Ppt1 Rtos.zmp

7/29/2019 Ppt1 Rtos.zmp

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Embedded System

Embedded Systems:

Electronics system that containsprocessor (uP/uC/DSP/ASIP etc.)and peripherals which isconfigured to perform specificdedicated application

Software embeds in ROM andnot accessible to the user

Mostly designed for timeconstrained applications (real timesystems)

either independent system or apart of larger system.

G P Computer System:

contains general purpose (GP)microprocessor, primary &

secondary memories, keyboards,

printer, scanner, monitor etc.

Programmed for GP applicationsthat includes word processing,

accounting, scientific computing,

email, multimedia, database system

etc.

user does have access to

software and decides which OS to

run and which application to launch.

## Small scale, medium scale and sophisticated Embedded System


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Elements of embedded systems

(1) Processors : Microprocessors : e.g.68HCxxx, 80x86, SPARC, PowerPC

Microcontrollers : 68HC11xx, 8051, PIC series, ARM series DSP : TMS series, SHARC

ASSP/ASIP : processor with hardwired logic customized to application

(2) Memories : ROM variants : application programs, code for RTOS, codes for booting,

initializing, initial input data and strings, pointers of service routines

RAM (int. & ext.) : variables during program run, I/O buffers Cache : copies of inst. and data in advance, intermediate results for fast

processing

(3) Built-in Modules : Serial/Parallel ports, ADC/DAC, PWM, Watchdog timer, RTC etc.

(4) I/O :

Inputs => keyboard, sensor circuits, comm. network interface etc. Outputs=> LED, LCD, Alarm, actuators, comm. network interface etc.


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Example (Telegraph) :

Network Data may be out of order, lost or arrive twice

Other computers on Network also want to print

Need to provide status of printer to computers

It has to work with different type of printer

It need to respond quickly and keep track of time

Aspects : throughput, response, handling uncommon events,debugablility


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Real Time System

system wherein execution of jobs (or tasks) are time constrained

Release time, response time, deadlines

Hard and Soft Real time System: Hard -> miss of deadline is fatal fault, consequences may be disastrous

Soft -> occasional misses of deadline will not do serious harm, only overallperformance is slightly degraded, job execution has value even if completedlate

Often described quantitatively in terms ofusefulness function as a function ofthe tardiness of job

Tardiness = lateness of job = completion time - deadline

Plot is a guide for selection type of scheduling to be used that best suits


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Validation > for Hard RTS, validation of all constraints (worst case) is must

> for Soft RTS, only demo of jobs meeting avg. case is suffice

Example of Hard RTS :- industrial process control, flight control

Example of Soft RTS :- Telephone system (delay in call setup not much

objectionable), multimedia (quality is not much affected if few frames, say 2

or 3, per minute are late or lost)

In soft RTS, since timing constraints are relaxed

we have more design options

we can make system better in other aspects

we can make system cost-effective

Performance measures for soft RTS : miss rate: % of tasks that are executed late (missed deadlines)

loss rate: % of tasks that are discarded (not executed at all)

invalid rate = miss rate + loss rate

We need to minimize invalid rate


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RTOS

When RTOS is needed? When not?

RTOS is platform for embedded system that performs various system tasks

such as handling system call, scheduling, inter process communication andsynchronization (IPC), interrupt handling, servicing timers etc.

Central concept in OS is process which is an abstraction of running program

Process (or Task) : Process is an action of execution of the program. Application software can be

defined as consisting of the processes

Process Status :- ready, running, blocked, terminated, sleeping


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RTOS

Context of process and context switching

a process can create one or more other processes (referred to as childprocesses)and these processes in turn can create child processes to formprocess tree.

When process is created, RTOS allocates memory space and instantiatesdata structure called Process Control Block (PCB)

PCB :- Context (Program status word, SP, PC & other CPU registers value),process ID, Current state, pointers to queue of messages, pointers to parentand daughter process, pointer to list of resources, pointer to access

permission descriptor for sharing resources globally and with other process User mode (application) and Kernel mode (RTOS) processes

No synchronization among processes without OS kernel

Threads : Threads are subunits of a process which may execute simultaneously

Process may contain multiple threads. All threads of a process run in addressspace of same process.

Have their own PC, stack and registers which are private and never shared

Advantages:- reduces context switching overhead, multiple control points ina process, improves responsiveness


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RTOS


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RTOS

Basic OS Functions:

[1] System Call: Application accesses kernel data and code via system calls

Upon receiving call, RTOS saves context of calling process and

switched kernel mode

it then executes the API function and switches back to user mode

[2] Servicing Interrupts: independent call from either h/w or s/w whose scheduling is controlled

either by ISR process instructions or RTOS

If ISR happens in non-reentrant function f1 and if ISR also contains call

to f1 then it modifies shared variables off1. When f1 is resumed, it runs

with modified variables which leads to unpredictable result. To avoid this

ISR must call only reentrant function.

Latency: sum of (i) time to process current instruction, flush the pipeline,

read interrupt vector & jump to handler part of OS kernel (ii) time kernel

takes to disable interrupts (iii) time required to complete service routine

of higher-priority interrupt if any (iv) time to save context of interrupted

process (v) time the kernel takes to start ISR


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Reentrant Functions:

A reentrantfunction is one that can be used by more than one process

concurrently without fear of data corruption. A reentrant function can be

interrupted at any time and resumed at a later time without loss of data. A function or routine is described as reentrant if it can be safely called again

before its previous invocation has been completed (i.e. it can be safely

executed concurrently).

Any function called within ISR should be reentrant

Only static and global variable affects re-entrance

Reentrant function forces compiler to allocate separate memory dynamically

for each instance of call and allows more than one task (process) to use it

concurrently without fear of data corruption

A reentrant function:

Does not hold static data over successive calls

Does not return a pointer to static data; all data is provided by the caller

of the function

Ensures protection of global shared data

Must not call any non-reentrant functions


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Considerstrtoupperfunction where global variable buffer used by multipleprocesses. Two reentrant function for


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The function reentrant by dynamically allocating memory to buffer for

every invocation of function. This will preserve the

Better solution => caller must provide the storage for input strings (in_str)

and output string (out_str).


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RTOS

According to OS kernel features, following explains 3 alternatives foundin 3 RTOSs for responding and synchronizing the ISR call

(1) Interrupt source calls ISR directly and ISR does complete servicing. Itmerely send a message to RTOS

(2) RTOS Kernel intercepts the interruptcall and initiate correspondingISR after saving context. ISR send message(s) to RTOS for initiatingprocesses. Now kernel schedules and runs processes according tomessages sent by ISR. RTOS only schedules process and ISR runsduring suspension of processes.

(3) RTOS treats ISR as a process. After interrupt call, RTOS calls ISRprocess after saving context. The ISR merely sends IPC containingparameters needed by RTOS. Then RTOS initiates and executes theprocesses and one of them is ISR

[3]Scheduling: Scheduler is a Part of RTOS kernel that schedules processes and allocates

resources to them according to scheduling policy

Preemptive and Non preemptive/Cooperative scheduling

online (run time) and offline (pre- run time) scheduling

Fixed priority and dynamic priority based scheduling


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Function:- (i) routine for performing actions as per parameter

passed to it and may be called from process, ISR or anotherfunction (nesting) (ii) synchronization among functions without OS or

scheduler

Process:- (i) independent entity scheduled/controlled by RTOS (ii)each process has distinct stack for saving context and distinct

memory block for storing PCB (iii) no synchronization among the

processes without OS kernel

Each process runs sequentially but executions of multiple processes

can be interleaved called multiprocessing


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Scheduling Algorithms

Timing parameters:

Release time: an instant the process is ready for execution Execution (run) time : time period to complete process execution

Absolute deadline (d) : an instant by which process must finish

Slack time = (d-t)remaining run time of process at time t

= deadline

estimated completion time (negative slackmeans missed deadline)


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[A] Non Preemptive Task(Process) scheduling:

(1)Round Robin (Circular Queue):

Processes are executed sequentially from ready linked list

RTOS keeps track of currently executing process. It updates linked list uponinsertion and removal of a process. New process is inserted at the end oflinked list

Circular FIFO buffer can be used to form ready list of processes

New process is inserted before the currently running one (last in circular list)

Simple architecture : no interrupts, no shared data, no latency concerns.

Attractive option for some jobs


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(2) Round Robin with Priority based Ready List

Processes in ready queue are ordered as per their priority but executed in turn

(circularly) from ordered list

Scheduling is still non-preemptive as no process is preempted by any other

Time to execute all processes in ready queue is termed as one cycle

The processes are ordered based on priority at the beginning of every cycle.

Process is allowed to run till the end of execution


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(3) Round Robin Scheduling with Time Slicing:

RTOS defines a time slot for each process for its execution Tslot

In a single schedule cycle ( time slice Tslice) processes are taken sequentially

If process doesnt finish in Tslotit is blocked (suspended) until it get its turn inthe next time slice

Tsliceis fixed whereas Tslot is variable

If process finishes before Tslot there is waiting period

Disadv. Of Non-Preemptive:

Long execution of low priority tasks make high priority task to wait long

Difficult to meet deadlines


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Scheduling Algorithms

[B] Preemptive Scheduling:

Process may be blocked at any instant to allow higher priority task torun. This is called preemption.

In preemptive scheduling, Priorities are assigned to processes usingvarious criteria.

Following scheduling algorithms are commonly used

Shortest Remaining Runtime First

Priorities are assigned such that shorter the execution time,

higher the priority

Earliest Deadline First (EDF)

Earlier the deadline, higher the priority Least Slack time First (LSF) or Least Laxity First (LLF)

Smaller the slack, higher the priority

Note: Schedulershould not blockany process running in critical region


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Example :- A scheduler is invoked every 1 unit of time and selects process fromready queue to run. (a) How processes are scheduled with Least Slack Time firstpolicy ? (b) How the processes are scheduled with Shortest Run time firstpolicy?(c) Find total time spent in wait by all process, total no. of context switches andno. of deadline missed for each scheduling.

In case of tie, following criteria would be used in that order (i) process alreadyrunning should be preferred over others (ii) process with earlier deadline shouldbe selected (iii) processes should be scheduled in Round robin fashion

Ans:

Slacktime = deadline - t- time remaining to finish process at t

The slack time of processes when they became ready (released) is given byP0=18 (at t=0), P1=4 (at t=10), P2=19 (at t=4) and for P3=5 (at t=5).

>slack time of running process remains constant while it decreases for waitingprocess

>to ensure proper scheduling using Round Robin, circular selection of processesfrom ready queue is required


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Periodic Process (Task) model

Periodic Processes (Tasks) are executed repetitively with periodpi

and execution time ei where iis process number

Assume that process needs to execute only one thread in a period

Threads (jobs) of a process Pi are refereed to as i1, i2, . . . being

ikbeing the kth thread in process Pi

LCM of allpifor i=1,2,3, . . . . . n is called hyper period H The ratio ui=ei/pithe utilization of the process (task) Pi is equal to

fraction of time of a processor is kept busy

The total utilization U of all the processes in the system is the sum of

utilizations of individual process in it

1ST thread of process Pi released at release time rimust completeDiunits of time afterri; where Di is relative deadline. If is not Dispecified, period of the process would be considered as deadline.


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Periodic Scheduling

Periodic Scheduling of Process (Taslk):

In general, Periodic threads of process i can be represented as 4-tuple(ri,pi,ei,Di) whereri phase of 1ST thread, pi period, ei execution timeand Di relative deadline.

For example P1= (1,12,4,6) is periodic task whose 1st job is released at time1 and must complete at time 7. The second job is ready at time 13 and mustbe completed by 19 and so on.

Some element of 4-tuple may be omitted for simplicity. For example

(12,4,6) => has zero phase (i.e. ri=0)

(12,4) => has ri=0 & Di=0. Here period becomes deadline for thread

e.g. P1(3,2) :- threads of process P1 are periodic with period (deadline) 3and execution time 2.

Some algorithms for periodic scheduling are:

Dynamic priority => EDF, Least Slack Time First, Shortest RemainingRuntime First

Fixed Priority => Rate Monotonic (RM), Deadline Monotonic (DM)


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Periodic Scheduling

i1

, i2

. . . are threads of process Pi

For P1, relative deadline is 6 for each threads. Therefore absolutedeadline for11,12,13 . . are 7, 17, 27, . . .

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