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PRIORITY BASED SCHEDULING: Assign each process a priority. Schedule highest priority first. All processes within same priority are FCFS. Priority may be determined by user or by some default mechanism. The system may determine the priority based on memory requirements, time limits, or other resource usage.
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Basic principles. A set of queues with diferent priorities. At any moment, a ready job is in at most one queue. Basic scheduling rules: Rule 1: If priority(A) > priority(B), then A runs (B doesn’t) Rule 2: If priority(A) == priority(B), RR is applied. MLFQ scheduling.
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Operating Systems — N/H/MWF@12. Course Aims • This course aims to: – explain the structure and functions of an operating system, – illustrate key operating system aspects by concrete example, and – prepare you for future courses. . . • At the end of the course you should be able to: – compare and contrast CPU scheduling algorithms ...
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- Chapter 6: CPU Scheduling
- Objectives
- Basic Concepts
- Dispatcher
- Scheduling Criteria
- Scheduling Algorithm Optimization Criteria
- Shortest-Job-First (SJF) Scheduling
- Example of Shortest-remaining-time-first
- Priority Scheduling
- Multilevel Feedback Queue
Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time CPU Scheduling Operating Systems Examples Algorithm Evaluation
To introduce CPU scheduling, which is the basis for multiprogrammed operating systems To describe various CPU-scheduling algorithms To discuss evaluation criteria for selecting a CPU-scheduling algorithm for a particular system To examine the scheduling algorithms of several operating systems
Maximum CPU utilization obtained with multiprogramming CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait
Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: switching context switching to user mode jumping to the proper location in the user program to restart that program Dispatch latency – time it takes for the dispatcher to stop one process and start another running
CPU utilization – keep the CPU as busy as possible Throughput – # of processes that complete their execution per time unit Turnaround time – amount of time to execute a particular process Waiting time – amount of time a process has been waiting in the ready queue Response time – amount of time it takes from when a request was submitted until the fi...
Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time
Associate with each process the length of its next CPU burst Use these lengths to schedule the process with the shortest time SJF is optimal – gives minimum average waiting time for a given set of processes The difficulty is knowing the length of the next CPU request Could ask the user
Now we add the concepts of varying arrival times and preemption to the analysis ProcessAarri Arrival TimeT
A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (smallest integer highest priority) Preemptive Nonpreemptive SJF is priority scheduling where priority is the inverse of predicted next CPU burst time Problem Starvation – low priority processes may never execute Solution ...
A process can move between the various queues; aging can be implemented this way Multilevel-feedback-queue scheduler defined by the following parameters: number of queues scheduling algorithms for each queue method used to determine when to upgrade a process method used to determine when to demote a process method used to determine which queue a pr...
10. Scheduling. Scheduling is the policy the operating system uses to time multiplexes the CPU between different tasks. A large number of scheduling policies have been developed over the years. Some of these are specialized for particular types of computing systems, while others aim to support more general-purpose systems.
3.1 Process Concept. An operating system executes a variety of programs that run as a process. Process – a program in execution; process execution must progress in sequential fashion. No parallel execution of instructions of a single process. Multiple parts.
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What is process scheduling?
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The process scheduling is the activity of the process manager that handles the removal of the running process from the CPU and the selection of another process on the basis of a particular strategy. Process scheduling is an essential part of a Multiprogramming operating system. Such operating systems allow more than one process to be loaded ...
