Process and Operating System (ch6-1)

8/12/2019 Process and Operating System (ch6-1)

1/38

2008 Wayne Wolf Overheads for Computers asComponents 2nded.

Processes and operating

systems

Multiple tasks and multiple processes.

Specifications of process timing.

Preemptive real-time operating systems.

Processes and UML.


2/38

2008 Wayne Wolf Overheads for Computers asComponents 2nded.

Reactive systems

Respond to external events.Engine controller.

Seat belt monitor.Requires real-time response.System architecture.

Program implementation.

May require a chain reaction amongmultiple processors.


3/38

Tasks and processes

A task is a functionaldescription of a

connected set ofoperations.

(Task can also meana collection of

processes.)

A process is a uniqueexecutionof a program.

Several copies of a

program may runsimultaneously or atdifferent times.

A process has its ownstate:

registers;

memory.

The operating systemmanages processes.

2008 Wayne Wolf Overheads forComputers as

Components 2nded.


4/38

2008 Wayne Wolf

Overheads for Computers as

Components 2nded.

Why multiple processes?

Multiple tasks means multiple processes.

Processes help with timing complexity:

multiple ratesmultimedia

automotive

asynchronous inputuser interfaces

communication systems


5/38

2008 Wayne Wolf


Components 2nded.

Multi-rate systems

Tasks may be synchronous orasynchronous.

Synchronous tasks may recur at differentrates.

Processes run at different rates based on

computational needs of the tasks.


6/38

2008 Wayne Wolf


Components 2nded.

Example: engine control

Tasks:

spark control

crankshaft sensingfuel/air mixture

oxygen sensor

Kalman filter

engine

controller


7/38


8/38

2008 Wayne Wolf


Components 2nded.

Real-time systems

Perform a computation to conform to externaltiming constraints.Deadline frequency:

Periodic.Aperiodic.

Deadline type:Hard: failure to meet deadline causes system failure.

Soft: failure to meet deadline causes degradedresponse.Firm: late response is useless but some late

responses can be tolerated.


9/38

2008 Wayne Wolf


Components 2nded.

Timing specifications on

processes

Release time: time at which processbecomes ready.

Deadline: time at which process mustfinish.


10/38

2008 Wayne Wolf


Components 2nded.

Release times and

deadlines

time

P1

initiatingevent

deadline

aperiodic processperiodic process initiated

at start of period

period

P1P1

deadline

periodperiodic process initiated

by event


11/38

Rate requirements on

processes

Period: intervalbetween processactivations.

Rate: reciprocal ofperiod.

Initiatino rate may be

higher than period---several copies ofprocess run at once.

2008 Wayne Wolf


Components 2nded.

time

P11

P12

P13

P14

CPU 1

CPU 2

CPU 3

CPU 4


12/38

2000 Morgan

Kaufman


Components

Timing violations

What happens if a process doesnt finishby its deadline?

Hard deadline: system fails if missed.Soft deadline: user may notice, but system

doesnt necessarily fail.


13/38

2008 Wayne Wolf


Components 2nded.

Example: Space Shuttle

software error

Space Shuttles first launch was delayedby a software timing error:

Primary control system PASS and backupsystem BFS.

BFS failed to synchronize with PASS.

Change to one routine added delay thatthrew off start time calculation.

1 in 67 chance of timing problem.


14/38

Task graphs

Tasks may have datadependencies---mustexecute in certain order.

Task graph showsdata/controldependencies betweenprocesses.

Task: connected set ofprocesses.

Task set: One or moretasks.

2008 Wayne Wolf


Components 2nded.

P3

P1 P2

P4

P5

P6

task 1 task 2

task set


15/38

Communication between

tasks

Task graph assumes thatall processes in each taskrun at the same rate,

tasks do notcommunicate.

In reality, some amountof inter-task

communication isnecessary.

Its hard to requireimmediate response formulti-rate communication.

2008 Wayne Wolf


Components 2nded.

MPEG

system

layer

MPEG

audio

MPEG

video


16/38

2008 Wayne Wolf


Components 2nded.

Process execution

characteristics

Process execution time Ti.

Execution time in absence of preemption.

Possible time units: seconds, clock cycles.

Worst-case, best-case execution time may be usefulin some cases.

Sources of variation:

Data dependencies.Memory system.

CPU pipeline.


17/38

2008 Wayne Wolf


Components 2nded.

Utilization

CPU utilization:

Fraction of the CPU that is doing useful work.

Often calculated assuming no schedulingoverhead.

Utilization:

U = (CPU time for useful work)/(total available CPU time)= [ St1 t t2T(t) ] / [t2t1]

= T/t


18/38

2008 Wayne Wolf


Components 2nded.

State of a process

A process can be inone of three states:

executingon the CPU;

readyto run;

waitingfor data.

executing

ready waiting

gets data

and CPU

needs

data

gets data

needs data

preemptedgets

CPU


19/38

2008 Wayne Wolf


Components 2nded.

The scheduling problem

Can we meet all deadlines?

Must be able to meet deadlines in all cases.

How much CPU horsepower do we needto meet our deadlines?


20/38

2008 Wayne Wolf


Components 2nded.

Scheduling feasibility

Resource constraintsmake schedulability

analysis NP-hard.Must show that the

deadlines are met forall timings of resource

requests.

P1 P2

I/O device


21/38

2008 Wayne Wolf


Components 2nded.

Simple processor feasibility

Assume:

No resource conflicts.

Constant processexecution times.

Require:

T SiTiCant use more than

100% of the CPU.

T1 T2 T3

T


22/38

2008 Wayne Wolf


Components 2nded.

Hyperperiod

Hyperperiod: least common multiple(LCM) of the task periods.

Must look at the hyperperiod schedule tofind all task interactions.

Hyperperiod can be very long if task

periods are not chosen carefully.


23/38

2008 Wayne Wolf


Components 2nded.

Hyperperiod example

Long hyperperiod:P1 7 ms.

P2 11 ms.

P3 15 ms.

LCM = 1155 ms.

Shorter hyperperiod:P1 8 ms.

P2 12 ms.

P3 16 ms.

LCM = 96 ms.


24/38

2004 Wayne Wolf


Components 2nded.

Simple processor feasibility

example

P1 period 1 ms, CPUtime 0.1 ms.



LCM 5.00E-03

peirod CPU time CPU time/L

P1 1.00E-03 1.00E-04 5.00E-04

P2 1.00E-03 2.00E-04 1.00E-03

P3 5.00E-03 3.00E-04 3.00E-04

total CPU/LCM 1.80E-03

utilization 3.60E-01


25/38

2004 Wayne Wolf


Components 2nded.

Cyclostatic/TDMA

Schedule in timeslots.

Same processactivationirrespective ofworkload.

Time slots may beequal size orunequal.

T1 T2 T3

P

T1 T2 T3

P


26/38

2008 Wayne Wolf


Components 2nded.

TDMA assumptions

Schedule based onleast commonmultiple (LCM) ofthe processperiods.

Trivial scheduler -

> very smallschedulingoverhead.

P1 P1 P1

P2 P2

PLCM


27/38

2008 Wayne Wolf


Components 2nded.

TDMA schedulability

Always same CPU utilization (assumingconstant process execution times).

Cant handle unexpected loads.Must schedule a time slot for aperiodic

events.


28/38

2008 Wayne Wolf


Components 2nded.

TDMA schedulability example

TDMA period = 10ms.

P1 CPU time 1 ms.P2 CPU time 3 ms.

P3 CPU time 2 ms.

P4 CPU time 2 ms.

TDMA period 1.00E-02

CPU time

P1 1.00E-03

P2 3.00E-03

P3 2.00E-03

P4 2.00E-03

total 8.00E-03

utilization 8.00E-01


29/38

2008 Wayne Wolf


Components 2nded.

Round-robin

Schedule processonly if ready.Always test

processes in thesame order.

Variations:Constant system

period.

Start round-robinagain after finishinga round.

T1 T2 T3

P

T2 T3

P


30/38

2008 Wayne Wolf


Components 2nded.

Round-robin assumptions

Schedule based on least common multiple(LCM) of the process periods.

Best done with equal time slots forprocesses.

Simple scheduler -> low scheduling

overhead.Can be implemented in hardware.


31/38

2008 Wayne Wolf


Components 2nded.

Round-robin schedulability

Can bound maximum CPU load.

May leave unused CPU cycles.

Can be adapted to handle unexpectedload.

Use time slots at end of period.

S h d l bilit d


32/38

2008 Wayne Wolf


Components 2nded.

Schedulability and

overhead

The scheduling process consumes CPUtime.

Not all CPU time is available for processes.Scheduling overhead must be taken into

account for exact schedule.

May be ignored if it is a small fraction of totalexecution time.

R i i di


33/38

2008 Wayne Wolf


Components 2nded.

Running periodic

processes

Need code to control execution ofprocesses.

Simplest implementation: process =subroutine.


34/38

while loop implementation

Simplestimplementation hasone loop.

No control overexecution timing.

while (TRUE) {

p1();

p2();}

2008 Wayne Wolf


Components 2nded.


35/38

Timed loop implementation

Encapuslate set of allprocesses in a singlefunction thatimplements the taskset,.

Use timer to control

execution of the task.No control over timing

of individualprocesses.

void pall(){

p1();

p2();}

2008 Wayne Wolf


Components 2nded.

M lti l ti


36/38

Multiple timers

implementation

Each task has its ownfunction.

Each task has its owntimer.

May not have enoughtimers to implement

all the rates.

void pA(){ /* rate A */

p1();

p3();

}

void B(){ /* rate B */

p2();

p4();p5();

}

2008 Wayne Wolf


Components 2nded.

Ti + t


37/38

Timer + counter

implementation

Use a software countto divide the timer.

Only works for cleanmultiples of the timerperiod.

int p2count = 0;

void pall(){

p1();

if (p2count >= 2) {

p2();

p2count = 0;

}else p2count++;

p3();

}

2008 Wayne Wolf


Components 2nded.


38/38

Implementing processes

All of these implementations areinadequate.

Need better control over timing.Need a better mechanism than

subroutines.

2008 Wayne Wolf


Components 2nd ed

Date post:	03-Jun-2018
Category:	Documents
Upload:	paulokimura
View:	221 times
Download:	0 times

Process and Operating System (ch6-1)

Documents