processor scheduling

OS/ProcSche/NI2008 1

Processor SchedulingScheduling is the activity of selecting the next Job / Process / Request to be handled by a Server/CPU.Preemption Scheduling: A request being processed is preempted in order to provide service to other requests in the system. It is for providing fair services to all the requests, or servicing the superior request. Non Preemptive Scheduling: A non preemptive scheduling / server always processes a scheduled request / job to completion.

CPU scheduling criteria CPU Utilization – Percentage of Time that the processor

is busy.

Throughput – how many jobs finished/unit time?• No of processes completed per unit of time.

Turnaround Time – how long from job submission to job termination?

Interval of time between the submission of a process and its completion. Includes actual execution time + time spent waiting for resources, including the processor.

Response Time – how long (on average) does it take to get a “response”?

For an interactive process, this is the time from submission of a request/job until the response begins to be received.

Missed deadlines – were any deadlines missed?

CPU scheduling criteria

Delay Time / CPU Latency Time: Delay caused in selecting a process for execution

Burst/Service Time: Actual time taken by CPU to completes the process

Weighted Turn around time: Ratio of the turn around time of a job/process to its

own execution requirements. Deadline:

Time by which a job/process must be completed Deadline Overrun:

The amount of time by which the deadline of a job/process is exceeded. It can be both positive or negative.

Schedule Length: The total time taken to complete a set of jobs/processes submitted to a server.

Scheduler options Priorities

May use priorities to determine who runs next

• Amount of memory, Order of arrival, etc.. Dynamic vs. Static algorithms

• Dynamically alter the priority of the tasks while they are in the system (possibly with feedback)

• Static algorithms typically assign a fixed priority when the job is initially started.

Preemptive vs. Nonpreemptive Preemptive systems allow the task to be

interrupted at any time so that the O.S. can take over again.

Types of Scheduling Scheduling is split into three

steps/levels/stages Long Term Scheduling / Admission Scheduler

– ( New state / Admit state) to Ready Medium Term Scheduling / Memory Scheduler

– ( Ready / Suspended / Blocked / waiting) Short Term Scheduler / CPU scheduler

– ( Preemptive / Non-Preemptive)– ( Ready to Running State)

Types of Scheduling Long Term Scheduling:

• When to initiate the processing of a job / process. Which programs are admitted to the system for processing. The others are kept in the input queue until they are selected.

• Allocates necessary resources • Make it ready to send to the Medium Term Scheduler.

Medium Term Scheduler:• MTS keeps track of states of processes admitted for

processing and prepares for execution on the CPU. • It determines which processes should be kept in

memory and which one kept on disk (virtual memory). • The decision to add to the number of processes that

are partially or fully in main memory. • Performs memory swapping in/out.

Types of Scheduling Short Term Scheduling: The short term scheduler is

invoked whenever an event occurs that may lead to the suspension of the current process or that may provide an opportunity to preempt a currently running process in favor of another. Event are .

• Clock interrupts• I/O interrupts• Operating system calls • Signals

STS picks one process from the list of ready processes for execution on the CPU, and hands it to the Dispatching Mechanism. Dispatching loads the state of the selected process – contents of the PSW and CPU registers – into the CPU and allows the CPU to resume execution of the process.

Scheduling policies / Algorithms First-Come, First Served (FIFO) Shortest Job First (non-preemeptive) Shortest Job First (with preemption)

( Shortest Remaining Time Next ) Highest Response Ratio Next Scheduling Shortest Job Next Deadline Scheduling Priority Scheduling Real-Time Scheduling Round-Robin Scheduling

Scheduling policies (Preemptive)

Round Robin Scheduling Heavily loaded system Two Queues Multiple Queues Guaranteed Scheduling Lottery Scheduling Fair Share scheduling

First-Come, First-Served (FIFO) Start jobs in the order they arrive (FIFO queue) Run each job until completion

First-Come, First-Served (FIFO) Start jobs in the order they arrive (FIFO queue) Run each job until completion Arrival Processing Process Time Time 1 0 3 2 2 6 3 4 4 4 6 5 5 8 2

First-Come, First-Served (FIFO) Start jobs in the order they arrive (FIFO queue) Run each job until completion Arrival Processing Turnaround Process Time Time Delay Time 1 0 3 2 2 6 3 4 4 4 6 5 5 8 2

Total time taken,from submission to

completion

0 5 10 15 20

Arrival Processing Turnaround Process Time Time Delay Time 1 0 3 2 2 6 3 4 4 4 6 5 5 8 2

completion

Arrival Times of the Jobs

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completion

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completion

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completion

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completion

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completion

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completion

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completion

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completion

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completion

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completion

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completion

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completion

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Arrival Processing Turnaround Process Time Time Delay Time 1 0 3 0 2 2 6 1 3 4 4 5 4 6 5 7 5 8 2 10

completion

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Arrival Processing Turnaround Process Time Time Delay Time 1 0 3 0 3 2 2 6 1 7 3 4 4 5 9 4 6 5 7 12 5 8 2 10 12

completion

Shortest Job First Select the job with the shortest (expected) running time Non-Preemptive

0 5 10 15 20

Same Job Mix

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Shortest Remaining Time Preemptive version of SJF

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Same Job Mix

Shortest Remaining Time• Preemptive version of SJF

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5: CPU-Scheduling 61

EXAMPLE DATA:

Process Arrival Service Time Time

1 0 8 2 1 4

3 2 9 4 3 5

0 8 12 21 26

P1 P2 P3 P4

Average Turnaround time = ( (8-0) + (12-1) + (21-2) + (26-3) )/4 = 61/4 = 15.25

CPU SCHEDULING

Residence Timeat the CPU

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.

Starvation occurs if a low priority process never runs. Solution: build aging into a variable priority.

Delicate balance between giving favorable response for interactive jobs, but not starving batch jobs.

PRIORITY BASED SCHEDULING:

Priority scheduling Assign a priority (number) to each process Schedule processes based on their priority Higher priority jobs processes get more CPU

time Managing priorities

Can use “nice” to reduce your priority Can periodically adjust a process’ priority

• Prevents starvation of a lower priority process• Can improve performance of I/O-bound

processes by basing priority on fraction of last quantum used

Priority scheduling

Idea: Each job is assigned a priority Schedule highest priority ready job May be preemptive or non-preemptive Priority may be static or dynamic

Advantages Static priorities work well for real time systems Dynamic priorities work well for general workloads

Disadvantages Low priority jobs can starve How to choose priority of each job?

Goal: Adjust priority of job to match CPU burst

Approximate SCTF by giving short jobs high priority

Highest Response Ratio Next (HRRN)

Non-preemptive, tries to get best average normalized turnaround time

Depends on Response Ratio• W = time spent waiting• S = expected service time• RR = (W + S) / S

Select process with highest RR Feedback

Starts in high-priority queue, moves down in priority as it executes

Lower-priority queues often given longer time slices Can starve long processes

• Move up processes if they wait too long

Highest Response Ratio Next (HRRN)

Choose next process with the greatest Response Ratio

( time spent waiting + expected service time)

expected service time

Fair-Share Scheduling

User’s application runs as a collection of processes (threads)

User is concerned about the performance of the application

Need to make scheduling decisions based on process sets

Traditional UNIX Scheduling

Multilevel feedback using round robin within each of the priority queues

If a running process does not block or complete within 1 second, it is preempted

Priorities are recomputed once per second

Base priority divides all processes into fixed bands of priority levels

Starvation

Effect on Processes

Overhead

Response Tim

Throughput

Preemptive

Selection Function

Penalizes short and I/O

processes

May be high

Emphasized

Max tim

aiting

Good for short

processes

Low if quantum

too sm

At tim

e quantum

- Round R

Possible

Penalizes Long processes

Can be H

Good for short

processes

Shortest process

Possible

Penalizes long processes

Can be H

At arrival

Min tim

e left

Good balance

Can be H

See Notes

Possible

May favor I/O

bound processes

Can be H

emphaisized

At tim

e quantum

See Notes

Feedback

Characteristics of V

arious Scheduling Policies (Table 9.3, page 402)

Round-Robin Scheduling

Goal: Enable interactivity Limit the amount of CPU that a process can

have at one time.

Time quantum Amount of time the OS gives a process before

intervention The “time slice” Typically: 1 to 100ms

ROUND ROBIN: Use a timer to cause an interrupt after a predetermined time.

Preempts if task exceeds it’s quantum. Train of events

DispatchTime slice occurs OR process suspends on eventPut process on some queue and dispatch next

Use numbers in last example to find queueing and residence times. (Use quantum = 4 sec.)

Definitions:– Context Switch Changing the processor from

running one task (or process) to another. Implies changing memory.

– Processor Sharing Use of a small quantum such that each process runs frequently at speed 1/n.

– Reschedule latency How long it takes from when a process requests to run, until it finally gets control of the CPU.

ROUND ROBIN

Choosing a time quantum

Too short - inordinate fraction of the time is spent in context switches.

Too long - reschedule latency is too great. If many processes want the CPU, then it's a long time before a particular process can get the CPU. This then acts like FCFS.

Adjust so most processes won't use their slice. As processors have become faster, this is less of an issue.

ROUND ROBIN

Round-Robin Scheduling•

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Arrival Processing Process Time Time 1 0 3 2 2 6 3 4 4 4 6 5 5 8 2

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Round-Robin Scheduling

Effectiveness of round-robin depends on The number of jobs, and The size of the time quantum.

Large # of jobs means that the time between scheduling of a single job increases

Slow responses Larger time quantum means that the time

between the scheduling of a single job also increases

Slow responses Smaller time quantum means higher processing

rates but also more overhead!

MULTI-LEVEL QUEUES:

Each queue has its scheduling algorithm. Then some other algorithm (perhaps priority

based) arbitrates between queues. Can use feedback to move between queues Method is complex but flexible. For example, could separate system processes,

interactive, batch, favored, unfavored processes

MULTI-LEVEL QUEUES

Multilevel Queue Ready queue is partitioned into separate

queues:foreground (interactive)background (batch)

Each queue has its own scheduling algorithm foreground – RR background – FCFS

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

foreground then from background). Possibility of starvation.

Time slice – 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

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 process

will enter when that process needs service

Example of Multilevel Feedback Queue

Three queues: Q0 – RR with time quantum 8 milliseconds Q1 – RR time quantum 16 milliseconds Q2 – FCFS

Scheduling A new job enters queue Q0 which is served FCFS.

When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1.

At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.

Multilevel Feedback Queues

Multi-Level Queue Scheduling

Multiple queues, each with its own priority. Equivalently: each priority has its own ready queue

Within each queue...Round-robin scheduling. Simplist Approach:

A Process’s priority is fixed & unchanging

High priority

Low priority

Multi-Level Feedback Queue Scheduling Problem: Fixed priorities are too restrictive

Processes exhibit varying ratios of CPU to I/O times. Dynamic Priorities

Priorities are altered over time, as process behavior changes!

Issue: When do you change the priority of a process and how often?

Solution: Let the amount of CPU used be an indication of how a process is to be handled

Expired time quantum more processing needed Unexpired time quantum less processing needed

Adjusting quantum and frequency vs. adjusting priority?

Here’s how the priorities are used in Windows

CPU SCHEDULING Using Priorities

Multi-Level Feedback Queue Scheduling n priority levels, round-robin scheduling within a

level Quanta increase as priority decreases Jobs are demoted to lower priorities if they do not

complete within the current quantum

High priority

Low priority

Lottery Scheduling

Scheduler gives each thread some lottery tickets To select the next process to run...

The scheduler randomly selects a lottery number The winning process gets to run

Example Thread A gets 50 tickets Thread B gets 15 tickets Thread C gets 35 tickets There are 100 tickets outstanding.

Lottery Scheduling

Scheduler gives each thread some lottery tickets. To select the next process to run...

The scheduler randomly selects a lottery number The winning process gets to run

Example Thread A gets 50 tickets 50% of CPUThread B gets 15 tickets 15% of CPUThread C gets 35 tickets 35% of CPUThere are 100 tickets outstanding.

Lottery Scheduling

Scheduler gives each thread some lottery tickets. To select the next process to run... The scheduler randomly selects a lottery

number The winning process gets to run Example Thread A gets 50 tickets 50% of CPU

Thread B gets 15 tickets 15% of CPUThread C gets 35 tickets 35% of CPUThere are 100 tickets outstanding.• Flexible• Fair• Responsive

MULTIPLE PROCESSOR SCHEDULING:

Different rules for homogeneous or heterogeneous processors.

Load sharing in the distribution of work, such that all processors have an equal amount to do.

Each processor can schedule from a common ready queue ( equal machines ) OR can use a master slave arrangement.

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.

CPU SCHEDULING

A Brief Look at Real-Time Systems

Assume processes are relatively periodic Fixed amount of work per period (e.g. sensor

systems or multimedia data)

systems or multimedia data) Two “main” types of schedulers... Rate-Monotonic Schedulers

Earliest-Deadline-First Schedulers

systems or multimedia data) Two “main” types of schedulers... Rate-Monotonic Schedulers

Assign a fixed, unchanging priority to each process No dynamic adjustment of priorities

• Less aggressive allocation of processor Earliest-Deadline-First Schedulers

Assign dynamic priorities based upon deadlines

Typically real-time systems involve several steps (that aren’t in traditional systems)

Admission control All processes must ask for resources ahead of time. If sufficient resources exist,the job is “admitted” into

the system. Resource allocation

Upon admission... the appropriate resources need to be reserved for

the task. Resource enforcement

Carry out the resource allocations properly

Rate Monotonic Schedulers

For preemptable, periodic processes (tasks)

Assigns a fixed priority to each task T = The period of the task C = The amount of processing per task period

In RMS scheduling, the question to answer is... What priority should be assigned to a given task?

Process P1T = 1 secondC = 1/2 second / period

P1PRI > P2PRI

P2PRI > P1PRI

P1PRI > P2PRI

P2PRI > P1PRI

The computation for P2 cannot be increased

Assign shortest period tasks to the highest priorities

Admission control is difficult...< 69% There is a standard formula> 69% It may be possible, but need to do more

complex analysis

Assumptions: Processes complete (yield) within their period Independent processes Same CPU requirements per burst Other non-periodic processes have no deadlines Instantaneous preemption with no overhead

Earliest Deadline First

When processes do not need periodic executionor have constant CPU requirements...

When processes have deadline specifications...

Unlike RMS, EDF uses dynamic priorities (based upon earliest deadline first)(+) 100% processor utilization …?(-) Need to keep track of deadlines

Admission Control Just check to see if 100% processor utilization. Sum the Ci/Ti’s and see if less than or equal to 1 What about overhead?

What are the main tasks of the scheduler? What is the difference between preemptive and non-

preemptive scheduling? What is the advantage of a shorter scheduling

quantum? What is the advantage of a longer scheduling quantum? Why is feedback scheduling useful for interactive jobs? Are these scheduling policies subject to starvation?

Shortest Job First scheduling? Round Robin scheduling? First Come First Served scheduling? Priority scheduling? Earliest Deadline First scheduling?

processor scheduling

response time

actual execution time

actual time

interval of time

total time

turnaround time

unit of time

executionburstservice

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