Chapter 4 CPU Scheduling Contents Scheduling concepts Types

Chapter 4 CPU Scheduling

Contents Scheduling concepts Types of scheduling – Long, medium, short CPU scheduler Dispatcher Scheduling criteria Kinds of scheduling Scheduling algorithms Scheduling algorithm evaluation

Scheduling • Key to multi-programming • Objective of multiprogramming is to maximize resource utilization • Not possible to achieve without proper scheduling • All resources are scheduled before use • CPU is primary resource and scheduling CPU is central to OS • Four types of schedulers: Long term, Short term, Medium term, I/O

CPU, I/O cycle • Process execution consists of cycle of CPU execution and I/O operation • Process alternate between these 2 states • Begins with CPU burst, followed by I/O burst, again CPU burst …. terminates after a CPU burst • I/O bound jobs have short CPU bursts • CPU bound jobs have long CPU bursts • Durations of CPU bursts have been extensively measured • Many jobs have more shorter CPU bursts

Process Scheduling

Long Term Scheduler • Also called as job scheduler • Determines which programs are to be submitted for processing • Controls the degree of multi-programming • Selects processes from pool and loads them into memory for execution • Selection is between I/O bound and CPU bound jobs • Executes much less frequently • Needs to be invoked when a process leaves the system

Medium Term Scheduler • Part of the swapping function • Usually based on the need to manage multiprogramming • The process can be reintroduced into memory and its execution can be continued where it left off • If no virtual memory, memory management is also an issue

Short Term Scheduler • Also called as CPU scheduler • Makes fine grained decisions of which job to execute next. That is which job actually gets to use the processor • Selects from among the processes that are ready to execute and allocates the CPU to one of them • This scheduler is called often and it executes at least once every milliseconds

• Selects a process from the processes in Ready queue • CPU scheduling decisions occur when a process: 1. switches from running to waiting state 2. switches from running to ready state (interrupt occurs) 3. switches from waiting to ready state (ex. after completion of I/O) 4. terminates • Scheduling under 1, 4 is non preemptive • Scheduling under 2 and 3 is preemptive

Preemptive and non-preemptive scheduling • New job has to be scheduled in cases of 1 and 4 – non preemptive • In case of 2 and 3 scheduling is optional – preemptive • Scheduling done when status of ready queue changes • Preemptive scheduling is expensive, hard • Requires extra hardware • Processes can share data • Should be careful and cautious -if one preempted by another and the shared data referred by the new process • At time of processing system calls, kernel may be busy on behalf of a process • If preempted, chaos is the result – unless such preemptions are taken care of the OS

Dispatcher • Dispatcher module gives control of the CPU to the process selected by the short-term scheduler and this involves: – switching context – switching to user mode – jumping to the proper location in the user program to restart that program • Dispatch latency – the time taken by dispatcher to stop one process and start another running • Dispatcher should execute fast because it is invoked during every process switch

Scheduling Criteria • CPU utilization – To what percentage CPU is utilized • 40% - lightly loaded and 90% - heavily loaded • Throughput – No. of processes that complete their execution per unit time (Degree of multiprogramming) • For long processes it could be 1 in an hour and for short processes it could be 10 per sec • Turnaround time –Time interval between the time of submission and completion (Execution time) • Includes also waiting times for CPU as well as I/O devices

Scheduling criteria • Waiting time – sum of all the time waiting in Ready queue l Does not take into account wait time for I/O and I/O operation time • Response time – amount of time it takes from the time of submission of a job until the first response is produced l In time shared systems small turn around time may not be enough l Response time should be small. It is the time taken to start responding. Does not include time to output that response

Optimization Criteria • Maximize CPU utilization and throughput • Minimize turnaround time, waiting time and response time • We optimize average times. Occasionally optimize extreme measures Ex. Minimize maximum response time • In time shared systems it is better if variance in response time is minimized • A system that has reasonable and predictable response time is better than system with small average response time and highly variable

Kinds of Scheduling • Non-preemptive scheduling: A process runs to completion when scheduled • Preemptive scheduling: A process may be preempted for another process which may be scheduled. A set of processes are processed in an overlapped manner

Non-preemptive and preemptive scheduling methods • Non-preemptive: • FCFS – First come first served • SJF – shortest job first and SRTF – shortest remaining time first • Priority scheduling • Preemptive: • SJF • Priority scheduling • Round robin • Multilevel queue

First Come First Served (FCFS) • Example: Process Burst Time P 1 24 P 2 3 P 3 3 • Assume processes arrive at time 0 The Gantt Chart for the schedule is: P 1 0 P 2 24 P 3 27 30 • Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 • Average waiting time: (0 + 24 + 27)/3 = 17

FCFS Scheduling Suppose processes arrive as: P 2 , P 3 , P 1. • The Gantt chart for the schedule is: P 2 0 P 3 3 P 1 6 30 • Waiting time for P 1 = 6; P 2 = 0; P 3 = 3 • Average waiting time: (6 + 0 + 3)/3 = 3 • Much better than previous case

FCFS scheduling • All I/O bound processes finish CPU bursts quickly and wait for I/O operations • Convoy effect - short processes wait for a long process to get off the CPU • CPU bound process is keeping I/O device – I/O operations are much slower than CPU operations • CPU is idle and all other jobs wait for I/O • Not good for time shared systems – each user has to get a share of CPU time frequently

Shortest-Job-First (SJF) Scheduling • In fact it is shortest next CPU burst • Assume CPU burst length for each process in ready queue are known • Two schemes: – Non-preemptive – once CPU assigned, process not preempted until its CPU burst completes – Can be preemptive – if a new process with CPU burst less than remaining time of current, preempt Shortest-Remaining-Time-First (SRTF) • SJF is optimal – gives minimum average waiting time for a given set of processes

Example of Non-Preemptive SJF Process Burst Time P 1 16 P 2 3 P 3 4 SJF (non-preemptive) The Gantt Chart for the schedule is: P 2 0 P 3 3 P 1 7 23 Here, the waiting time for P 1 is 7 ms, P 2 is 0 ms, P 3 is 3 ms. • Average waiting time = (7 + 0 + 3 )/3 =10/3=3. 33 ms.

Example of Preemptive SJF Process Arrival time Burst Time P 1 0 8 P 2 1 4 P 3 2 9 P 4 3 5 SJF (preemptive) The Gantt Chart for the schedule is: p 1 0 p 2 1 p 4 5 p 1 10 p 3 17 19 • Average waiting time = (10 -1) +(17 -2)+(5 -3)/4 = 26/4 =6. 5 ms.

SJF / SRTF scheduling • Provably optimal – gives minimum average wait time • By moving short jobs first it decreases wait times of these more than increase the wait times of long ones • Difficulties: CPU bursts should be known in advance • Good for long term scheduling – programmers estimate run time and submit with job request • Can predict CPU burst time based on the previous and past burst times of the process • Predictions of bursts can be used to do SJF – this is approximate SJF scheduling

Burst time prediction • • Exponential average formula Tn+1 = a * tn + (1 -a) * Tn : 0 a 1 Tn stores the past history tn contains most recent information of burst time • Set values for ‘a’ suitably • Tn+1 = a * tn + (1 -a)*a* tn-1+(1 -a)2* a* tn-1 …

Priority Scheduling • Priority number (integer) associated with process • SJF and SRTF are special cases of general priority scheduling • Larger the burst time lower is the priority • CPU allocated to process with highest priority • Can be preemptive or non preemptive • Preemptive priority scheduling will preempt the CPU if a high priority job arrives to ready queue • Problem: Starvation / indefinite blocking low priority processes may never execute • Solution: Aging increasing gradually the priority of processes which are waiting in the system for CPU for a long time • Ex: If priority is 127 (low) decrement by 1 for every 15 minutes of wait – takes 32 hours to get the priority 0.

Example of priority scheduling Process Burst Time (ms) P 1 8 P 2 2 P 3 1 P 4 3 p 5 4 p 2 0 p 4 1 p 1 5 priority 3 1 3 2 4 p 3 13 p 5 14 18 • Average waiting time = wait times of (p 1+p 2+p 3+p 4+p 5)/5 = (5+0+13+2+14)/5= 34/5 = 6. 8 ms

Round robin scheduling • Designed for time-sharing systems • Jobs get the CPU for a fixed time (quantum time or time slice) • Similar to FCFS, but with preemption - CPU interrupted at regular intervals • Needs hardware timer • Ready queue treated as a circular buffer • Process may use less than a full time slice • They terminate and scheduling take place • If process is incomplete at the end of time slice, they join end of ready queue • With n processes and quantum = q, each process waits for at most (n-1)*q

Dynamics of Round Robin (RR) • Decision mode: preemptive – A process is allowed to run until the time slice period called time quantum, is reached – Then a clock interrupt occurs and the running process is dispatched • If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units.

Performance of RR • • • Depends on quantum Extremes: Very large – FCFS Very small (1 ms) – processor sharing Processes get a feel that each one owns a processor with speed = 1/n of the single processor Context switching effect to be considered Small quantum – Context switches are frequent CPU will spend lot of time on switching If context switch time is 10% of quantum, 10% of CPU time spent on switches

Performance of RR • Turn around time also depends on time quantum • Increase in quantum time - less frequent executions of switching – lesser turn around time • Average turn around time (TT) does not always improve with increase in quantum time • TT can be improved if most processes can finish their jobs in one quantum • With 3 processes of 10 ms each, and time slice = 1 ms, average turn around time = 29 ms (excluding switching time) • If time slice = 10, average turn around time = 20 ms (plus switching time)

Time quantum vs turn around time • Suppose that time quantum = 1 ms • CPU bursts of 4 jobs all arriving at time 0 be as follows: P 1 P 2 P 3 P 4 6 3 1 7 15 9 3 17 are turn around times Average TT = 11 ms • If time slice = 2, 3, 4, 5, 6 and 7 we get average TT to be 11. 5, 10. 8, 11. 5, 12. 2, 10. 5

Round-Robin Example Process Arrival Time Service Time 1 0 3 2 2 6 3 4 4 4 6 5 5 8 2

Example of RR with Time Quantum = 20 Process P 1 P 2 P 3 P 4 Burst Time 53 17 68 24 • The Gantt chart is: P 1 0 P 2 20 37 P 3 P 4 57 P 1 77 P 3 P 4 P 1 P 3 97 117 121 134 154 162

Features of RR • All processes gets equal share of the CPU • Typically RR gives longer turnaround time than SJF, but better response time – good for time-sharing • Short time slices gives better response time • Simple, low overhead, works for interactive systems If quantum too small, too much time wasted in context switching If too large, reduces to FCFS Typical value: 10 - 100 ms Rule of thumb: Choose quantum so that large majority (80 -90%) of jobs finish CPU burst in one quantum

Multilevel queue scheduling • This scheduling partitions the ready queue into several separate queues. Ex: Ready queue can be logically divided into separate queues based on the idea that jobs can be categorized as: - foreground (interactive) - background (batch) • Assign high priority for type 1 jobs - externally • These two categories have different response time requirements – make 2 queues • Each queue has its own scheduling algorithm: foreground – RR background – FCFS • Method is complex but flexible

Multilevel queue scheduling • Scheduling must be done among the queues –fixed priority preemptive • All foreground jobs are served and then background attended to only when foreground queue empty • Another method -using time slice between the queues: • Each queue gets a certain amount of CPU time which it can schedule amongst its processes i. e. , 80% to foreground in RR; 20% to background in FCFS

Multilevel Queue Scheduling

Multilevel feedback queue • Preemptive scheduling with dynamic priorities • A process can move between the various queues • Multilevel-feedback-queue scheduler defined by the following parameters: – number of queues – scheduling algorithms for each queue – method used to determine which queue a process will enter when that process needs service – method used to determine when to upgrade process – method used to determine when to demote process • Most general CPU scheduling – most complex • Several parameters to set – if chosen properly could be the best for target system

Multilevel Feedback Queues

Example of Multilevel Feedback Queue • Three queues: – Q 0 : time quantum 8 milliseconds – Q 1 : time quantum 16 milliseconds – Q 2 : FCFS • Scheduling : – A new job enters queue Q 0 which is served FCFS. When it gets CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q 1. – At Q 1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q 2. – Could be also vice versa.

Example of Multilevel Feedback Queue

Comparison of algorithms • Which one is best ? • The answer depends on: – on the system workload (extremely variable) – hardware support for the dispatcher – relative weighting of performance criteria (response time, CPU utilization, throughput. . . ) – The evaluation method used (each has its limitations. . . ) • Hence the answer depends on too many factors to give any. . .

Scheduling Algorithm Evaluation • • Deterministic modeling Queuing models Simulation Implementation

Algorithm Evaluation • Analytic evaluation: - Deterministic modeling : takes a particular predetermined workload and compares the performance of each algorithm for that workload. Simple and easy to do; but requires exact numbers for input which is hard to get for any given system. Not realistic. • Queuing models (stochastic model ) : - Knowing the job arrival rates and service rates we can compute CPU utilization, average queue length, average waiting time, etc. - This is a separate area called queuing-network analysis.

• Experimental method: - Simulation : - Involves programming a model of the system - Results are of limited accuracy • Implementation: - Implement the algorithm and study the performance - It is the best way to study and compare the performance of different algorithms - It is expensive and time consuming

Evaluation of CPU Schedulers by Simulation

Evaluation by implementation • Best way to tune parameters of OS is to implement and let it be in use and collect statistics • Change parameters if needed • Difficulties: • Changing environment – Jobs may not be of same type and number always • Clever users can change style of code • Non-interactive programs can be changed as interactive (dummy inputs) so that his job gets high priority • Big jobs may be broken down to several small ones • Flexible OS will allow system managers change settings and priorities • If pay checks are needed urgently, this job’s priority can be changed temporarily and set back to low after the job is done • Such systems are rarely available