CISC3595

CPU Scheduling

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Basic ConceptsMultiprogramming

Load multiple program into memory, and run them simultaneously

A process (or kernel-level thread) is executed until it must wait (for I/O completion, child exiting…), OS then switches to run another process (or thread)

Goal: improve CPU utilization

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Process SchedulingShort-term scheduling (CPU scheduling) Select one process from ready queue to

be executedMedium-term scheduling, Long-term scheduling

What is ready queue ? Contains PCB for all processes that lines

up waiting for CPU Not necessarily FIFO queue

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Process Control Block (PCB): review

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Information associated with each process Process state Program counter:

address of next instruction to be executed CPU registers:

Contents of CPU registers CPU scheduling information

Process priority, … Memory-management information

Value of base and limit registers, page/segmentation tables (to be studied later)

Accounting information Amount of CPU time used …

I/O status information I/O device allocated, list of open files…

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Process Control Block (PCB): review

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Context Switch: review

6P0, P1 are running concurrently, i.e., their executions are interleaved.

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Process Scheduling Queues: review

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OS maintain multiple process queues for scheduling purpose:

Job queue – set of all processes in the system Ready queue – set of all processes residing in

main memory, ready and waiting to execute Device queues – set of processes waiting for

an I/O device Processes migrate among the various

queues

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Ready Queue And I/O Device Queues: review

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CPU and I/O Bursts• Process execution consists of a cycle of CPU execution and I/O wait• CPU burst:

• Data processing, calculation

• E.g. sorting, searching• I/O burst:

• Read from I/O device• File system access

•What about the myCopy program?

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Histogram of CPU-burst Times

A large number short CPU burst

Small number of long CPU burst

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CPU Scheduler Selects one process from ready queue, and

allocates CPU to execute it When to make CPU scheduling :

1. a process switches from running to waiting state

2. a process switches from running to ready state

3. a Process switches from waiting to ready4. a process terminates

Nonpreemptive scheme: only run CPU scheduling under 1 and 4 i.e., current running process gives up CPU

Otherwise, preemptive scheduling scheme

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Dispatcher module Scheduler => Dispatcher Dispatcher module: gives control of CPU to the

process selected by short-term scheduler 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 Including context switching latency

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CPU Scheduling AlgorithmsFirst-come, First-Served SchedulingShortest-Job-First SchedulingPriority SchedulingRound-Robin SchedulingMultilevel Queue SchedulingMultilevel Feedback Queue Scheduling

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CPU Scheduling Criteria CPU utilization – percentage of time CPU is busy

(not idle) Throughput – # of processes that complete their

execution per time unit Turnaround time – amount of time to execute a

particular process From process submission to time of completion

Waiting time – amount of time a process has been waiting in ready queue

Response time – amount of time it takes from when a request was submitted until first response is produced (for time-sharing environment)

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Optimization Criteria Maximize CPU utilization Maximize throughput Minimize turnaround time Minimize waiting time Minimize response time

Achieved performance measure is random variable depending on the statistics of workload

Usually, optimize average measure (average case performance)

Sometimes, optimize minimum or maximum value (worst case performance)

Or sometimes, variance of the measure

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First-Come First-Served SchedulingServe processes in the order of arrivals, non-preemptive

Process Burst TimeP1 24 P2 3 P3 3

Suppose that processes arrive in the order: P1 , P2 , P3 Gantt Chart for FCFS schedule is:

Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17

P1 P2 P3

24 27 300

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FCFS Scheduling (Cont.)Process Burst TimeP1 24 P2 3 P3 3

Suppose that processes arrive in the order P2 , P3 , P1

The Gantt chart for FCFS schedule is:

Waiting time for P1 = 6; P2 = 0; P3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3 Much better than previous case

Short process behind long process

P1P3P2

63 300

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Exercise Draw Gantt chart for FCFS schedule for the

following set of processes arrived in the order of P1, P2, P3, P4.

Process Burst TimeP1 10P2 4P3 6

P4 5 Average waiting time What order of arrival of these processes will give smallest

average waiting time ?

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FCFS Scheduling What will happen if the system has

one CPU-bound process many I/O-bound processes

A Convoy effect happens when a set of processes need to use a resource for a short

time one process holds the resource for a long time,

blocking all of the other processes Result: poor utilization of the other resources in the

system. Other problems of FCFS ?

Ill-suited for time-sharing systems (interactive environment)

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Shortest-Job-First (SJF) Scheduling Schedule process with shortest-next-CPU-

burst (i.e., the length/duration of next CPU burst)

Two schemes: nonpreemptive – once CPU given to the process it

cannot be preempted until completes its CPU burst

preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. Also called Shortest-Remaining-Time-First (SRTF)

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Shortest-Job-First (SJF) Scheduling Among non-preemptive scheduling algorithms, SJF

achieves minimum average waiting time for a given set of processes that arrives at the same time. Suppose the duration of CPU burst of n processes is l1,l2,…ln , li<lj for i<j, (it’s sorted in ascending order

Assume the schedule that achieves minimum average waiting time is not SJF, it means that there exists at least a pair of processes, j and k, such that lj<lk, but process j is scheduled to run after process k

The schedule: … lk … lj … We can switch lk and lj to decrease avg. waiting time. Assumption is wrong, and SJF achieves minimum avg. waiting

time.

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Example of Non-Preemptive SJFProcess Arrival Time Burst Time

P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4

SJF (non-preemptive)

Average waiting time = (0 + 6 + 3 + 7)/4 = 4

P1 P3 P2

73 160

P4

8 12

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Example of Preemptive SJFProcessArrival Time Burst TimeP1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4

SJF (preemptive)

Average waiting time = (9 + 1 + 0 +2)/4 = 3

P1 P3P2

42 110

P4

5 7

P2 P1

16

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Length of Next CPU Burst ? Predict next CPU burst length

Using length of previous CPU bursts, using exponential moving averaging

: Define4.10 , 4.

lengthburst CPU 1)(n predicted 3.

burst CPU of length actual 2.

burst CPU n thedenote tonsubscript use 1.

th1

th

n

thn nt

.1 1 nnn t

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Exponential Average Prediction

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Examples of Exponential Averaging =0

n+1 = n,, recent history does not count

=1 n+1 = tn, only previous

CPU burst counts Expand formula:

n+1 = tn+(1 - ) tn -1 + … +(1 - )j tn -j + … +(1 - )n +1 0

each successive term has less weight than its predecessor Both and (1 - ) are less than or equal to 1

.1 1 nnn t

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Priority Scheduling CPU is allocated to process with the highest

priority A priority number (integer) is associated with each

process smallest integer highest priority

Can be Preemptive or Nonpreemptive Preemptive: newly arrived higher priority process

preempt current running process Priority can be internal or external

Internal: decided based on some quantities of process resource requirement, ratio of I/O burst and avg burst,

next CPU bursts length External: importance, funds paid,…

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Priority Scheduling Problem of priority scheduling

Indefinite blocking, starvation: low priority processes may never execute

Solution Aging – as time progresses increase the priority of the

waiting process E.g., increase the priority of a waiting process by 1

every 15 minutes…

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Round Robin (RR) So far, we see FCFS, SJF, Priority Scheduling

How they perform for time-sharing system ? 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.

If there are n processes in ready queue and time quantum is q, then each process gets 1/n of CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units.

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Example of RR with Time Quantum = 20ProcessBurst Time

P1 53 P2 17 P3 68 P4 24

The Gantt chart is:

Typically, higher average turnaround than SJF, but better response

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162

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Round Robin (RR) Choosing q, time quantum

q large FIFO q small processor sharing

Downside of having very small q ? q must be large with respect to context switch,

otherwise overhead is too high

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Quantum and Context Switch

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Turnaround Time Varies vs Time Quantum

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Multilevel Queue Ready queue is partitioned into separate queues

Realtime process foreground (interactive) process background (batch) process

Each queue has its own scheduling algorithm Realtime – Earliest Deadline First foreground – RR background – FCFS

Scheduling must be done between queues Fixed priority scheduling; (i.e., serve all from realtime,

then foreground, then from background). Low priority process might experience starvation

Time slice – each queue gets a certain amount of CPU time i.e., 80% to foreground in RR, 20% to background in FCFS

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Multilevel Queue Scheduling

How do we assign process to appropriate queue ?

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Multilevel Feedback Queue Scheduling Compared to multilevel queue scheduling

A process can move between various queues; aging can be implemented this way

To favor shorter jobs, or process with short CPU burst Penalize process that have been run longer by

moving it to lower priority queue … Feed the process back to a different queue

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Example of Multilevel Feedback Queue

Q0 – RR with quantum 8 millisecondsQ1 – RR quantum 16 millisecondsQ2 – FCFS Strict priority among queues Q1>Q2>Q3

Process becomes ready

I/O, terminate

I/O, terminate

I/O, terminate

Timer goes off

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Multilevel Feedback Queue Effect of multilevel feedback queue

I/O bound and interactive processes stay in highest priority queue

CPU bound process stays in low priority queue 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 process will

enter when that process needs service

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Multiple-Processor Scheduling CPU scheduling more complex when multiple

CPUs are available Homogeneous processors within a multiprocessor Two appraches

Symmetric multiprocessing: each processor is self-scheduling Share a common queue, or private queue

Asymmetric multiprocessing: one processor (master server) handles scheduling decision, I/O Processing, and other system activities – only one processor accesses the system data structures,

alleviating the need for data sharing

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Multiple-Processor Scheduling: considerations Recall L1 cache ?

To avoid cache invalidation and repopulation, scheduling taking into account processor affinity: a process has an affinity to the processor that it is currently running on Avoid migrating a process to other processor

Load balancing: keep workload balanced among processors Needed if each processor has its private queue Push migration and pull migration

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Real-Time Scheduling Hard real-time systems – required to

complete a critical task within a guaranteed amount of time

Soft real-time computing – requires that critical processes receive priority over less fortunate ones

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Thread Scheduling Local Scheduling – How the threads library

decides which thread to put onto an available LWP

Global Scheduling – How the kernel decides which kernel thread to run next

Case studies: Windows XP and Linux

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Homework: comparing CPU scheduling used by Windows XP and Linux

Details to come Evaluation of different CPU scheduling

algorithms By hand Through mathematical model Through simulation studies Actual Implementation and measurement