RTS: Kernel Design

1. RTS: Kernel Design

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

3. 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

4. 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: for(;;) { process_1(); process_2(); process_3(); process_3(); }

5. Cyclic Executives: example: Interactive games • 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.

6. 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 fanous CICS from IBM : Customer Information Control System IBM’s OS/2 uses this in Windows presentation management. Finite state automata and Co-routine based kernels

7. 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)

8. void main() { init(); while(TRUE); } void int1(void){ save (context); taks1(); retore (context);} void int1(void){ save (context); taks1(); restore (context);} Foreground/background systems is a variation of this where main does some useful task in the background; Interrupt driven systems: code example

9. 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.

10. 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.

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. (See notes given in class) 11

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 slot. 12

13. Blind Bingo Display(); Read input(); Loop: update display(); If all done exit(); Read input(); End Loop; A c b g k V n m L s E t y w f D v z x e 13

14. Period, Frame and Hyper-period: Cyclic Executive Design • See class notes • Design the slots • Table-driven cyclic executive