In computing, scheduling is the action .
- 1. Process
- 2. A program under execution is called process. When a program is loaded to the main memory and starts its execution then it is called a process.
- 6. States of the process:-
- 7. 1. New: The process is being created. 2. Running: Instructions are being executed. 3. Waiting: The Process is waiting for some event to occur(such as an I/O completion or reception of a signal). 4. Ready: The Process is waiting to be assigned to a processor. 5. Terminate: The process has finished execution.
- 13. CPU Scheduling is a process of determining which process will own for execution while another process is on hold.
- 15. CPU Scheduler • CPU scheduler will select a process from the ready process from the memory and will allocate the CPU to that process. CPU scheduling decisions occur when a process:- • Switches from "Running" to "Waiting" state. • Switches from "Running" to "Ready" state. • Switches from "Waiting" to "Ready". • Terminates.
- 16. Types of CPU Scheduling
- 17. Types of CPU scheduling Algorithm
- 19. 1.First Come First Serve (FCFS) 2.Shortest-Job-First (SJF) 3.Shortest Remaining Time 4.Priority Scheduling 5.Round Robin Scheduling 6.Multilevel Queue Scheduling
- 20. 1. Burst time (BT): The time required for the execution of the process 2. Arrival time (AV): The time at which the process enters the ready queue. 3. Completion Time/Finish Time /Exit time (CT) :it is when a process finishes execution and is no longer being processed by the CPU. It is the summation of the arrival, waiting, and burst times. 4. Turnaround time (TAT): time elapsed between the arrival of a process and its completion is known as turnaround time. That is, the duration it takes for a process to complete its execution and leave the system. Turnaround Time = Completion Time – Arrival Time / (WT+BT) 5. Waiting time (WT): The time spent by the process in ready state i.e., the difference between turnaround time and burst time. 6. Response time: The time taken by a process, to be allocated to the CPU, for the first time after entering ready queue i.e., the difference between the time a process first gets CPU and the arrival time 7. Throughput: This is a way to find the efficiency of a processor. It is the number of processes executed by the CPU in a given amount of time.
- 21. TAT = (CT-AT) or (WT+BT) WT = TAT-BT RT = AT- Time allocated to the CPU, for the first time
- 23. First Come First Serve (FCFS)
- 25. Problems with FCFS Scheduling It is Non Pre-emptive algorithm, which means the process priority doesn't matter Not optimal Average Waiting Time. Resources utilization in parallel is not possible, which leads to Convoy Effect, and hence poor resource(CPU, I/O etc) utilization.
- 26. What is Convoy Effect? Convoy Effect is a situation where many processes, who need to use a resource for short time are blocked by one process holding that resource for a long time. This essentially leads to poor utilization of resources and hence poor performance.
- 30. Types of Priority Scheduling Algorithm Priority scheduling can be of two types: 1.Preemptive Priority Scheduling: If the new process arrived at the ready queue has a higher priority than the currently running process, the CPU is preempted, which means the processing of the current process is stopped and the incoming new process with higher priority gets the CPU for its execution. 2.Non-Preemptive Priority Scheduling: In case of non-preemptive priority scheduling algorithm if a new process arrives with a higher priority than the current running process, the incoming process is put at the head of the ready queue, which means after the execution of the current process it will be processed.
- 31. Characteristics of Priority Scheduling •Schedules processes on the basis of priority. •Used to perform batch processes. •In the case of two processes with similar priorities, we use FCFS and Round-Robin to choose between them. •A number is given to each process to indicate its priority level. •Lower is the number assigned, higher is the priority level of a process. •If a higher priority task arrives when a lower priority task is executing, the higher priority task replaces the one with lower priority and the •latter is put on hold until it completes execution.
- 32. Problem with Priority Scheduling Algorithm ->In priority scheduling algorithm, the chances of indefinite blocking or starvation. ->A process is considered blocked when it is ready to run but has to wait for the CPU as some other process is running currently. ->But in case of priority scheduling if new higher priority processes keeps coming in the ready queue then the processes waiting in the ready queue with lower priority may have to wait for long durations before getting the CPU for execution. ->In 1973, when the IBM 7904 machine was shut down at MIT, a low- priority process was found which was submitted in 1967 and had not yet been run.
- 33. Using Aging Technique with Priority Scheduling To prevent starvation of any process, we can use the concept of aging where we keep on increasing the priority of low-priority process based on the its waiting time. For example, if we decide the aging factor to be 0.5 for each day of waiting, then if a process with priority 20(which is comparitively low priority) comes in the ready queue. After one day of waiting, its priority is increased to 19.5 and so on. Doing so, we can ensure that no process will have to wait for indefinite time for getting CPU time for processing.
- 34. Solve problem in class
- 40. scheduling algorithm used in operating systems to manage processes efficiently by categorizing them into multiple queues based on their properties and priorities
- 43. Advantages of Multilevel Queue Scheduling 1.You can use multilevel queue scheduling to apply different scheduling methods to distinct processes. It will have low overhead in terms of scheduling. Disadvantages 1.There is a risk of starvation for lower priority processes. 2.It is rigid in nature.
- 45. Multilevel Feedback Queue (MLFQ) scheduling is a dynamic scheduling algorithm used in operating systems to manage processes by placing them in multiple queues and using preemption and priority adjustments based on the behavior of the processes.
- 47. Advantages of MFQS •This is a flexible Scheduling Algorithm •This scheduling algorithm allows different processes to move between different queues. •In this algorithm, A process that waits too long in a lower priority queue may be moved to a higher priority queue which helps in preventing starvation. Disadvantages of MFQS •This algorithm is too complex. •As processes are moving around different queues which leads to the production of more CPU overheads. •In order to select the best scheduler this algorithm requires some other means to select the values
- 48. Threads
- 49. •A thread is a basic unit of execution within a process, enabling concurrent execution of code. •Threads within a process share the process's memory space, file handles, and other resources. •Threads are also known as Lightweight processes. •Threads are a popular way to improve the performance of an application through parallelism.
- 52. Process Thread A Process simply means any program in execution. Thread simply means a segment of a process. The process consumes more resources Thread consumes fewer resources. The process requires more time for creation. Thread requires comparatively less time for creation than process. The process is a heavyweight process Thread is known as a lightweight process The process takes more time to terminate The thread takes less time to terminate. Processes have independent data and code segments A thread mainly shares the data segment, code segment, files, etc. with its peer threads. The process takes more time for context switching. The thread takes less time for context switching. Communication between processes needs more time as compared to thread. Communication between threads needs less time as compared to processes. For some reason, if a process gets blocked then the remaining processes can continue their execution In case if a user-level thread gets blocked, all of its peer threads also get blocked.
- 53. Advantages of Thread 1.Responsiveness 2.Resource sharing, hence allowing better utilization of resources. 3.Economy. Creating and managing threads becomes easier. 4.Scalability. One thread runs on one CPU. In Multithreaded processes, threads can be distributed over a series of processors to scale. 5.Context Switching is smooth. Context switching refers to the procedure followed by the CPU to change from one task to another. 6.Enhanced Throughput of the system.
- 54. Types of Thread There are two types of threads: 1.User Threads 2.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.
- 55. User Level threads Kernel Level Threads These threads are implemented by users. These threads are implemented by Operating systems These threads are not recognized by operating systems, These threads are recognized by operating systems, In User Level threads, the Context switch requires no hardware support. In Kernel Level threads, hardware support is needed. These threads are mainly designed as dependent threads. These threads are mainly designed as independent threads. In User Level threads, if one user-level thread performs a blocking operation then the entire process will be blocked. On the other hand, if one kernel thread performs a blocking operation then another thread can continue the execution. Example of User Level threads: Java thread, POSIX threads. Example of Kernel level threads: Window Solaris. Implementation of User Level thread is done by a thread library and is easy. While the Implementation of the kernel-level thread is done by the operating system and is complex. This thread is generic in nature and can run on any operating system. This is specific to the operating system.
- 56. 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
- 57. Many to One Model
- 58. Many to One Model •In the many to one model, many user-level threads are all mapped onto a single kernel thread. •Thread management is handled by the thread library in user space, which is efficient in nature. •In this case, if user-level thread libraries are implemented in the operating system in some way that the system does not support them, then the Kernel threads use this many-to-one relationship model.
- 59. One to One Model
- 60. •The one to one model creates a separate kernel thread to handle each and every user thread. •Most implementations of this model place a limit on how many threads can be created. •Linux and Windows from 95 to XP implement the one-to-one model for threads. •This model provides more concurrency than that of many to one Model.
- 61. Many to Many Model
- 62. •The many to many model multiplexes any number of user threads onto an equal or smaller number of kernel threads, combining the best features of the one-to-one and many-to-one models. •Users can create any number of threads. •Blocking the kernel system calls does not block the entire process. •Processes can be split across multiple processors.
- 63. Thread Libraries - It provide programmers with API for the creation and management of threads. - It may be implemented either in user space or in kernel space. user space involves API functions implemented solely within the user space, with no kernel support - kernel space involves system calls and requires a kernel
- 64. Three types of Thread 1.POSIX Pitheads may be provided as either a user or kernel library, as an extension to the POSIX standard. 2.Win32 threads are provided as a kernel-level library on Windows systems. 3.Java threads: Since Java generally runs on a Java Virtual Machine, the implementation of threads is based upon whatever OS and hardware the JVM is running on, i.e. either Pitheads or Win32 threads depending on the system.
- 65. Multithreading Issues -Thread Cancellation Thread cancellation means terminating a thread before it has finished working. There can be two approaches for this, one is Asynchronous cancellation, which terminates the target thread immediately. The other is Deferred cancellation allows the target thread to periodically check if it should be canceled. -Signal Handling Signals are used in UNIX systems to notify a process that a particular event has occurred. Now in when a Multithreaded process receives a signal, to which thread it must be delivered? It can be delivered to all or a single thread.
- 66. fork() System Call fork() is a system call executed in the kernel through which a process creates a copy of itself. Now the problem in the Multithreaded process is, if one thread forks, will the entire process be copied or not? Security Issues Yes, there can be security issues because of the extensive sharing of resources between multiple threads.