protothread and its usage in contiki OS
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Protothreads provide a lightweight threading mechanism for memory-constrained embedded systems by allowing sequential flow of control without full multi-threading or complex state machines. Protothreads use a single stack that is reused for each thread, requiring less memory than traditional multithreading. They provide blocking wait functionality through macros that expand the code to use C switch statements instead of stack switching or other complex mechanisms. This allows for conditional blocking within functions to simplify programming of event-driven systems.
protothread and its usage in contiki OS
- 1. 1 Protothreads – Simplifying Programming of Memory-Constrain ed Embedded Systems By Salah Adopted from Adam Dunkels et. al., http://dunkels.com/adam/pt/expansion.html
- 2. 2 Protothreads uProtothreads are a extremely lightweight, stackless type of threads that provides a blocking context on top of an event-driven system, without the overhead of per-thread stacks. uThe purpose of Protothreads is to implement sequential flow of control without complex state machines or full multi-threading. u Protothreads provides conditional blocking inside C functions..
- 3. 3 Why not just use multithreading? Multithreading the basis of (almost) all embedded OS/RTOSes! ◦ WSN community: Mantis, BTNut (based on multithreading); Contiki (multithr eading on a per-application basis) Nothing wrong with multithreading ◦ Multiple stacks require more memory ◦ Networked = more concurrency than traditional embedded ◦ Can lead to more expensive hardware ◦ Preemption ◦ Threads: explicit locking; Protothreads: implicit locking Protothreads are a new point in the design space ◦ Between event-driven and multithreaded
- 4. 4 Threads require per-thread stack memory Four threads, each with its own stack Thread 1 Thread 2 Thread 3 Thread 4
- 5. 5 Events require one stack Four threads, each with its own stack Thread 1 Thread 2 Thread 3 Thread 4 Eventhandler 1Eventhandler 2Eventhandler 3 Stack is reused for every event handler Threads require per-thread stack memory ● Four event handlers, one stack Eventhandler 4
- 6. 6 Protothreads require one stack Four threads, each with its own stack Thread 1 Thread 2 Thread 3 Thread 4 Threads require per-thread stack memory ● Four protothreads, one stack Events require one stack ● Four event handlers, one stack Protothread 1Protothread 2Protothread 3Protothread 4 Just like events
- 7. 7 Main features u No machine specific code since the protothreads library is pure C u Does not use error-prone functions such as longjmp() u Very small RAM overhead - only two bytes per protothread u Can be used with or without an OS u Provides blocking wait without full multi-threading or stack- switching
- 8. 8 Examples applications: - Memory constrained systems - Event-driven protocol stacks - Deeply embedded systems - Sensor network nodes
- 9. 9
- 10. 10 Pt file structure Data Structures struct Pt Initialization #define PT_INIT(pt) Initialize a protothread. Declaration and definition #define PT_THREAD(name_args) Declaration of a protothread. #define PT_BEGIN(pt) Declare the start of a protothread inside the C functi on implementing the protothread. #define PT_END(pt) Declare the end of a protothread.
- 11. 11 Pt file structure Blocked wait #define PT_WAIT_UNTIL(pt, condition) Block and wait until condition is true. #define PT_WAIT_WHILE(pt, cond) Block and wait while condition is true. Hierarchical protothreads #define PT_WAIT_THREAD(pt, thread) Block and wait until a child protothread completes. #define PT_SPAWN(pt, child, thread) Spawn a child protothread and wait until it exits.
- 12. 12 Pt file structure Exiting and restarting #define PT_RESTART(pt) Restart the protothread. #define PT_EXIT(pt) Exit the protothread. Calling a protothread #define PT_SCHEDULE(f) Schedule a protothread.
- 13. 13 Pt file structure Yielding from a protothread #define PT_YIELD(pt) Yield from the current protothread. #define PT_YIELD_UNTIL(pt, cond) Yield from the protothread until a condition occurs. Defines #define PT_WAITING 0 #define PT_YIELDED 1 #define PT_EXITED 2 #define PT_ENDED 3
- 14. 14 Protothread scheduling A protothread runs in a C function We schedule a protothread by invoking its function We can invoke the protothread from an event handler ◦ Protothreads as blocking event handlers We can let the operating system invoke our protothreads ◦ Contiki Protothreads can invoke other protothreads ◦ Can wait until a child protothread completes ◦ Hierarchical protothreads
- 15. 15 How do we implement protothreads?
- 16. 16 Simple example program using protothread --waits for a counter to reach a certain threshold, -prints out a message, and resets the counter. - This is done in a while() loop that runs forever. - The counter is increased in the main() function.
- 18. 18 C preprocessor expand the above code,
- 21. 21 C-switch expansion-another example int a_protothread(struct pt *pt) { PT_BEGIN(pt); PT_WAIT_UNTIL(pt, condition1); if(something) { PT_WAIT_UNTIL(pt, condition2); } PT_END(pt); } int a_protothread(struct pt *pt) { switch(pt->lc) { case 0: pt->lc = 5; case 5: if(!condition1) return 0; if(something) { pt->lc = 10; case 10: if(!condition2) return 0; } } return 1; } Line numbers