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 (last lectures)
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. Blind Bingo (Homework 2)
Display();
Read input();
Loop:
update display();
If all done exit();
End Loop;
A c b g k
V n m L s
E t y w f
D v z x e 13
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14. Cyclic Executive Design 1 (pages 83-87)
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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15. 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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16. 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
Learn how the function in the table are dispatched
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