![Objectives of Process Scheduling
Algorithm
• Max CPU utilization [Keep CPU as busy as possible]
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Fair allocation of CPU.
•
Max throughput [Number of processes that complete their execution
per time unit]
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Min turnaround time [Time taken by a process to finish execution]
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Min waiting time [Time a process waits in ready queue]
•
Min response time [Time when a process produces first response]](https://image.slidesharecdn.com/unit2-process-230509102222-920b6060/85/PROCESS-pptx-20-320.jpg)
PROCESS.pptx
- 2. Process Management A process is basically a program in execution. The execution of a process must progress in a sequential fashion. A process is defined as an entity which represents the basic unit of work to be implemented in the system. To put it in simple terms, we write our computer programs in a text file and when we execute this program, it becomes a process which performs all the tasks mentioned in the program. When a program is loaded into the memory and it becomes a process, it can be divided into four sections ─ stack, heap, text and data. The following image shows a simplified layout of a process inside main memory −
- 3. Stack The process Stack contains the temporary data such as method/function parameters, return address and local variables. Heap This is dynamically allocated memory to a process during its run time. Text This includes the current activity represented by the value of Program Counter and the contents of the processor's registers. Data This section contains the global and static variables.
- 4. Program A program is a piece of code which may be a single line or millions of lines. A computer program is usually written by a computer programmer in a programming language. For example, here is a simple program written in C programming language −
- 5. • A computer program is a collection of instructions that performs a specific task when executed by a computer. • When we compare a program with a process, we can conclude that a process is a dynamic instance of a computer program. • A part of a computer program that performs a well-defined task is known as an algorithm. • A collection of computer programs, libraries and related data are referred to as a software.
- 6. Process Life Cycle (Process State) • When a process executes, it passes through different states. These stages may differ in different operating systems, and the names of these states are also not standardized. • In general, a process can have one of the following five states at a time.
- 7. 1 Start This is the initial state when a process is first started/created. 2 Ready The process is waiting to be assigned to a processor. Ready processes are waiting to have the processor allocated to them by the operating system so that they can run. Process may come into this state after Start state or while running it by but interrupted by the scheduler to assign CPU to some other process. 3 Running Once the process has been assigned to a processor by the OS scheduler, the process state is set to running and the processor executes its instructions. 4 Waiting Process moves into the waiting state if it needs to wait for a resource, such as waiting for user input, or waiting for a file to become available. 5 Terminated or Exit Once the process finishes its execution, or it is terminated by the operating system, it is moved to the terminated state where it waits to be removed from main memory.
- 9. THREADS: • A thread is a flow of execution through the process code, with its own program counter that keeps track of which instruction to execute next, system registers which hold its current working variables, and a stack which contains the execution history. • A thread shares with its peer threads few information like code segment, data segment and open files. When one thread alters a code segment memory item, all other threads see that.
- 10. • A thread is also called a lightweight process. • Threads provide a way to improve application performance through parallelism. • Threads represent a software approach to improving performance of operating system by reducing the overhead • Each thread belongs to exactly one process and no thread can exist outside a process. • Each thread represents a separate flow of control. • The following figure shows the working of a single-threaded and a multithreaded process.
- 12. Difference between Process and Thread
- 13. Advantages of Thread • Threads minimize the context switching time. • Use of threads provides concurrency within a process. • Efficient communication. • It is more economical to create and context switch threads. • Threads allow utilization of multiprocessor architectures to a greater scale and efficiency.
- 14. Context Switching • A context switch is the mechanism to store and restore the state or context of a CPU in Process Control block so that a process execution can be resumed from the same point at a later time. • Using this technique, a context switcher enables multiple processes to share a single CPU. • Context switching is an essential part of a multitasking operating system features. • When the scheduler switches the CPU from executing one process to execute another, the state from the current running process is stored into the process control block. • After this, the state for the process to run next is loaded from its own PCB and used to set the PC, registers, etc. • At that point, the second process can start executing.
- 16. CPU Scheduling Algorithms • A Process Scheduler schedules different processes to be assigned to the CPU based on particular scheduling algorithms. • There are six popular process scheduling algorithms in which we are going to discuss four algorithms in this chapter − • First-Come, First-Served (FCFS) Scheduling • Shortest-Job-First (SJF) Scheduling • Round Robin (RR) Scheduling • Multiple-Level Queues Scheduling • Multi-level Feedback Scheduling
- 17. • These algorithms are either non-preemptive or preemptive. • Non-preemptive algorithms are designed so that once a process enters the running state, it cannot be preempted until it completes its allotted time, whereas the preemptive scheduling is based on priority where a scheduler may preempt a low priority running process anytime when a high priority process enters into a ready state.
- 18. CPU Scheduling in Operating Systems Scheduling of processes/work is done to finish the work on time. Below is the different time with respect to a process. Arrival Time: Time at which the process arrives in the ready queue. Completion Time: Time at which process completes its execution. Burst Time: Time required by a process for CPU execution. Turn Around Time: Time Difference between completion time and arrival time. Turn Around Time = Completion Time – Arrival Time Waiting Time (W.T): Time Difference between turn around time and burst time. Waiting Time = Turn Around Time – Burst Time
- 19. Why do we need scheduling? • A typical process involves both I/O time and CPU time. In a uni- programming system like MS-DOS, time spent waiting for I/O is wasted and CPU is free during this time. • In multi programming systems, one process can use CPU while another is waiting for I/O. This is possible only with process scheduling.
- 20. Objectives of Process Scheduling Algorithm • Max CPU utilization [Keep CPU as busy as possible] • Fair allocation of CPU. • Max throughput [Number of processes that complete their execution per time unit] • Min turnaround time [Time taken by a process to finish execution] • Min waiting time [Time a process waits in ready queue] • Min response time [Time when a process produces first response]
- 21. First-Come, First-Served (FCFS) Scheduling • First Come First Serve is the full form of FCFS. It is the easiest and most simple CPU scheduling algorithm. • In this type of algorithm, the process which requests the CPU gets the CPU allocation first. • This scheduling method can be managed with a FIFO queue. As the process enters the ready queue, its PCB (Process Control Block) is linked with the tail of the queue. • So, when CPU becomes free, it should be assigned to the process at the beginning of the queue.
- 22. Characteristics of FCFS method: • It offers non-preemptive scheduling algorithm. • Jobs are always executed on a first-come, first-serve basis • It is easy to implement and use. • However, this method is poor in performance, and the general wait time is quite high.
- 23. Program for FCFS CPU Scheduling | Set 1 • Given n processes with their burst times, the task is to find average waiting time and average turn around time using FCFS scheduling algorithm. First in, first out (FIFO), also known as first come, first served (FCFS), is the simplest scheduling algorithm. • FIFO simply queues processes in the order that they arrive in the ready queue. In this, the process that comes first will be executed first and next process starts only after the previous gets fully executed. Here we are considering that arrival time for all processes is 0.
- 25. Important Points: 1.Non-preemptive 2.Average Waiting Time is not optimal 3.Cannot utilize resources in parallel: Results in Convoy effect (Consider a situation when many IO bound processes are there and one CPU bound process. The IO bound processes have to wait for CPU bound process when CPU bound process acquires CPU. The IO bound process could have better taken CPU for some time, then used IO devices).
- 26. Convoy Effect in OS • Convoy effect is a phenomenon which occurs in First Come First Serve Scheduling Algorithm. Consider a process that is too large that occurs at the very start of the ready queue. Due to this long process other processes in the queue may starve for a very long time. • To understand this, consider an example. Where a very long train is passing by and smaller cars and other vehicles may have to wait for a very long time to pass by. • So, as in First Come First Serve Algorithm (FCFS), Jobs are executed as they come. As FCFS is a non-preemptive algorithm, meaning jobs can’t be preempted (stopped) if they have begun execution, so unless the process does not get finished other process can’t get CPU resources. • This situation can be likened to a convoy passing through the road. So, unless the convoy passes completely, other people can’t use the road.
- 28. • Advantages – • It is simple and easy to understand. • Disadvantages – • The process with less execution time suffers i.e. waiting time is often quite long. • Favors CPU Bound process then I/O bound process. • Here, first process will get the CPU first, other processes can get CPU only after the current process has finished its execution. Now, suppose the first process has large burst time, and other processes have less burst time, then the processes will have to wait more unnecessarily, this will result in more average waiting time, i.e., Convoy effect. • This effect results in lower CPU and device utilization. • FCFS algorithm is particularly troublesome for time-sharing systems, where it is important that each user get a share of the CPU at regular intervals.
- 29. Shortest-Job-First (SJF) Scheduling • Shortest Job First Scheduling (Non-Preemptive Algorithm) • Shortest Job First (SJF) is a Scheduling Algorithm where the process is executed in ascending order of their burst time, that is, the process having the shortest burst time is executed first and so on. • The processor knows burst time of each process in advance. • It can be thought of as shortest-next-CPU-burst algorithm, as Scheduling depends on length of the next CPU burst of a process. (The duration for which a process gets control of the CPU, is the Burst time for a process.) • SJF can be Pre-emptive or Non- preemptive. Under Non-preemptive Scheduling, once a process has been allocated to CPU, the process keeps the CPU until the process has finished its execution.
- 30. • Let us try to understand SJF Non-Preemptive Scheduling with an example – • Here, processes are assumed to be arrived at the same time. • See the below Example –
- 32. • Steps – 1.Since Burst time for P3 (Burst = 1 sec) is lowest it is executed first. 2.Then Burst time for P1 is lowest in order thus it gets executed the 2nd time. 3.Similarly, P4 and then P2 • Waiting time – 1.P1 waiting time = 1 2.P2 waiting time = 7 3.P3 waiting time = 0 4.P4 waiting time = 3 • The average waiting time is = (1+ 7 + 0 + 3) / 4 = 2.75 ms
- 33. • Features of SJF • SJF is a greedy Algorithm • It has Minimum average waiting time among all scheduling algorithms. • Difficulty of SJF is knowing the length of next CPU request. • It’s used frequently in Long-term scheduling in Batch System as in this, the time limit is provided by the user specifying the process. SJF can’t be implemented in Short-term scheduling as one can’t know the exact length of next CPU burst. • Drawback • In SJF Scheduling, a process with high burst time may suffer starvation. Starvation is the process in which a process with higher burst time is kept on waiting and waiting, but is not allocated to the CPU. It’s prevented by aging. • Total execution time must be known beforehand of a process.
- 34. • What is aging? • Aging is a technique which is used to reduce starvation of the processes. • Aging takes into account the waiting time of the process in the ready queue and gradually increases the priority of the process. • Hence, as the priority of a process gets increased this ensures that all process get completed eventually.