1. RTS: Kernel Design and Cyclic Executives
Chapter 4
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2. Kernel & Device drivers
Servers (application ~, web ~, component ~)
Shell
XWin
Thread lib
ftp
User applications
System call interface
Process, memory, file system, network managers.
Kernel
Device drivers
Hardware/controller
Devices
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3. Task characteristics of real workload
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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4. Simple kernels
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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5. Simple kernels: cyclic executives
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:
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6. Cyclic Executives: Example: Interactive games
Space invaders:
for(;;) {
check_for_keypressed();
move_aliens();
check_collision();
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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7. Finite state automata and Co-routine based kernels
void process_a(void){ for(;;) { switch (state_a) { case 1: phase_a1(); | case 2: phase_a2(); | …. case n: phase_an();}}} void process_b(void){ 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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8. Interrupt driven systems
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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9. Interrupt driven systems: code example
void main() { init(); while(TRUE); } void int1(void){ save (context); task1(); retore (context);} restore (context);}
Foreground/background systems is a variation of this where main does some useful task in the background;
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10. Process scheduling
Scheduling is a very important function in a real-time operating system.
Two types: pre-run-time and run-time
Pre-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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11. More on Cyclic Executives
Simple loop cyclic executive
Frame/slots
Table-based predetermined schedule cyclic executive
Periodic, aperiodic and interrupt-based task
Lets design a cyclic-executive with multiple periodic tasks. 11
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12. The basic systems
Several functions are called in a prearranged sequence
Some kind of cooperative scheduling
You a have a set of tasks and a scheduler that schedules these tasks
Types of tasks: base tasks (background), interrupt tasks, clock tasks
Frame of slots, slots of cycles, each task taking a cycle, burn tasks to fill up the left over cycles in a frame. 12
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13. Cyclic Executive Design 1 (pages 81-87)
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 processing
Design the slots
Table-driven cyclic executive
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14. Cyclic executive Each task implemented as a function
All tasks see global data and share them
Cyclic executive for three priority level
The 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 of your text
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15. RT Cyclic Executive Program
Lets examine the code:
Identify the tasks
Identify the cyclic schedule specified in the form of a table
Observe how the functions are specified as table entry
Understand the scheduler is built-in
Learn how the function in the table are dispatched
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16. 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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17. Implementation (contd.)
void three() { printf("Task 3 running\n"); sleep(1); } void four() { printf("Task 4 running\n"); void five() { printf("Task 5 running\n");
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18. 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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19. Implementation (contd.)
void (*ttable[SLOTX][CYCLEX])() = { {one, two, burn, burn, burn}, {one, three, four, 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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20. 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 simple change table configuration, and rewrite the dummy functions we used.
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