Uniprocessor Scheduling

Chapter 9

Processor Scheduling• Processor scheduling determines the assignment of

processes to be executed by the processor over time• Goals of scheduling:

– High processor utilization• less idle time

– Fast throughput• large number of processes completed per unit time

– Quick response time• small elapsed time from the submission of a request to the beginning of

the response

– Fairness

– Low overhead

Classification of Scheduling Activity

• Long-term: which process to admit

• Medium-term: which process to swap in or out

• Short-term: which ready process to execute next

Queuing Diagram for Scheduling

Long-Term Scheduling• Determines which programs are admitted to the

system for processing• Controls the degree of multiprogramming• Batch jobs:

– Whether to create a new process? If more processes are admitted

• better CPU utilization - less likely that all processes will be blocked

• each process has smaller fraction of the CPU time

– Which job to admit next? May be based on priority, I/O requirements, etc.

• The long term scheduler may attempt to keep a mix of processor-bound and I/O-bound processes

Long-Term Scheduling II

• Interactive programs:– E.g. user connecting to time-sharing system– Requests not queued up– Will accept new process requests until the system is

saturated– When the system is full a new connection request is

refused with an error message

Medium-Term Scheduling

• Swapping decisions based on the need to manage multiprogramming

• On systems with no virtual memory, the size of the process is also a criterion

• Done by memory management software and discussed in chapters 7 and 8

Short-Term Scheduling• Determines which process is going to execute

next (also called CPU scheduling)• The short term scheduler is known as the

dispatcher• The scheduler is invoked whenever an event

occurs that may lead to blocking or preemption of the current process. Example events:– clock interrupts– I/O interrupts– operating system calls– signals

Short-Term Scheduling Criteria• Allocate processor time to optimize certain aspects of

system behavior such as the following (Table 9.2)• User-oriented criteria: perceived by a user or process

– response time (interactive jobs), turn-around time (batch jobs)

• System-oriented criteria– processor utilization, throughput

• Performance-oriented (quantitative): readily measured– response time, throughput

• Non-performance-oriented (qualitative)– predictability, fairness

• Thus, the design of a scheduling policy involves compromising among several competing requirements and depends on the nature and use of the system

Definition of terms• Turnaround time

– Interval of time between the submission of a process and its completion

• Response time– Interval of time from submission of a request until the response

begins to be received

• Throughput– Number of processes completed per unit time

• Processor utilization– Percentage of time the processor is busy

• Fairness– In the absence of priorities, processes should be treated the

same; no process should starve

Using Priorities• Implemented by having multiple ready queues to

represent each level of priority• Scheduler will always choose a process of higher

priority over one of lower priority• Problem: Lower-priority may suffer starvation• Then allow a process to change its priority based

on its age or execution history • Our first scheduling algorithms will not make use

of priorities• We will then consider other algorithms that use

dynamic priority mechanisms

Characteristics of Scheduling Policies• The selection function: determines which process

in the ready queue is selected next for execution• The decision mode: specifies the instants in time

at which the selection function is exercised– Non-preemptive

• Once a process is in the running state, it will continue until it terminates or blocks itself for I/O

– Preemptive• Currently running process may be interrupted and moved to

the Ready state by the OS• Allows for better service since any one process cannot

monopolize the processor for very long• Greater overhead than non-preemptive ones

ProcessArrivalTime

ServiceTime

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Service time = total processor time needed in one CPU-I/O cycleOrService time = total execution time required

Example to discuss various scheduling policies

• Selection function: the process that has been waiting the longest in the ready queue (hence, FCFS); also known as FIFO

• Decision mode: non-preemptive– a process runs until it terminates or blocks itself

First Come First Served (FCFS)

FCFS drawbacks

• A process that does not perform any I/O will monopolize the processor

• Favors CPU-bound processes– I/O-bound processes have to wait until CPU-bound

process completes– They may have to wait even when their I/O are

completed (poor device utilization)– We could have kept the I/O devices more busy by

giving a bit more priority to I/O bound processes

• FCFS is not an attractive policy on its own; combine with priority scheme

• Selection function: same as FCFS

• Decision mode: preemptive– a process is allowed to run until the time slice period

(quantum, typically from 10 to 100 ms) has expired– then a clock interrupt occurs and the running process

is put on the ready queue

Round-Robin (Time Slicing)

Time Quantum for Round Robin

• Length of the time slice is the principal design issue for round robin

• Very short time quanta should be avoided because there is processing overhead involved in handling the clock interrupt and dispatching function

• Should be slightly larger than a typical interaction time to get good response time

Round Robin: Critique• Effective in a time-sharing or transaction processing

system • Still favors CPU-bound processes

– A I/O bound process uses the CPU for a time less than the time quantum and then is blocked waiting for I/O

– A CPU-bound process runs for all its time slice and is put back into the ready queue (thus getting in front of blocked processes)

• A solution: virtual round robin (VRR)– When I/O is completed, the blocked process is moved to an

auxiliary queue which has preference over the main queue– A process dispatched from the auxiliary queue runs no longer

than the basic time quantum minus the time spent running since it was selected from the ready queue

Queuing for Virtual Round Robin

• Selection function: the process with the shortest expected processing time

• Decision mode: non-preemptive• Short processes are moved ahead of longer jobs

in the queue• We need to estimate the required processing time

(CPU burst time) for each process

Shortest Process Next (SPN)

• Simple averaging

– Ti : execution time for ith instance of this process (total execution time for batch job; processor burst time for interactive job)

– Si : predicted value for ith instance

– S1 : predicted value for first instance; not calculated

nn

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in Sn

nT

nT

nS

111

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Estimating average CPU burst

• Exponential averaging– Sn+1 = Tn + (1 - )Sn (0 1)

– Sn+1 = Tn + (1 - )Tn-1 + … + (1 - )iTn-i + … + (1 - )nS1

– when is close to 1, weight is given to most recent observations

• quickly reflect rapid change in observed quantity

– when is smaller, the averaging is spread over several observations

– Eg: = 0.8• Sn+1 = 0.8Tn + 0.16Tn-1 + 0.032Tn-2 +0.0064Tn-3 + …

– S1 = 0 gives greater priority to new processes

Estimating average CPU burst

S1 = 0

S1 = 0

Shortest Process Next: Critique

• Possibility of starvation for longer processes as long as there is a steady supply of shorter processes

• Lack of preemption is not suited for time sharing environments– CPU bound process gets lower priority (as it should)

but a process doing no I/O could still monopolize the CPU if it is the first one to enter the system

• SPN implicitly incorporates priorities: shortest jobs are given preferences

Shortest Remaining Time (SRT)

• Selection function: the process with the shortest expected remaining processing time

• Decision mode: preemptive• When a new process with a smaller remaining

time enters the ready queue, the running process is preempted for the new process

Shortest Remaining Time: Critique

• Can be considered a preemptive version of SPN• Does not have a bias for longer processes• Risk of starvation of longer processes still exists• As with SPN, need an estimate of processing

time• Also, elapsed service times must be recorded;

adds overhead• Gives better turnaround time than SPN because

short jobs are given immediate preference

Highest Response Ratio Next (HRRN)

• Selection function: the process with largest value of response ratio (RR)

– w: time spent waiting for the processor– s: expected service time

• Decision mode: non-preemptive

s

swRR

Highest Response Ratio Next: Critique

• Expected service time must be estimated as before

• Shorter jobs are favored because they have smaller denominator (larger ratio)

• Age of the process is also considered; therefore, longer jobs do get a chance to get past competing shorter jobs – waiting time for longer jobs increases, thus RR

increases and the longer job gets scheduled

Multilevel Feedback Scheduling• Preemptive scheduling (using time slices) with dynamic

priorities• Several ready to execute queues with decreasing

priorities: P(RQ0) > P(RQ1) > ... > P(RQn)• A new process is placed in RQ0 • When it is preempted (finishes its time quantum), it is

placed in RQ1. Next time it is preempted it is placed in RQ2 and so on, until it reaches RQn

• I/O-bound (short) processes will stay in higher priority queues. CPU-bound jobs will drift downward.

• Dispatcher chooses a process for execution in RQi only if RQi-1 to RQ0 are empty, hence longer jobs may starve

Multiple Feedback Queues

• FCFS is used in each queue except for lowest priority queue where Round Robin is used

• With a fixed quantum time, the turnaround time of longer processes can stretch out alarmingly

• To compensate we can increase the time quantum according to the depth of the queue (RQi = 2i)

• Longer processes may still suffer starvation. Possible fix: promote a process to higher priority after some time

Time Quantum for feedback Scheduling

Algorithm Comparison

• Which one is best?

• The answer depends on various factors such as– the system workload (extremely variable)– relative weighting of performance criteria (response

time, CPU utilization, throughput...)

• Summary of algorithms: See– Table 9.3– Table 9.5, Figure 9.5