Operating System Concepts and Techniques Lecture 5 Scheduling-1 M. Naghibzadeh Reference M. Naghibzadeh, Operating System Concepts and Techniques, First ed., iUniverse Inc., To order: or

2 Terms Scheduling policy describes the guideline for selecting the next process to use a computer resource such as CPU. Scheduling algorithm is a finite set of clearly stated unambiguous stepwise operations that when applied to a finite set of processes waiting to use a resource, either one or more of these processes are selected or a time table for the resource usage is produced Scheduling mechanism determines how to do every step of the scheduling algorithm For example, the exact steps to be taken to modify a process-queue and process state

Scheduling Criteria Properties which influence the order in which active entities are selected to run request Time Processing Time Priority  Static priority  dynamic priority Deadline Wait Time Preemptability 3

Scheduling Objectives Throughput Deadline hard real-time soft real-time Turnaround Time Average turnaround time Normalized Response Time Priority Processor Utilization System Balance Fairness 4

Scheduling levels High-Level: select requests Medium-level: decide on process swapping Low-level: decide on giving CPU time to ready processes Input/output scheduling: decide on disk read/write order 5

First-Come-First-Served (FCFS) FCFS: take the request which arrived earliest Nonpreemptive Most natural, link banks, airport check-ins, post offices, and barber shops Irregularity in arrival requires a queue Simple to implement  A single-linked list with two pointers; each link points from each node to the node of the previous request Ddisadvantages: a long request can increase others wait time 6

Shortest Job Next (SJN) SJN (SPN): take the request which needs the least execution time Non-preemptive Executes higher number of requests in one unit of time To implement, a double-linked list with two pointers; keeps requests ordered Disadvantages: need to know execution time, update overhead time of the list, and starvation 7

SRTN & HRRN Shortest Remaining Time Next (SRTN) Nonpreemptive Scheduling decision right after a process is completed Disadvantages similar to SPN Highest Response Rattio Next (HRRN) RR=(w+e)/e; it increases as w increases Nonpreemptive Scheduling decision right after a process is completed Disadvantages similar to SPN 8

Two more Fair-Share Scheduling All users get equal shares of CPU  Prevents a user to get a big chunk of CPU time by creating many child processes or threads Priority scheduling Processes have priorities; A higher priority is executed before a lower priority one Preemptive Disadvantage: starvation of low priority request 9

Round Robin (RR) The most widely used scheduling policy  A time quantum is defined and an interval timer is set to send an interrupt whenever the time quantum has passed, since the timer is set or reset  Time quantum (slice) is very small, one millisecond or less  Upon timer interrupt a process switching is done  If a process cannot use its slice, processor switches to the next process and resets timer  All (same priority) processes share CPU time 10

RR advantages/disadvantages Advantages:  Quick response to interactive requests  Longer waiting time for a process that requires longer CPU time  Possibility of defining many priority levels, Disadvantage:  Task switching overhead 11

Process Scheduling in Linux Many levels of priorities (100 or more) Three schedulers SCHED_FIFO: Nonpreemptive, highest priority, say from , for different processes. SCHED_RR: Preemptive, second highest priority, say from Same level highest priority RR processes are scheduled using round robin  Priorities are assigned so that any SCHED_FIFO process has higher priority than any SCHED_RR; any SCHED_RR has higher priority than any SCHED_OTHER 12

Process Scheduling in Linux… SCHED_OTHER Every process has a static priority Time is divided into variable size frames called epoch At start of every epoch, scheduler calculates every SCHED_OTHER process’ CPU share in integer number of scheduling-clock ticks; called credit, epoch length is calculated Credit is related to static priority Epoch length is related to how many processes are ready, what their priorities are etc. 13

SCHED_OTHER... If a process cannot use its credit it is saved for the next time (see details in the book) Goodness shows which process is executed next For SCHED_FIFO and SCHED_RR, goodness=priority+1000 For SCHED_OTHER, goodness=credit 14

Summary It is not always the case that every service request arrives after the previous service request has been completely served. Service requests may pile up, in which case we would like to serve requests in such an order so as to increase some metrics. This process is called scheduling. Scheduling methodologies and algorithms for single-processor systems are considered Many scheduling strategies are studied, FCFS, SJN, SRTN, HRRN, and RR The Linux operating system was selected to present real- world schedulers 15

16 Find out What an interactive process is The block structure of an interval timer The time slice of your computer’s processor Whether Linux scheduler is a complete one for real-time tasks Why epoch length is not fixed The goals of Linux schedulers The difference between SCHED_FIFO and SCHED_RR

17 Any questions?