1 cpu scheduling & deadlock operating systems lecture 4

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CPU SchedulingCPU Scheduling&&

DeadlockDeadlock

Operating Systems Lecture 4

OS1 - Lecture 2 – Scheduling – Paul Flynn2

Process ManagementProcess Management

Concept of a Process Context-Change Process Life Cycle Process Creation Process Spawning


Process State DiagramsProcess State Diagrams

Three State Model Ready Running Blocked

Five State Model Ready Running Blocked Ready Suspended Blocked Suspended


CPU SchedulingCPU Scheduling

Scheduling the processor among all ready processes

The goal is to achieve: High processor utilization High throughput

number of processes completed per of unit time Low response time

time elapsed from the submission of a request until the first response is produced


Classification of Scheduling ActivityClassification 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 SchedulingQueuing Diagram for Scheduling


Long-Term SchedulingLong-Term Scheduling

Determines which programs are admitted to the system for processing

Controls the degree of multiprogramming Attempts to keep a balanced mix of

processor-bound and I/O-bound processes CPU usage System performance


Medium-Term SchedulingMedium-Term Scheduling

Makes swapping decisions based on the current degree of multiprogramming Controls which remains resident in memory

and which jobs must be swapped out to reduce degree of multiprogramming


Short-Term SchedulingShort-Term Scheduling

Selects from among ready processes in memory which one is to execute next The selected process is allocated the CPU

It is invoked on events that may lead to choose another process for execution: Clock interrupts I/O interrupts Operating system calls and traps Signals


Characterization of Scheduling PoliciesCharacterization of Scheduling Policies

The selection function determines which ready process is selected next for execution

The decision mode specifies the instants in time the selection function is exercised Nonpreemptive

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

Preemptive Currently running process may be interrupted and

moved to the Ready state by the OS Prevents one process from monopolizing the

processor


Short-Term SchedulerShort-Term SchedulerDispatcherDispatcher

The dispatcher is the module that gives control of the CPU to the process selected by the short-term scheduler

The functions of the dispatcher include: Switching context Switching to user mode Jumping to the location in the user program to

restart execution The dispatch latency must be minimal


The CPU-I/O CycleThe CPU-I/O Cycle

Processes require alternate use of processor and I/O in a repetitive fashion

Each cycle consist of a CPU burst followed by an I/O burst A process terminates on a CPU burst

CPU-bound processes have longer CPU bursts than I/O-bound processes


Short-Term Scheduling CriteriaShort-Term Scheduling Criteria

User-oriented criteria Response Time: Elapsed time between the

submission of a request and the receipt of a response

Turnaround Time: Elapsed time between the submission of a process to its completion

System-oriented criteria Processor utilization Throughput: number of process completed per unit

time fairness


Scheduling AlgorithmsScheduling Algorithms

First-Come, First-Served Scheduling Shortest-Job-First Scheduling

Also referred to as Shortest Job Next Highest Response Ratio Next (HRN) Shortest Remaining Time (SRT) Round-Robin Scheduling Multilevel Feedback Queue Scheduling


Process Mix ExampleProcess Mix Example

ProcessArriva

lTime

ServiceTime

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2

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Service time = total processor time needed in one (CPU-I/O) cycleJobs with long service time are CPU-bound jobs and are referredto as “long jobs”


First Come First Served (FCFS)First Come First Served (FCFS)

Selection function: the process that has been waiting the longest in the ready queue (hence, FCFS)

Decision mode: non-preemptive a process runs until it blocks for an I/O


FCFS drawbacksFCFS drawbacks

Favors CPU-bound processes A CPU-bound process monopolizes the processor I/O-bound processes have to wait until completion

of CPU-bound process I/O-bound processes may have to wait even after

their I/Os are completed (poor device utilization) Better I/O device utilization could be achieved if

I/O bound processes had higher priority


Shortest Job First (Shortest Job First (Shortest Process NextShortest Process Next))

Selection function: the process with the shortest expected CPU burst time I/O-bound processes will be selected first

Decision mode: non-preemptive The required processing time, i.e., the CPU burst

time, must be estimated for each process


SJF / SPN CritiqueSJF / SPN Critique

Possibility of starvation for longer processes Lack of preemption is not suitable in a time

sharing environment SJF/SPN implicitly incorporates priorities

Shortest jobs are given preferences CPU bound process have lower priority, but a

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


Highest Response Ratio Next (HRN)Highest Response Ratio Next (HRN)

Based on SJF with formula introduced Priority Based - P Time Waiting + Run Time / Run Time = P The process with the HIGHEST Priority will

be selected for running Non-Preemptive Reduces SJF bias against Short Jobs


PrioritiesPriorities

Implemented by having multiple ready queues to represent each level of priority

Scheduler the process of a higher priority over one of lower priority

Lower-priority may suffer starvation To alleviate starvation allow dynamic

priorities The priority of a process changes based on its

age or execution history


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

a clock interrupt occurs and the running process is put on the ready queue

Round-RobinRound-Robin


RR Time QuantumRR Time Quantum

Quantum must be substantially larger than the time required to handle the clock interrupt and dispatching

Quantum should be larger then the typical interaction but not much larger, to avoid penalizing I/O

bound processes


Round Robin: critiqueRound Robin: critique

Still favors CPU-bound processes An I/O bound process uses the CPU for a time less

than the time quantum before it is blocked waiting for an I/O

A CPU-bound process runs for all its time slice and is put back into the ready queue May unfairly get in front of blocked processes


Multilevel Feedback SchedulingMultilevel Feedback Scheduling

Preemptive scheduling with dynamic priorities

N ready to execute queues with decreasing priorities:

Dispatcher selects a process for execution from RQi only if RQi-1 to RQ0 are empty


Multilevel Feedback SchedulingMultilevel Feedback Scheduling

New process are placed in RQ0

After the first quantum, they are moved to RQ1 after the first quantum, and to RQ2

after the second quantum, … and to RQN after the Nth quantum

I/O-bound processes remain in higher priority queues. CPU-bound jobs drift downward. Hence, long jobs may starve


Multiple Feedback Queues Multiple Feedback Queues

Different RQs may have different quantum values


Algorithm ComparisonAlgorithm Comparison

Which one is the best? The answer depends on many factors:

the system workload (extremely variable) hardware support for the dispatcher relative importance of performance criteria

(response time, CPU utilization, throughput...) The evaluation method used (each has its

limitations...)

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DeadlocksDeadlocks 3.1. Resource 3.2. Introduction to deadlocks 3.3. The ostrich algorithm 3.4. Deadlock detection and recovery 3.5. Deadlock avoidance 3.6. Deadlock prevention 3.7. Other issues

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Deadlock RecapDeadlock Recap

Process is deadlocked if it is waiting for an event that will never occur

Most common situation is where two processes are involved on is holding resource required by the other and also looking for resource held by the other process.

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Deadlock ExampleDeadlock Example

Airline booking example. Alan wants to books seat on flight AB123 so locks this file. Same time Brian wants to book flight on AB456 and locks this file. Alan wants to book return flight on AB456 and Brian wants to book return flight on AB123. No each user as a file locked and is requesting a file locked by the other.

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Deadlock Example contDeadlock Example cont

Alan locks AB123 Brian lock AB456 Alan requests AB456 Brian requests AB123

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Conditions for deadlockConditions for deadlock

There are 4 conditions necessary for deadlock Mutual exclusion Resource holding No premption Circular wait

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Conditions explainedConditions explained

Mutual Exclusion only one process can use a resource at a time

Resource holding process can hold a resource while requesting another

No premption resources cannot be forcibly removed from a process

Circular wait closed circle exists where each process is holding a resource required by another.

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Dealing with deadlockDealing with deadlock

Three ways of dealing with deadlocks Deadlock prevention Deadlock avoidance Deadlock detection

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Deadlock preventionDeadlock prevention

Prevent any one of the 4 conditions from occuring will prevent deadlock

Mutual exclusion – cannot prevent Resource holding – to prevent this a

process must be allocated all it resources at once called one shot allocation very inefficient. Process may have to wait for resources it might not need. Process may hold resources for long time without using.

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Deadlock prevention contDeadlock prevention cont

No premption to deny this condition two ways If a process is holding a resource requests

another it could be forces to give up the resource it is holding.

A resources required by a process and held by a second could be forcibly removed from the second. Not possible with serially reusable resources.

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Deadlock preventionDeadlock prevention

Circular wait – this condition can be prevented if resources are organsied in a particual order and require that resources follow an order.

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Deadlock avoidanceDeadlock avoidance

Deadlock prevention means that deadlock will not occur due to fact that we deny one of the 4 conditions necessary. Innefficient

Deadlock avoidance attempts to predict the possibility of deadlock as each resources request is made.

Example if process A requests a resource held by process B then make sure that process B is not waiting for resource held by A

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Bankers AlgorithmBankers Algorithm

The most common method of deadlock avoidance is to use the bankers algorithm. Uses banking anology, banker will only grant loan if he can meet the needs of customers based on their projected future loan requirements.

Example three processes P1,P2 & P3 and 10 resources available. Table shows requirements

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Example cont.Example cont.

Process Max need Current usage

P1 8 3

P2 5 1

P3 8 2

Total maximum needs =21 means that allocation cannot be met at one time. We need a sequence of allocations which will allow all processes to finish. If we start with P2 when it is finished it will release 5 resources. Next if we allow P1 to run it will release 8 resources and so P3 will be able to finish.

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Problems with deadlock avoidanceProblems with deadlock avoidance

Each process has to pre-declare its maximum rersource requirements. This is not realistic for interactive systems.

The avoidance algorithm must be executed every time a resource request is made. For a multi-user system with a large number of user processes, the processing overhead would be severe.

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Deadlock detectionDeadlock detection

A deadlock detection strategy accepts the risk of deadlock occuring and periodically executes a procedure to detect any in place.

Breaking a deadlock implies that a process must be aborted or resources prempted from processes, either could result in loss of work.

1 cpu scheduling & deadlock operating systems lecture 4

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