UNIX 2 Scheduler

8/11/2019 UNIX 2 Scheduler

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PROCESS

&

CPU SCHEDULING


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Diagram of Process State


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Representation of Process Scheduling


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CONTEXT SWITCH

Save the state of the old process

Load the saved state for the new process.

Context-switch time is overhead; the system does no usefulwork while switching.

Time dependent on hardware support.


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CPU Scheduler

A CPU scheduler, running in the dispatcher,isresponsible for selecting of the next runningprocess. Based on a particular strategy

When does CPU scheduling happen? A process switches from the running state to waiting state

(e.g. I/O request)

A process switches from the running state to the ready state.

A process switches from waiting state to ready state

(completion of an I/O operation)

A process terminates


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Scheduling queues

CPU schedulers use various queues in thescheduling process: Job queue: consists of all processes

All jobs (processes), once submitted, are in the job queue.

Some processes cannot be executed (e.g. not in memory). Ready queue

All processes that are ready and waiting for execution are inthe ready queue.

Usually, a long-term scheduler/job scheduler selectsprocesses from the job queue to the ready queue.

CPU scheduler/short-term scheduler selects a process fromthe ready queue for execution.

Simple systems may not have a long-term job scheduler


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Scheduling queues

Device queue

When a process is blocked in an I/Ooperation, it is usually put in a device queue

(waiting for the device). When the I/O operation is completed, the

process is moved from the device queue tothe ready queue.


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Performance metrics for CPUscheduling

CPU u ti l izat ion: percentage of the time that CPUis busy.

Throughput: the number of processes completedper unit time

Turnaround t ime: the interval from the time ofsubmission of a process to the time of completion.

Wait t ime: the sum of the periods spent waiting in

the ready queue Response time: the time of submission to the

time the first response is produced


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Goal of CPU scheduling

Other performance metrics:

fairness: It is important, but harder to definequantitatively.

Goal:

Maximize CPU utilization, Throughput, and

fairness. Minimize turnaround time, waiting time, and

response time.


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Which metric is used more?

CPU utilization

Trivial in a single CPU system

Throughput, turnaround time, wait time,and response time

Can usually be computed for a givenscenario.


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Deterministic modelingexample:

Suppose we have processes A, B, and C,submitted at time 0

We want to know the response time, waiting

time, and turnaround time of process A

A B C A B C A C A C Time

response time = 0

+ +wait time

turnaround time

Gantt chart: visualize how processes execute.


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Deterministic modeling example



time, and turnaround time of process B


response time

+wait time

turnaround time


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Deterministic modeling example



time, and turnaround time of process C


response time+ ++wait time

turnaround time


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Preemptive versusnonpreemptive scheduling

Many CPU scheduling algorithms have bothpreemptive and nonpreemptive versions: Preemptive: schedule a new process even when the

current process does not intend to give up the CPU

Non-preemptive: only schedule a new process when thecurrent one does not want CPU any more.

When do we perform non-preemptive scheduling? A process switches from the running state to waiting state

(e.g. I/O request)

A process switches from the running state to the ready state. A process switches from waiting state to ready state

(completion of an I/O operation)

A process terminates


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Scheduling Policies

FIFO (first in, first out)

Round robin

SJF (shortest job first)

Priority Scheduling

Multilevel feedback queues

Many more...


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FIFO

FIFO: assigns the CPU based on theorder of requests

Non-preempt ive: A process keeps running

on a CPU until it is blocked or terminated Also known as FCFS (first come, first serve)

- Simple

- Short jobs can get stuck behind long jobs- Turnaround time is not ideal


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Round Robin

Round Rob in (RR) periodically releasesthe CPU from long-running jobs

Based on timer interrupts so short jobs can get

a fair share of CPU time Pre-emptive: a process can be forced to

leave its running state and replaced byanother running process

Time sl ice: interval between timer interrupts


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More on Round Robin

If time slice is too long

Scheduling degrades to FIFO

If time slice is too short

Throughput suffers

Context switching cost dominates


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FIFO vs. Round Robin

With zero-cost context switch, is RRalways better than FIFO?


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Suppose we have three jobs of equal length

A B C A B C TimeA B C

turnaround time of A

turnaround time of B

turnaround time of C

Round Robin

A B TimeC

turnaround time of Aturnaround time of B


FIFO


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Round Robin

+ Shorter response time

+ Fair sharing of CPU

- Not all jobs are preemptable

- Not good for jobs of the same length

- More precisely, not good in terms of the turnaroundtime.


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Shortest Job First (SJF)

SJFruns whatever job puts the leastdemand on the CPU, also known as STCF(shortest t ime to complet ion f i rs t )

+ Probably optimal in terms of turn-around time .+ Great for short jobs

+ Small degradation for long jobs


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SJF Illustrated

A B TimeC

turnaround time of A

turnaround time of B


Shortest Job First

response time of A = 0

response time of B

response time of C

wait time of A = 0wait time of B

wait time of C


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Drawbacks of Shortest Job First

- Starvation: constant arrivals of short jobscan keep long ones from running

- There is no way to know the completion

time of jobs (most of the time)

Some solutions

Ask the user, who may not know any better

If a user cheats, the job is killed


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Priority Scheduling (MultilevelQueues)

Prior i ty schedu l ing: The process withthe highest priority runs first

Priority 0:

Priority 1:

Priority 2:

Assume that low numbers represent high

priority

A

B

C

A B TimeC

Priority Scheduling


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Multilevel Feedback Queues

Mult i level feedback queuesuse multiplequeues with different priorities

Round robin at each priority level

Run highest priority jobs first Once those finish, run next highest priority, etc

Jobs start in the highest priority queue

If time slice expires, drop the job by one level If time slice does not expire, push the job up

by one level


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Priority 0 (time slice = 1):



A B C

time = 0

Time


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C

time = 2

A B

A B Time


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C

time = 3

A B

A B C Time


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C

time = 3

A B

A B C Time

suppose process A is blocked on an I/O


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C

time = 3

A

B

A B C Time

suppose process A is blocked on an I/O


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time = 5

BA B C Time

C

A

suppose process A is returned from an I/O


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time = 6

BA B C Time

C

A


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time = 8

BA B C TimeA C

C


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time = 9

BA B C TimeA C C

UNIX 2 Scheduler

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