a condition of running then that has to be maintained by the OS for appropriate progress of the process to be gained. 12/31/2023 Process and Thread 10

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12. Process Model Running:  It means a process that is currently being executed. Assuming that there is only a single processor in the below execution process, so there will be at most one processor at a time that can be running in the state. Ready:  It means a process that is prepared to execute when given the opportunity by the OS. Blocked/Waiting:  It means that a process cannot continue executing until some event occurs like for example, the completion of an input-output operation. New:  It means a new process that has been created but has not yet been admitted by the OS for its execution. A new process is not loaded into the main memory, but its process control block (PCB) has been created. Exit/Terminate:  A process or job that has been released by the OS, either because it is completed or is aborted for some issue. 12/31/2023 Process and Thread 12

13. Possible State Transitions Null -> New:  A new process is created for the execution of a process. New -> Ready:  The system will move the process from new to ready state and now it is ready for execution. Here a system may set a limit so that multiple processes can’t occur otherwise there may be a performance issue. Ready -> Running:  The OS now selects a process for a run and the system chooses only one process in a ready state for execution. Running -> Exit:  The system terminates a process if the process indicates that is now completed or if it has been aborted. 12/31/2023 Process and Thread 13

14. Possible State Transitions Running -> Ready:  The reason for which this transition occurs is that when the running process has reached its maximum running time for uninterrupted execution. An example of this can be a process running in the background that performs some maintenance or other functions periodically. Running -> Blocked:  A process is put in the blocked state if it requests for something it is waiting. Like, a process may request some resources that might not be available at the time or it may be waiting for an I/O operation or waiting for some other process to finish before the process can continue. Blocked -> Ready:  A process moves from blocked state to the ready state when the event for which it has been waiting. Ready -> Exit:  This transition can exist only in some cases because, in some systems, a parent may terminate a child’s process at any time. 12/31/2023 Process and Thread 14

15. Process Control Block While creating a process, the operating system performs several operations. To identify the processes, it assigns a process identification number (PID) to each process. As the operating system supports multi-programming, it needs to keep track of all the processes. For this task, the  process control block  (PCB) is used to track the process’s execution status. Each block of memory contains information about the process state, program counter, stack pointer, status of opened files,  scheduling algorithms , etc. 12/31/2023 Process and Thread 15

16. Process Control Block All this information is required and must be saved when the process is switched from one state to another. When the process makes a transition from one state to another, the operating system must update information in the process’s PCB. A process control block (PCB) contains information about the process, i.e. registers, quantum, priority, etc. The process table is an array of PCBs, that means logically contains a  PCB  for all of the current processes in the system. 12/31/2023 Process and Thread 16

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18. Process Control Block Pointer:  It is a stack pointer that is required to be saved when the process is switched from one state to another to retain the current position of the process. Process state:  It stores the respective state of the process. Process number:  Every process is assigned a unique id known as process ID or PID which stores the process identifier. Program counter:  It stores the counter, :  which contains the address of the next instruction that is to be executed for the process. Register:  These are the CPU registers which include the accumulator, base, registers, and general-purpose registers. Memory limits:  This field contains the information about memory management system used by the operating system. This may include page tables, segment tables, etc. Open files list   :  This information includes the list of files opened for a process. 12/31/2023 Process and Thread 18

19. Introduction to Thread A thread is the subset of a process and is also known as the lightweight process. A process within a process is called thread. A process can have more than one thread, and these threads are managed independently by the scheduler. All the threads within one process are interrelated to each other. Threads have some common information, such as  data segment, code segment, files, etc.,  that is shared to their peer threads. But contains its own registers, stack, and counter. 12/31/2023 Process and Thread 19

20. Features of Thread Threads share data, memory, resources, files, etc., with their peer threads within a process. One system call is capable of creating more than one thread. Each thread has its own stack and register. Threads can directly communicate with each other as they share the same address space. Threads need to be synchronized in order to avoid unexpected scenarios. 12/31/2023 Process and Thread 20

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22. How does thread work? When a process starts, OS assigns the memory and resources to it. Each thread within a process shares the memory and resources of that process only. Threads are mainly used to improve the processing of an application. In reality, only a single thread is executed at a time, but due to fast context switching between threads gives an illusion that threads are running parallelly. If a single thread executes in a process, it is known as  a single-threaded  And if multiple threads execute simultaneously, then it is known as  multithreading. 12/31/2023 Process and Thread 22

23. Types of Thread 1. User Level Thread: As the name suggests, the user-level threads are only managed by users, and the kernel does not have its information. These are faster, easy to create and manage. The kernel takes all these threads as a single process and handles them as one process only. The user-level threads are implemented by user-level libraries, not by the system calls. 12/31/2023 Process and Thread 23

24. Advantages of User-level threads The user threads can be easily implemented than the kernel thread. User-level threads can be applied to such types of operating systems that do not support threads at the kernel-level. It is faster and efficient. Context switch time is shorter than the kernel-level threads. It does not require modifications of the operating system. User-level threads representation is very simple. The register, PC, stack, and mini thread control blocks are stored in the address space of the user-level process. It is simple to create, switch, and synchronize threads without the intervention of the process. 12/31/2023 Process and Thread 24

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26. Types of Thread 2. Kernel-Level Thread: The kernel-level threads are handled by the Operating system and managed by its kernel. These threads are slower than user-level threads because context information is managed by the kernel. To create and implement a kernel-level thread, we need to make a system call. 12/31/2023 Process and Thread 26

27. Advantages of Kernel-level threads The kernel-level thread is fully aware of all threads. The scheduler may decide to spend more CPU time in the process of threads being large numerical. The kernel-level thread is good for those applications that block the frequency. 12/31/2023 Process and Thread 27

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29. Feature User Level Threads Kernel Level Threads Implemented by It is implemented by the users. It is implemented by the OS. Context switch time Its time is less. Its time is more. Multithreading Multithread applications are unable to employ multiprocessing in user-level threads. It may be multithreaded. Implementation It is easy to implement. It is complicated to implement. Blocking Operation If a thread in the kernel is blocked, it blocks all other threads in the same process. If a thread in the kernel is blocked, it does not block all other threads in the same process. Recognize OS doesn't recognize it. It is recognized by OS. Thread Management Its library includes the source code for thread creation, data transfer, thread destruction, message passing, and thread scheduling. The application code on kernel-level threads does not include thread management code, and it is simply an API to the kernel mode. Hardware Support It doesn't need hardware support. It requires hardware support. Creation and Management It may be created and managed much faster. It takes much time to create and handle. Examples Some instances of user-level threads are Java threads and POSIX threads. Some instances of Kernel-level threads are Windows and Solaris. Operating System Any OS may support it. The specific OS may support it. 12/31/2023 Process and Thread 29

30. Components of Threads Program counter Register set Stack space 12/31/2023 Process and Thread 30

31. Benefits of Threads Enhanced throughput of the system:  When the process is split into many threads, and each thread is treated as a job, the number of jobs done in the unit time increases. That is why the throughput of the system also increases. Effective Utilization of Multiprocessor system:  When you have more than one thread in one process, you can schedule more than one thread in more than one processor. Faster context switch:  The context switching period between threads is less than the process context switching. The process context switch means more overhead for the CPU. Responsiveness:  When the process is split into several threads, and when a thread completes its execution, that process can be responded to as soon as possible. 12/31/2023 Process and Thread 31

32. Benefits of Threads Communication:  Multiple-thread communication is simple because the threads share the same address space, while in process, we adopt just a few exclusive communication strategies for communication between two processes. Resource sharing:  Resources can be shared between all threads within a process, such as code, data, and files. Note: The stack and register cannot be shared between threads. There is a stack and register for each thread. 12/31/2023 Process and Thread 32

33. Process vs Program Process Program The process is basically an instance of the computer program that is being executed. A Program is basically a collection of instructions that mainly performs a specific task when executed by the computer. A process has a  shorter lifetime . A Program has a  longer lifetime . A Process requires resources such as memory, CPU, Input-Output devices. A Program is stored by hard-disk and does not require any resources. A process has a dynamic instance of code and data A Program has static code and static data. Basically, a process is the  running instance  of the code. On the other hand, the program is the  executable code . 12/31/2023 Process and Thread 33

34. Process Scheduling When there are two or more runnable processes then it is decided by the Operating system which one to run first then it is referred to as Process Scheduling. A scheduler is used to make decisions by using some scheduling algorithm. Given below are the properties of a  Good Scheduling Algorithm : 12/31/2023 Process and Thread 34

35. Response time should be minimum for the users. The number of jobs processed per hour should be maximum i.e. Good scheduling algorithm should give maximum throughput. The utilization of the CPU should be 100%. Each process should get a fair share of the CPU. 12/31/2023 Process and Thread 35

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37. Process scheduling (FCFS, SJF, RR, Priority, Real-time scheduling) The Purpose of a Scheduling algorithm Maximum CPU utilization Fare allocation of CPU Maximum throughput Minimum turnaround time Minimum waiting time Minimum response time 12/31/2023 Process and Thread 37

38. CPU Scheduling   CPU Scheduling  is a process of determining which process will own CPU for execution while another process is on hold. The main task of CPU scheduling is to make sure that whenever the CPU remains idle, the OS at least select one of the processes available in the ready queue for execution. The selection process will be carried out by the CPU scheduler. It selects one of the processes in memory that are ready for execution. Types of CPU Scheduling Preemptive Scheduling Non Preemptive Scheduling 12/31/2023 Process and Thread 38

39. CPU Scheduling   CPU scheduling is the basic of multiprogram med operating system. By switching the CPU among processes, the operating system can make the computer more productive. In a single processor system, only one process run at a time. Any other must wait until the CPU is free and can be rescheduled. 12/31/2023 Process and Thread 39

40. Scheduling Criteria CPU Utilization: We want to keep the CPU as busy as possible at all the time. Conceptually, CPU utilization can range from 0 to 100%. In a real system, it should range from 40%( for a lightly loaded system) to 90% (for a heavily used system) Throughput: The number of processes that are completed per unit time. If the CPU is busy executing processes, then work is being done. Turnaround time: The interval from the time of submission of a process to the time of completion is the turnaround time. Waiting Time: The CPU scheduling algorithm does not affect the amount of time during which a process executes or does I/O ; it affects only the amount of time that process spends waiting in the ready queue. Waiting time is the sum of the periods spent waiting in the ready queue. 12/31/2023 Process and Thread 40

41. CPU Scheduling   CPU scheduling is a process of determining which process will own CPU for the execution while another process is on hold. 12/31/2023 Process and Thread 41

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43. CPU Scheduling   12/31/2023 Process and Thread 43

44. Preemptive Scheduling In Preemptive Scheduling, the tasks are mostly assigned with their priorities. Sometimes it is important to run a task with a higher priority before another lower priority task, even if the lower priority task is still running. The lower priority task holds for some time and resumes when the higher priority task finishes its execution. 12/31/2023 Process and Thread 44

45. Non Preemptive Scheduling In this type of scheduling method, the CPU has been allocated to a specific process. The process that keeps the CPU busy will release the CPU either by switching context or terminating. It is the only method that can be used for various hardware platforms. That’s because it doesn’t need special hardware (for example, a timer) like preemptive scheduling. When scheduling is Preemptive or Non-Preemptive? A process switches from the running to the waiting state. Specific process switches from the running state to the ready state. Specific process switches from the waiting state to the ready state. Process finished its execution and terminated. 12/31/2023 Process and Thread 45

46. When scheduling is Preemptive or Non-Preemptive? Only conditions 1 and 4 apply, the scheduling is called non- preemptive. All other scheduling are preemptive. 12/31/2023 Process and Thread 46

47. Important CPU scheduling Terminologies Burst Time/Execution Time:  It is a time required by the process to complete execution. It is also called running time. Arrival Time:  when a process enters in a ready state Finish Time:  when process complete and exit from a system Multiprogramming:  A number of programs which can be present in memory at the same time. Jobs:  It is a type of program without any kind of user interaction. User:  It is a kind of program having user interaction. Process:  It is the reference that is used for both job and user. CPU/IO burst cycle:  Characterizes process execution, which alternates between CPU and I/O activity. CPU times are usually shorter than the time of I/O. 12/31/2023 Process and Thread 47

48. Process Scheduling Algorithm There are six popular process scheduling algorithms which we are going to discuss in this chapter − First-Come, First-Served (FCFS) Scheduling Shortest-Job-Next (SJN) Scheduling Priority Scheduling Shortest Remaining Time Round Robin(RR) Scheduling Multiple-Level Queues Scheduling 12/31/2023 Process and Thread 48

49. First Come First Serve (FCFS) Jobs are executed on first come, first serve basis. It is a non-preemptive, pre-emptive scheduling algorithm. The simple CPU Scheduling algorithm Easy to understand and implement. Its implementation is based on FIFO queue. Poor in performance as average wait time is high. 12/31/2023 Process and Thread 49

50. Convoy Effect 12/31/2023 Process and Thread 50

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52. FCFS The shaded box represents the idle time of CPU. 12/31/2023 Process and Thread 52

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55. Shortest Job First (SJF) /Shortest Job Next 12/31/2023 Process and Thread 55

56. (Shortest job first) SJF is a full form of (Shortest job first) is a scheduling algorithm in which the process with the shortest execution time should be selected for execution next. This scheduling method can be preemptive or non-preemptive. It significantly reduces the average waiting time for other processes awaiting execution. 12/31/2023 Process and Thread 56

57. Shortest job first It is associated with each job as a unit of time to complete. In this method, when the CPU is available, the next process or job with the shortest completion time will be executed first. It is Implemented with non-preemptive policy. This algorithm method is useful for batch-type processing, where waiting for jobs to complete is not critical. It improves job output by offering shorter jobs, which should be executed first, which mostly have a shorter turnaround time. 12/31/2023 Process and Thread 57

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60. Example for Shortest Job First Process ID Arrival Time Burst time Completion Time TAT=CT-AT WT=TAT-BT P1 8 8 =8-0=8 =8-8=0 P2 1 4 12 =12-1=11 =11-4=7 P3 2 9 32 =32-2=30 =30-9=21 P4 3 5 17 17-3=14 =14-5=9 P5 4 6 23 23-4=19 =19-6=13 12/31/2023 Process and Thread 60

61. Mode: Non preemptive/No Interruption Criteria : Burst Time Process will be executing completely. In above Figure, At the time zero(0), only one process that is P1 will be available in ready queue. Total Burst Time: 8+4+9+5+6=32 Turnaround Time=Completion Time-Arrival Time…………(1) Waiting Time(WT): TAT-Burst Time………………………………..(2) 12/31/2023 Process and Thread 61

62. Average Turnaround Time: (8+11+30+14+19)/5=82/5=16.4ms Average Waiting Time: 0+7+21+9+13=50/5=10ms Questions: Calculate the average TAT and WT. [Class Work] 12/31/2023 Process and Thread 62 PROCESS ID ARRIVAL TIME BURST TIME COMPLETION TIME TURN AROUND TIME=CT-AT WAITING TIME=TAT-BT P1 1 21 ? 22 ? ? P2 2 3 ?27 ? ? P3 3 6 ? 33 ? ? P4 4 2 ? 24 ? ?

63. Solution: Total TAT=96 Average TAT=96/4=24ms Waiting Time=64. Average WT=64/4=16ms. Wa Let us construct the Gantt Chart: 12/31/2023 Process and Thread 63

64. SHORTEST JOB FIRST(SJF) Mode: Preemptive Mode Criteria: Burst Time There will be an interruption If one process is being executed by the CPU, at the same time, if any other process with lower burst time will appear , then automatically executing process will be send back to the ready state and high PRIORITY PROCESS will be given to the CPU; which is called Preemption. Concept: Lower Burst Time Should be executed First. 12/31/2023 Process and Thread 64

65. Example 1 PROCESS ID ARRIVAL TIME BURST TIME COMPLETION TIME TAT=CT-AT WT=TAT-BT P1 8, 1 second has been already completed executed, so P1 Burst time is: 8-1=7 20 20-0=20 =20-8=12 P2 1 1 /0 2 2-1=1 =1-1=0 P3 2 3 /2/0 5 5-2=3 =3-3=0 P4 3 2 7 7-3=4 =4-2=2 P5 4 6 13 13-4=9 =9-6=3 12/31/2023 Process and Thread 65

66. SHORTEST JOB FIRST(SJF) At time zero(0), only process P1 is available to the ready queue. After 1 sec, P2 has arrived into the ready state. P2 should be allocated to the CPU, because it has the lowest burst time (1ms). P3 is allocated to the CPU, having the lowest burst time (3ms). If any two process having the same burst time, then select the process which is having the lowest ID. What is ATAT and AWT? 12/31/2023 Process and Thread 66

67. Gantt Chart 12/31/2023 Process and Thread 67

68. PRIORITY SCHEDULING 12/31/2023 Process and Thread 68

69. PRIORITY SCHEDULING The lower the value higher the priority. 12/31/2023 Process and Thread 69

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71. PRIORITY SCHEDULING Each process has its own priority. Out of all available process, highest priority process gets the CPU. Two Types: Static Priority( Does not change throughout the execution of program) Dynamic(Change after some interval of time) Version: Non Preemptive and Preemptive Remember: Lesser the number higher the priority. 12/31/2023 Process and Thread 71

72. Round Robin Algorithm( Mode: Preemptive) Used in time sharing system. Time Sharing is the main emphasis of the algorithm. Each step of this algorithm is carried out cyclically. The system defines a specific time slice, known as a time quantum . Similar to FCFS with time quantum. Criteria: Time Quantum Ready Queue(RAM, select process from RAM) To Running Queue (CPU). Sequences of processes in ready queue (Context Switching) Two Queue: Ready Queue and Running Queue. 12/31/2023 Process and Thread 72

73. Round Robin Algorithm Context Switching: Context switching occurs when the time quantum expires or when a process voluntarily yields the CPU. The operating system saves the current state (context) of the running process, including register values, program counter, and other relevant information. The next process in the ready queue is selected, and its saved context is loaded. Control is transferred to the newly loaded process, and it continues its execution from where it was last interrupted. Assign a fixed time unit, known as a time quantum or time slice . 12/31/2023 Process and Thread 73

74. Summary of Round Robin Algorithm Round Robin Scheduling is the preemptive scheduling algorithm. We assign a fixed time to all  processes for execution , this time is called time quantum. All processes can execute only until their time quantum and then leave the  CPU  and give a chance to other processes to complete their execution according to time quantum. Context switching mechanisms store the  states of preempted processes . 12/31/2023 Process and Thread 74

75. Formula The formula of Round robin  Turn Around Time = Completion Time – Arrival Time The formula of Round robin  Waiting Time(W.T): Time Difference between the turnaround and the burst time. The formula of Round robin  Waiting Time = Turn Around Time – Burst Time 12/31/2023 Process and Thread 75

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79. Advantage of Round-robin Scheduling Round-robin Scheduling avoids starvation or convoy effect. In Round-robin Scheduling , all the jobs get a fair and efficient allocation of CPU. Round-robin Scheduling don’t care about priority of the processes. Round-robin Scheduling gives the best performance in terms of average response time. We can assume the worst case response time for the process, If we know the total number of processes on the running queue. Round-robin Scheduling is easily implementable on the system because it does not depend upon burst time. Round-robin Scheduling promotes Context switching to save the states of the preempted processes. 12/31/2023 Process and Thread 79

80. Disadvantages of Round-robin Scheduling Round-robin Scheduling utilize more time on context switching which is not good in some cases. The processor output will be reduced in Round-robin Scheduling, If slicing time of OS is low. If the time quantum is low, then it increases the context switching time in Round-robin Scheduling . Round-robin Scheduling can leads to decreases the comprehension In Round-robin Scheduling , it’s a difficult task to decide a correct time quantum to increase the efficiency and speed. In Round-robin Scheduling , We can’t set priorities for the processes. 12/31/2023 Process and Thread 80

81. Practice Session Look The Videos: https://www.youtube.com/watch?v=6PEyXwdxeIc 12/31/2023 Process and Thread 81

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92. Round Robin Algorithm (Numerical) PID AT BT CT TAT WT RT P1 5 P2 1 6 P3 2 3 P4 3 1 P5 4 5 P6 6 4 IF CPU SCHEDULING POLICY IS ROUND ROBIN WITH TIME QUANTUM=2 UNITS, CALCULATE THE AVERAGE WAITING TIME (AWT) and AVERAGE TAT. 12/31/2023 Process and Thread 92

93. In the following example, there are six processes named as P1, P2, P3, P4, P5 and P6. Their arrival time and burst time are given below in the table. The time quantum of the system is 4 units. According to the algorithm, we have to maintain the ready queue and the Gantt chart. The structure of both the data structures will be changed after every scheduling. Ready Queue: Initially, at time 0, process P1 arrives which will be scheduled for the time slice 4 units. Hence in the ready queue, there will be only one process P1 at starting with CPU burst time 5 units. 12/31/2023 Process and Thread 93

94. GANTT chart The P1 will be executed for 4 units first. Ready Queue Meanwhile the execution of P1, four more processes P2, P3, P4 and P5 arrives in the ready queue. P1 has not completed yet, it needs another 1 unit of time hence it will also be added back to the ready queue. 12/31/2023 Process and Thread 94

95. GANTT chart After P1, P2 will be executed for 4 units of time which is shown in the Gantt chart. Ready Queue During the execution of P2, one more process P6 is arrived in the ready queue. Since P2 has not completed yet hence, P2 will also be added back to the ready queue with the remaining burst time 2 units. 12/31/2023 Process and Thread 95

96. GANTT chart After P1 and P2, P3 will get executed for 3 units of time since its CPU burst time is only 3 seconds. Ready Queue Since P3 has been completed, hence it will be terminated and not be added to the ready queue. The next process will be executed is P4. 12/31/2023 Process and Thread 96

97. GANTT chart After, P1, P2 and P3, P4 will get executed. Its burst time is only 1 unit which is lesser then the time quantum hence it will be completed. Ready Queue The next process in the ready queue is P5 with 5 units of burst time. Since P4 is completed hence it will not be added back to the queue. 12/31/2023 Process and Thread 97

98. GANTT chart P5 will be executed for the whole time slice because it requires 5 units of burst time which is higher than the time slice. Ready Queue P5 has not been completed yet; it will be added back to the queue with the remaining burst time of 1 unit. 12/31/2023 Process and Thread 98

99. 12/31/2023 Process and Thread 99 GANTT Chart The process P1 will be given the next turn to complete its execution. Since it only requires 1 unit of burst time hence it will be completed. Ready Queue P1 is completed and will not be added back to the ready queue. The next process P6 requires only 4 units of burst time and it will be executed next.

100. GANTT chart P6 will be executed for 4 units of time till completion. Ready Queue Since P6 is completed, hence it will not be added again to the queue. There are only two processes present in the ready queue. The Next process P2 requires only 2 units of time. 12/31/2023 Process and Thread 100

101. GANTT Chart P2 will get executed again, since it only requires only 2 units of time hence this will be completed. Ready Queue Now, the only available process in the queue is P5 which requires 1 unit of burst time. Since the time slice is of 4 units hence it will be completed in the next burst. 12/31/2023 Process and Thread 101

102. GANTT chart P5 will get executed till completion. the completion time, Turnaround time and waiting time will be calculated as shown in the table below. As, we know, TURN AROUND TIME = COMPLETION TIME - ARRIVAL TIME    WAITING TIME = TURN AROUND TIME - BURST TIME     12/31/2023 Process and Thread 102

103. Process ID Arrival Time Burst Time Completion Time Turn Around Time Waiting Time 1 5 17 17 12 2 1 6 23 22 16 3 2 3 11 9 6 4 3 1 12 9 8 5 4 5 24 20 15 6 6 4 21 15 11 12/31/2023 Process and Thread 103 Avg Waiting Time = (12+16+6+8+15+11)/6 = 76/6 units

104. Round Robin Algorithm 12/31/2023 Process and Thread 104

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109. Inter- process communication "Inter-process communication is used for exchanging useful information between numerous threads in one or more processes (or programs).“ The main aim or goal of this mechanism is to provide communications in between several processes. In short, the intercommunication allows a process letting another process know that some event has occurred. Cooperating process or threads that needs inter process communication 12/31/2023 Process and Thread 109

110. Inter- process communication Cooperating process or threads that needs inter process communication Data Transfer Sharing Data Even notification Resources sharing Process Control Processes executing concurrently in the operating system may be either independent process or cooperating processes. 12/31/2023 Process and Thread 110

111. Inter- process communication Independent Processes: They cannot affect or be affected by the other processes executing in the system. Cooperating Processes: They can affect or be affected by the other process executing in the system. A process that share the data with other processes is called cooperating processes. 12/31/2023 Process and Thread 111

112. APPROACH IN IPC Pipes Shared Memory Message Queue Direct Communication Indirect communication Message Passing FIFO 12/31/2023 Process and Thread 112

113. Pipe:- The pipe is a type of data channel that is unidirectional in nature. It means that the data in this type of data channel can be moved in only a single direction at a time. Still, one can use two-channel of this type, so that he can able to send and receive data in two processes. Typically, it uses the standard methods for input and output. These pipes are used in all types of POSIX systems and in different versions of window operating systems as well. 12/31/2023 Process and Thread 113

114. Shared Memory:- It can be referred to as a type of memory that can be used or accessed by multiple processes simultaneously. It is primarily used so that the processes can communicate with each other. Therefore the shared memory is used by almost all POSIX( Portable Operating System Interface) and Windows operating systems as well. 12/31/2023 Process and Thread 114

115. Message Queue:- In general, several different messages are allowed to read and write the data to the message queue. In the message queue, the messages are stored or stay in the queue unless their recipients retrieve them. In short, we can also say that the message queue is very helpful in inter-process communication and used by all operating systems. To understand the concept of Message queue and Shared memory in more detail, let's take a look at its diagram given below: 12/31/2023 Process and Thread 115

116. 12/31/2023 Process and Thread 116

117. Message Passing:-  is a type of mechanism that allows processes to synchronize and communicate with each other. However, by using the message passing, the processes can communicate with each other without restoring the hared variables. Usually, the inter-process communication mechanism provides two operations that are as follows: send (message) received (message) 12/31/2023 Process and Thread 117

118. Message Passing Direct Communication: In this type of communication process, usually, a link is created or established between two communicating processes. However, in every pair of communicating processes, only one link can exist. Indirect Communication : Indirect communication can only exist or be established when processes share a common mailbox, and each pair of these processes shares multiple communication links. These shared links can be unidirectional or bi-directional. 12/31/2023 Process and Thread 118

119. ASSIGNMENT NO:2 Define an operating system. Explain its role and importance in computer systems. Describe the process model in operating systems. What are the different states a process can be in, and explain the transitions between these states? Explain the concept of the Process Control Block (PCB). What information does it contain, and how is it used during context switching? Define a thread. What is the difference between a process and a thread? Explain the advantages of using threads in a multitasking environment. 12/31/2023 Process and Thread 119

120. ASSIGNMENT NO:2 Compare and contrast kernel-level threads and user-level threads. What are the advantages and disadvantages of each approach? Discuss the concept of multiprogramming and parallel processing. How do they enhance system performance and resource utilization? Define critical sections. Explain the challenges of race conditions in concurrent programming. How can mutual exclusion be achieved using busy waiting? Explain the concept of semaphores in operating systems. How do they provide synchronization between processes? 12/31/2023 Process and Thread 120

121. ASSIGNMENT NO:2 Define monitors and describe how they address the issues of semaphores in concurrent programming. Compare and contrast preemptive and non-preemptive scheduling algorithms. Discuss the advantages and disadvantages of each approach in a multitasking environment. Define First-Come-First-Serve (FCFS) scheduling. What are its strengths and weaknesses? Provide an example to illustrate. 12/31/2023 Process and Thread 121

122. ASSIGNMENT NO:2 Explain Shortest Job First (SJF) scheduling. How does it minimize waiting time, and what challenges may arise in its implementation? Discuss Round Robin (RR) scheduling. What is the significance of the time quantum, and how does it affect system performance? Define Priority scheduling. How does it work, and what considerations should be taken into account when assigning priorities to processes? Briefly explain Real-Time scheduling. What distinguishes it from other scheduling algorithms, and what are the key requirements for real-time systems? 12/31/2023 Process and Thread 122

123. Critical Section Critical Section is the part of a program which tries to access shared resources. That resource may be any resource in a computer like a memory location, Data structure, CPU or any IO device. The critical section cannot be executed by more than one process at the same time; operating system faces the difficulties in allowing and disallowing the processes from entering the critical section. The critical section problem is used to design a set of protocols which can ensure that the Race condition among the processes will never arise. 12/31/2023 Process and Thread 123

124. Critical Section The critical section in a code segment where the shared variables can be accessed. Atomic action is required in a critical section i.e. only one process can execute in its critical section at a time. All the other processes have to wait to execute in their critical sections. do{ Entry Section Critical Section Exit Section Remainder Section } while (TRUE); 12/31/2023 Process and Thread 124

125. 12/31/2023 Process and Thread 125 Figure: General Structure of Typical Process

126. Critical Section Each process must request permission to enter its critical section. The section of code implementing this request is the entry section. The critical Section may be followed by an exit section. The remaining code is the remainder section. 12/31/2023 Process and Thread 126

127. Requirements of Synchronization mechanisms(The solution to the critical section problem) Primary: Mutual Exclusion: Our solution must provide mutual exclusion. By Mutual Exclusion, we mean that if one process is executing inside critical section then the other process must not enter in the critical section. Progress : Progress means that if one process doesn't need to execute into critical section then it should not stop other processes to get into the critical section. The main job of progress is to ensure one process is executing in the critical section at any point in time (so that some work is always being done by the processor). This decision cannot be “postponed indefinitely” – in other words, it should take a limited amount of time to select which process should be allowed to enter the critical section. If this decision cannot be taken in a finite time, it leads to a deadlock. 12/31/2023 Process and Thread 127

128. Progress 12/31/2023 Process and Thread 128

129. Progress 12/31/2023 Process and Thread 129

130. Requirements of Synchronization mechanisms 12/31/2023 Process and Thread 130

131. Requirements of Synchronization mechanisms Secondary Bounded Waiting We should be able to predict the waiting time for every process to get into the critical section. The process must not be endlessly waiting for getting into the critical section. Architectural Neutrality Our mechanism must be architectural natural. It means that if our solution is working fine on one architecture then it should also run on the other ones as well. 12/31/2023 Process and Thread 131

132. Semaphore Semaphore proposed by Edsger Dijkstra, is a technique to mange concurrent processes by using a simple integer value, which is known as a semaphore. Semaphore is simply a variable which is non-negative and shared between threads. This variable is used to solve the critical section problem and to achieve process synchronization in the multiprocessing environment. A semaphore S is an integer variable that, apart from initialization, is accessed only through two standard atomic operations: wait ( ) and signal ( ). 12/31/2023 Process and Thread 132

133. Semaphore Wait ( )  P [From the Dutch word proberen , which means “to test”] Signal ( )  V [From the Dutch word verhogen , which means to “increment”] Semaphore does not supports the busy waiting. Semaphore is the Synchronization tool that does not require the busy waiting The least value for a Semaphore is zero (0). The Maximum value of a Semaphore can be anything. 12/31/2023 Process and Thread 133

134. Semaphore Operation The Semaphores usually have two operations. The two operations have the capability to decide the values of the semaphores. The two Semaphore Operations are: Wait ( ) Signal ( ) 12/31/2023 Process and Thread 134

135. Wait ( ) Operation The Wait Operation works on the basis of Semaphore or Mutex Value. Here, if the Semaphore value is greater than zero or positive then the Process can enter the Critical Section Area. P(Semaphore S) { While (S<=0) ;// no operation S--; } 12/31/2023 Process and Thread 135

136. Wait ( ) Operation Do not allow the process  if  the value of Semaphore is less than zero or equal to zero. The wait operation decrements the value of the semaphore. The Wait Operation is used for deciding the condition for the process to enter the critical state or wait for execution of process. Here, the wait operation has many different names. The different names are: P operation is also called wait, sleep, decrease or down operation. 12/31/2023 Process and Thread 136

137. signal ( ) Operation The Signal Semaphore Operation is used to update the value of Semaphore. The Semaphore value is updated when the new processes are ready to enter the Critical Section. The Signal Operation is also known as: Wake up Operation, Up Operation, Increase Operation, V Function (most important alias name for signal operation). V(Semaphore S){ S++; } 12/31/2023 Process and Thread 137

138. signal ( ) Operation All the modifications to the integer value of the semaphore in the wait ( ) and signal ( ) operations must be executed indivisibly. That is, when one process modifies the semaphore value, no other process can simultaneously modify that some semaphore value. 12/31/2023 Process and Thread 138

139. Types of Semaphores 1. Binary Semaphore [0 and 1 only] Here, there are only two values of Semaphore in Binary Semaphore Concept. The two values are 1 and 0. If the Value of Binary Semaphore is 1, then the process has the capability to enter the critical section area. If the value of Binary Semaphore is 0 then the process does not have the capability to enter the critical section area. 12/31/2023 Process and Thread 139

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141. 2. Counting Semaphore (- ∞ to ∞ ) Here, there are two sets of values of Semaphore in Counting Semaphore Concept. The two types of values are values greater than and equal to one and other type is value equal to zero. If the Value of Binary Semaphore is greater than or equal to 1, then the process has the capability to enter the critical section area. If the value of Binary Semaphore is 0 then the process does not have the capability to enter the critical section area. Its value can range over an unrestricted domain. It is used to control access to a resource that has multiple instances. 12/31/2023 Process and Thread 141

142. Race Condition A race condition is a situation that may occur inside a critical section. This happens when the result of multiple thread execution in critical section differs according to the order in which the threads execute. Race conditions in critical sections can be avoided if the critical section is treated as an atomic instruction. Also, proper thread synchronization using locks or atomic variables can prevent race conditions. It occurs when two or more processes are executed at the same time, not scheduled in proper sequence and not executed in critical section correctly which causes data inconsistency and data loss. 12/31/2023 Process and Thread 142

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145. Monitor 12/31/2023 Process and Thread 145