Cpu scheduling
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This document discusses various CPU scheduling algorithms such as FCFS, SJF, priority scheduling, and round robin. It covers the basic concepts of scheduling, criteria for evaluating algorithms, examples of single processor and multiprocessor scheduling, and examples of scheduling in operating systems like Windows and Mac OS. The goal of scheduling is to allocate CPU time effectively among competing processes or threads.
Cpu scheduling
- 1. CPU SCHEDULING GUIDED BY: MR. VENKATESWARAN.R (ASST. PROFESSOR M.TECH) CREATED BY: VINOTH.J SARAVANA KUMAR.R KARTHICK SEKAR.S ASIR ALLOSANAI GANA SEKAR MUTHURAMALINGAM.E MUNEESWARAN.M INFANT JESUS COLLEGE OF ENGINEERING
- 2. OVERVIEW Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Operating Systems Examples
- 3. OVERVIEW In computer science, scheduling is the method by which threads, processes or data flows are given access to system resources (e.g. processor time, communications bandwidth). This is usually done to load balance a system effectively or achieve a target quality of service.
- 4. OVERVIEW Throughput: No of processes that complete the execution per unit time Response time: Amount of time it takes from when a request was submitted until the first response is produced Waiting time:Amount of time a process has been waiting in the ready queue Response time = waiting time + execution time
- 5. Types of scheduling methods Preemptive scheduling:A higher priority process can preempt a currently running low priority process to avoid priority inversion Nonpreemptive scheduling:A higher priority process waits a currently running process to finish regardless of the priority e.g., FCFS
- 6. Types of scheduling methods Long and medium term scheduling Long term scheduling: which determines which programs are admitted to the system for execution and when, and which ones should be exited. Medium term scheduling: which determines when processes are to be suspended and resumed; Short term scheduling (or dispatching): which determines which of the ready processes can have CPU resources, and for how long.
- 7. Single Processor Scheduling Algorithms First Come, First Served (FCFS) Shortest Job First (SJF) Priority Round Robin (RR)
- 8. First Come, First Served (FCFS) First Come, First Served (FCFS), is the simplest scheduling algorithm, FIFO simply queues processes in the order that they arrive in the ready queue. Throughput can be low, since long processes can hold the CPU It is based on Queuing No prioritization occurs, thus this system has trouble meeting process deadlines.
- 9. Example
- 10. MERITS AND DEMERITS MERITS:Easy to understand and easy to program It is fair DEMERITS: Does not perform well in real systems Ignores the service time request and all other criteria
- 11. Shortest-Job-First (SJR) Scheduling 1. non preemptive – once CPU given to the process it cannot be preempted until completes its CPU burst. 2. preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF). SJF is optimal – gives minimum average waiting time for a given set of processes.
- 12. Example
- 13. MERITS AND DEMERITS MERITS: SJF is optimal – gives minimum average waiting time for a given set of processes DEMERITS: Need to have a good heuristic to guess the next CPUexecution time Short duration processes will starve longer ones
- 14. Priority Scheduling The SJF algorithm is a special case of the general priority scheduling algorithm A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (smallest integer = highest priority)
- 15. Example
- 16. MERITS AND DEMERITS MERITS: Simplicity Reasonable support for priority. Suitable for applications with varying time and resource requirements. DEMERITS: Indefinite blocking or starvation. A priority scheduling can leave some low priority waiting processes indefinitely for CPU.
- 17. Round Robin (RR) Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. Performance 1. q large _ FIFO 2. q small _ q must be large with respect to context switch, otherwise overhead is too high.
- 18. Example
- 19. MERITS AND DEMERITS MERITS: Fair allocation of CPU across jobs Low average waiting time when job lengths vary DEMERITS: It is suitable for task with varied burst time. But for task with same time makes trouble.
- 21. Multilevel Feedback Queue A process can move between the various queues; Multilevel-feedback-queue scheduler defined by the following parameters: 1. number of queues 2. scheduling g algorithms for each queue
- 22. Example of Multilevel Feedback Queue Scheduling algorithm CPU Overhead Throughput Turnaround time Response time First In First Out Low Low High Low Shortest Job First Medium High Medium Medium Priority based Medium Low High High scheduling Round-robin scheduling High Medium Medium High Multilevel Queue High High Medium Medium scheduling
- 23. Multiprocessor Scheduling '' On a multiprocessor, scheduling is two dimensional. The scheduler has to decide which process to run and which CPU to run it on. This extra dimension greatly complicates scheduling on multiprocessors.
- 24. OPERATING SYSTEM EXAMPLES Windows Very early MS-DOS and Microsoft Windows systems were non-multitasking, and as such did not feature a scheduler. Windows 3.1x used a non-preemptive scheduler, meaning that it did not interrupt programs.
- 25. OPERATING SYSTEM EXAMPLES Mac OS Mac OS 9 uses cooperative scheduling for threads, where one process controls multiple cooperative threads, and also provides preemptive scheduling for MP tasks.
- 26. CONCLUSION The above mentioned are the various CPU scheduling used in the present day. These algorithms are inherited based on the recuirement of the processor.