Embedded Systems Unit III

8/12/2019 Embedded Systems Unit III

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Example: engine control Tasks:

spark control crankshaft sensing

fuel/air mixture oxygen sensor filter

enginecontroller


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Task graphs Tasks may have data

dependencies---mustexecute in certain order.

Task graph showsdata/controldependencies betweenprocesses.

Task : connected set ofprocesses.

Task set : One or moretasks.

P3

P1 P2

P4

P5

P6

task 1 task 2

task set


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Communication between tasks Task graph assumes that

all processes in each taskrun at the same rate,tasks do notcommunicate.

In reality, some amount ofinter-task communicationis necessary. Its hard to require

immediate response formulti-rate communication.

MPEGsystem

layer

MPEG

audio

MPEG

video


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Buggy cooperative multitasking system

Void process 1 ()

{If(input 1==0)

{

subA();

}

Else

{

subB();Switch();S

}

}


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Process2 ()

{

X =global 1; // global 1 is input to the process

While(x


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Processes and operatingsystems

Scheduling policies: RMS; EDF.

Scheduling modeling assumptions.


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Rate monotonic scheduling RMS (Liu and Layland): widely-used,

analyzable scheduling policy. Analysis is known as Rate Monotonic

Analysis (RMA).


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RMA model All process run on single CPU. Zero context switch time.

No data dependencies betweenprocesses. Process execution time is constant.

Deadline is at end of period. Highest-priority ready process runs.


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Rate-monotonic analysis Response time : time required to finish

process. Critical instant : scheduling state that gives

worst response time. Critical instant occurs when all higher-

priority processes are ready to execute.


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RMS priorities Optimal (fixed) priority assignment:

shortest-period process gets highest priority; priority inversely proportional to period; break ties arbitrarily.

No fixed-priority scheme does better.


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RMS CPU utilization, contd. RMS cannot use 100% of CPU, even with

zero context switch overhead. Must keep idle cycles available to handle

worst-case scenario. However, RMS guarantees all processes

will always meet their deadlines.


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RMS implementation Efficient implementation:

scan processes; choose highest-priority active process.


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E li d dli fi


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Earliest-deadline-firstscheduling

EDF : dynamic priority scheduling scheme. Process closest to its deadline has highest

priority. Requires recalculating processes at every

timer interrupt.


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EDF analysis EDF can use 100% of CPU. But EDF may fail to miss a deadline.


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EDF implementation On each timer interrupt:

compute time to deadline; choose process closest to deadline.

Generally considered too expensive to usein practice.


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EARLIEST DEADLINE FIRST SCHEDULING

CONSIDER THE FOLLOWING PROCESSES

PROCESS EXECUTION TIME PERIOD

P1 1 3

P2 1 4

P3 2 5


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P1=3,1 P2=4,1 P3=5,3

TIME RUNNING PROCESS DEADLINE

0 P11 P2

2 P3 P1

3 P3 P24 P1 P3

5 P2 P1

6 P17 P3 P2

8 P3 P1

9 P1 P3


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P1=3,1 P2=4,1 P3=5,3

TIME RUNNING PROCESS DEADLINE

10 P2

11 P3 P1,P2

12 P3 P1

13 P1 P2

14 P2 P1,P3

15 P1 P2

16 P2

17 P3 P1

18 P3 P1


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Fixing scheduling problems What if your set of processes is

unschedulable? Change deadlines in requirements. Reduce execution times of processes. Get a faster CPU.


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Priority inversion Priority inversion : low-priority process

keeps high-priority process from running. Improper use of system resources can

cause scheduling problems: Low-priority process grabs I/O device. High-priority device needs I/O device, but

cant get it until low -priority process is done. Can cause deadlock.


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Solving priority inversion Give priorities to system resources. Have process inherit the priority of a

resource that it requests. Low-priority process inherits priority of device

if higher.


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Data dependencies Data dependencies

allow us to improveutilization.

Restrict combination ofprocesses that can runsimultaneously.

P1 and P2 cant run

simultaneously.

P1

P2


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Context-switching time Non-zero context switch time can push

limits of a tight schedule. Hard to calculate effects---depends on

order of context switches. In practice, OS context switch overhead is

small (hundreds of clock cycles) relative tomany common task periods (ms ms).


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Interprocess communication

Interprocess communication (IPC ): OSprovides mechanisms so that processescan pass data.

Two types of communications: blocking : sending process waits for response; non-blocking : sending process continues.


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IPC styles

Shared memory: processes have some memory in common; must cooperate to avoid destroying/missing

messages. Message passing:

processes send messages along a

communication channel---no commonaddress space.


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Shared memory

Shared memory on a bus:

CPU 1 CPU 2memory

R di i i h d


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Race condition in sharedmemory

Problem when two CPUs try to write thesame location: CPU 1 reads flag and sees 0. CPU 2 reads flag and sees 0. CPU 1 sets flag to one and writes location. CPU 2 sets flag to one and overwrites

location.


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Critical regions

Critical region : section of code that cannotbe interrupted by another process.

Examples: writing shared memory; accessing I/O device.


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Message passing

Message passing on a network:

CPU 1 CPU 2

message message

message


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Process data dependencies

One process may notbe able to start untilanother finishes.

Data dependenciesdefined in a taskgraph .

All processes in onetask run at the samerate.

P1 P2

P3

P4

INTERPROCESS COMMUNICATION MECHANISMSSIGNALS POSIX


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SIGNALS POSIX

Signal Description

SIGABRT Abnormal termination signal such asissued by the abort() function.

SIGALRM Timeout signal such as issued by thealarm() function.

SIGHUP Death of session leader, or hangupdetected on controlling terminal.

SIGILL Detection of an invalid hardwareinstruction. Note that if a second faultoccurs while your process is in a signalhandler for this fault, the process will beterminated.

SIGINT Interactive attention signal ( Break ).

SIGKILL Termination signal should be used onlyfor emergency situations. This signalcannot be caught or ignored .

SIGPIPE Attempt to write on a pipe with noreaders.

SIGQUIT Interactive termination signal.

INTERPROCESS COMMUNICATION MECHANISMSSIGNALS
http://www.qnx.com/developers/docs/6.4.1/neutrino/lib_ref/a/abort.htmlhttp://www.qnx.com/developers/docs/6.4.1/neutrino/lib_ref/a/alarm.htmlhttp://www.qnx.com/developers/docs/6.4.1/neutrino/lib_ref/a/alarm.htmlhttp://www.qnx.com/developers/docs/6.4.1/neutrino/lib_ref/a/abort.html


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SIGNALS

Signal Description

SIGSEGV

Detection of an invalid memory reference.Note that if a second fault occurs whileyour process is in a signal handler for thisfault, the process will be terminated.

SIGSTOP Stop process (the default).Thi s signal

cannot be caught or ignored . SIGSYS Bad argument to system call.

SIGTERM Termination signal.

SIGUSR1 Reserved as application-defined signal 1.

SIGUSR2 Reserved as application-defined signal 2.

SIGWINCH Window size changed.


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CPU power consumption Most modern CPUs are designed with

power consumption in mind to somedegree.

Power vs. energy: heat depends on power consumption; battery life depends on energy consumption.


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CMOS power consumption Voltage drops : power consumption

proportional to V 2. Toggling : more activity means more

power. Leakage : basic circuit characteristics; can

be eliminated by disconnecting power.


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CPU power-saving strategies Reduce power supply voltage. Run at lower clock frequency. Disable function units with control signals

when not in use. Disconnect parts from power supply when

not in use.


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low power features Parallel execution units---longer idle shutdowntimes. Multiple data widths:

16-bit ALU vs. 40-bit ALU. Instruction caches minimizes main memory

accesses. Power management:

Function unit idle detection. Memory idle detection.


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Power management styles Static power management : does not

depend on CPU activity. Example: user-activated power-down mode.

Dynamic power management : based onCPU activity. Example: disabling off function units.


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Power optimization Power management : determining how

system resources are scheduled/used tocontrol power consumption.

OS can manage for power just as itmanages for time.

OS reduces power by shutting down units. May have partial shutdown modes.

Power management and


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gperformance

Power management and performance areoften at odds.

Entering power-down mode consumes energy, time.

Leaving power-down mode consumes energy, time.

Simple power management


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p p gpolicies

Request-driven : power up once request isreceived. Adds delay to response.

Predictive shutdown : try to predict how

long you have before next request. May start up in advance of request in

anticipation of a new request. If you predict wrong, you will incur additional

delay while starting up. Ex :- pagers


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Probabilistic shutdown Assume service requests are probabilistic. Optimize expected values:

power consumption; response time.

Simple probabilistic: shut down after timeTon , turn back on after waiting for T off .

Advanced Configuration and


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gPower Interface

ACPI : open standard for powermanagement services.

Hardware platform

devicedrivers

ACPI BIOS

OS kernel

applications power

management


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ACPI global power statesFive basic global power states

G3: mechanical off G2: soft off

S1: low wake-up latency with no loss of context S2: low latency with loss of CPU/cache state S3: low latency with loss of all state except memory S4: lowest-power state with all devices off

G1: sleeping state G0: working state Legacy state

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Embedded Systems Unit III