CHAPTER 4

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RTS: Kernel Design and Cyclic Executives

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Kernel & Device drivers

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Shell XWin Thread lib ftp User applications

System call interface

Devices

Process, memory, file system, network managers.

Device drivers

Kernel

Servers (application ~, web ~, component ~)

Hardware/controller

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Operating System Models

I. Full-blown operating system like Windows or Linux

II. Kernels with core functions (eg. Xinu)III. Small systems with dedicated functions (eg.

wii), xbox)IV. Devices with systems optimized for one or

more functions (eg. mp3 player)V. Cyclic executive (simple task loops.. Repeating:

heart pace maker, handheld games, whole new area called “serious games/gamification”)

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Task characteristics of real workload

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Each task Ti is characterized by the following temporal parameters:

Precedence constraints: specify any tasks need to precede other tasks.

Release or arrival time: ri,j: jth instance of ith task Phase Φi: release time of first instant of ith task Response time: time between activation and completion Absolute deadline: instant by which task must complete Relative deadline: maximum allowable response time Period Pi: maximum length of intervals between the release

times of consecutive tasks. Execution time: the maximum amount of time required to

complete a instance of the task assuming all the resources are available.

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Simple kernels

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Polled loop: Say a kernel needs to process packets that are transferred into the DMA and a flag is set after transfer:

for(;;) { if (packet_here) { process_data(); packet_here=0; }}Excellent for handling high-speed data channels, a processor is

dedicated to handling the data channel.Disadvantage: cannot handle bursts

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Simple kernels: cyclic executives

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Illusion of simultaneity by taking advantage of relatively short processes in a continuous loop:

for(;;) { process_1(); process_2(); process_3(); … process_n();}Different rate structures can be achieved by repeating tasks in the list:for(;;) { process_1(); process_2(); process_3(); process_3();}

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Cyclic Executives: Example: Interactive games

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Space invaders:for(;;) { check_for_keypressed(); move_aliens(); check_for_keypressed(); check_collision(); check_for_keypressed(); update_screen(); }}check_keypressed() checks for three button pressings: move

tank left or right and fire missiles.If the schedule is carefully constructed we could achieve a very

efficient game program with a simple kernel as shown above.

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Finite state automata and Co-routine based kernels

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void process_a(void){for(;;) { switch (state_a) { case 1: phase_a1(); | case 2: phase_a2(); |…. case n: phase_an();}}}

void process_b(void){for(;;) { switch (state_b) { case 1: phase_b1(); | case 2: phase_b2(); |…. case n: phase_bn();}}}

state_a and state_b are state counters;

Communication between coroutines thru’ global variables;

Example: the famous CICS from IBM : Customer Information Control System

IBM’s OS/2 uses this in Windows presentation management.

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Interrupt driven systems

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Main program is a simple loop.Various tasks in the system are schedules via

software or hardware interrupts;Dispatching performed by interrupt handling

routines.Hardware and software interrupts.

Hardware: asynchronous Software: typically synchronous

Executing process is suspended, state and context saved and control is transferred to ISR (interrupt service routine)

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Interrupt driven systems: code example

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void main() { init(); while(TRUE);}

void int1(void){ save (context); task1(); retore (context);}

void int1(void){ save (context); task1(); restore (context);}

Foreground/background systems is a variation of this where main does some useful task in the background;

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Process scheduling

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Scheduling is a very important function in a real-time operating system.

Two types: pre-run-time and run-timePre-run-time scheduling: create a feasible schedule

offline to meet time constraints, guarantee execution order of processes, and prevents simultaneous accesses to shared resources.

Run-time scheduling: allows events to interrupt processes, on demand allocation of resources , and used complex run-time mechanisms to meet time constraints.

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More on Cyclic ExecutivesSimple loop cyclic executiveFrame/slots Table-based predetermined schedule

cyclic executivePeriodic, aperiodic and interrupt-based

taskLets design a cyclic-executive with

multiple periodic tasks.

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The basic systemsSeveral functions are called in a

prearranged sequenceSome kind of cooperative schedulingYou a have a set of tasks and a scheduler

that schedules these tasksTypes of tasks: base tasks (background),

interrupt tasks, clock tasksFrame of slots, slots of cycles, each task

taking a cycle, burn tasks to fill up the left over cycles in a frame.

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Blind Bingo ( A Simple Example)

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A c b g k

V n m L s

E t y w f

D v z x e

Display();Read input();Loop: update display(); If all done exit(); Read input();End Loop;

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Cyclic Executive Design 1 (pages 81-87)

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Base tasks, clock tasks, interrupt tasks Base: no strict requirements, background activity Clock: periodic with fixed runtime Interrupt: event-driven preemption, rapid response

but little processingDesign the slotsTable-driven cyclic executive

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Cyclic executive

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Each task implemented as a functionAll tasks see global data and share themCyclic executive for three priority levelThe execution sequence of tasks within a cyclic

executive will NOT vary in any unpredictable manner (such as in a regular fully featured Operating Systems)

Clock tasks, clock sched, base tasks, base sched, interrupt tasks

Each clock slot executes, clock tasks, at the end a burn task that is usually the base task

Study the figures in pages 83-86 of your text

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RT Cyclic Executive Program

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Lets examine the code:Identify the tasksIdentify the cyclic schedule specified in the

form of a tableObserve how the functions are specified as

table entryUnderstand the scheduler is built-inLearn how the function in the table are

dispatched

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Implementation of a cyclic executive

#include <stdio.h>#include <ctype.h>#include <unistd.h>#include <sys/times.h>#define SLOTX 4#define CYCLEX 5#define SLOT_T 5000

int tps,cycle=0,slot=0;clock_t now, then;struct tms n;void one() { printf("Task 1 running\n"); sleep(1);}void two() { printf("Task 2 running\n"); sleep(1); }

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Implementation (contd.)

void three() { printf("Task 3 running\n"); sleep(1);}

void four() { printf("Task 4 running\n"); sleep(1);}

void five() { printf("Task 5 running\n"); sleep(1);}

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Implementation (contd.)

void burn() { clock_t bstart = times(&n); while ((( now = times(&n)) - then) < SLOT_T * tps / 1000) { } printf (" brn time = %2.2dms\n\n", (times(&n)-bstart)*1000/tps); then = now; cycle = CYCLEX;}

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Implementation (contd.)

void (*ttable[SLOTX][CYCLEX])() = {{one, two, burn, burn, burn},{one, three, four, burn, burn},{one, two, burn, burn, burn},{one, five, four, burn, burn}};

main() { tps = sysconf(_SC_CLK_TCK); printf("clock ticks/sec = %d\n\n", tps); then = times(&n); while (1) { for (slot=0; slot <SLOTX; slot++) for (cycle=0; cycle<CYCLEX; cycle++) (*ttable[slot][cycle])(); }}

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Summary

The cyclic executive discussed the scheduler is built-in. You can also use clock ticks RTC etc to schedule the tasks

In order use the cyclic executive discussed here in other applications simply change table configuration, and rewrite the dummy functions we used.

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