1. Software Engineering Lecture 5 Multiprogramming and Scheduling ASPI8-4 Anders P. Ravn March 2004

2. Overview • Concurrent processes - Java Threads • Mutual exclusion • Semaphores • Monitors - Java synchronized, wait, notify • Ada rendezvous

3. A kernel specification /* kernel.h Interface to a lightweight kernel that implements concurrent processes and a release primitive `pause'. Anders P. Ravn, DTU, Denmark 24 April 1998 apr@it.dtu.dk*/ typedef void (*Program)(void); /* A program text is a function without parameters*/ typedef void * Thread; /* identifier for a process */ extern Thread create(Program p,unsigned int stacksize); /* creates a process with a stackof the specified size and starts it executing theprogram. If there is insufficientmemory, the result is NULL */ extern void pause(void); /* release the processor */

4. Multiprogramming #include ”kernel.h” void process() { /* do something */ pause(); /* do something */ } void main() { Thread p1, p2; p1 = create(&process,2000); /* p1 is started */ p2 = create(&process,1000); /* p2 is started */ /* the kernel will see to it that main is left when both p1 and p2 has exited */ }

5. A kernel implementation I typedef unsigned long Register; typedef struct x {struct x* next; Register esp;} Threaddescriptor; static Threaddesriptor* ready; /* queue of threads linked cyclically; ready points to last, the first is current */ #define current ready->next ready: current esp1 esp2 esp3

6. A kernel implementation II void pause() { Register sp; __asm__(”pushal movl %%esp,%sp"); /* sp -> |saved registers ... | eip return from call */ DISABLE; /* scheduling */ current->esp = sp; ready = current; sp = current->esp; __asm__(" movl %sp,%%esp popal" ); ENABLE; } stack1 stack2 ready: current esp1 esp2 esp3

7. A kernel implementation III pause: pushl %ebp movl %esp,%ebp pushal movl %esp,%ecx sp = esp movl ready,%eax movl (%eax),%edx current->esp movl %ecx,4(%edx) = sp movl %edx,ready ready = current movl (%edx),%edx movl 4(%edx),%ecx sp = current->esp movl %ecx,%esp popal leave ret

8. A kernel implementation IV pause: pushal movl ready,%eax movl (%eax),%edx current->esp movl %esp,4(%edx) = esp movl %edx,ready ready = current movl (%edx),%edx movl 4(%edx),%esp esp = current->esp popal ret

9. Java Threads import java.awt.*; class ProcessextendsThread { public Customer(...){ ...} public void run(){... // do something } ... Process p1 = new Process(); p1.start(); ...

10. Shared Variables class Banking { /* shared variable balance and private withdrawals */ public int balance; public int[] wd; public Banking() { balance = 2000; wd = new int[2]; } // Invariant: // balance+wd[0]+wd[1] == 2000 }

11. Critical Section class Customer extends Thread { int id; Lock critical; Banking bank; public void run() { do { sleep(800-400*id); critical.enter(id); int local = bank.balance; sleep(shortdelay); bank.balance = local-1; critical.leave(id); bank.wd[id]++; } while (true); }}

12. Semaphore public class Semaphore { int count; public Semaphore(int initial_value){ count= initial_value; } public synchronized void Wait(){ while(count == 0) wait(); --count; } public synchronized void signal(){ if (count++ == 0) notify(); } }

13. Rendezvous

14. Scheduling • Periodic processes – cyclic executive • Fixed Priority Scheduling – Rate Monotonic • Response Time Analysis • Sporadic Processes • Blocking and priority inversion • Priority Ceiling protocols • Real-Time Java

15. Cyclic executive loop wait 25msinterrupt; a(); b(); c(); wait 25ms interrupt; a(); b(); d(); e(); wait 25ms interrupt; a(); b(); c(); wait 25ms interrupt; a(); b(); d(); end loop;

16. Utilization tests Utilization U = C/T Priority is rate (1/T) monotonic U1 + ... + UN  N( N2 – 1)  0.693 (FPS) U1 + ... + UN  1 (Earliest Deadline First !?) Liu & Layland JACM, 1973

17. Response Time Analysis Response time R= C + I -- Interference Ii = Ri /TN CN + ... + Ri /Ti+1Ci+1(FPS) Joseph & Pandya Computer Journal 1986

18. Sporadic Processes Deadline D < T Priority is deadline (1/D) monotonic

19. Blocking Critical Regions V and Q locked by eg a semaphore. d(Q,V): EEEEBQ-----------BQQVVEE c(V) : EEVV----VVEE b() : ------------EEEE a(Q) : EEQQ----------------QQQQQQ------EE Priority Inversion

20. Response Time Analysis Response time R= C + B + I K = (k1,..., km): resources used by a process of lower priority and by a process with a higher or equal prority Bi = Ck1 + ... + Ckm

21. Immediate Ceiling Protocol A resource uses the maximual priority of any process using it. K = (k1,..., km): resources used by a process of lower priority and by a process with a higher or equal prority Bi = max C(k), k  K

22. Blocking ICPP d(Q,V): EEEEBQ-----------BQQVVEE c(V) : EEVV----VVEE b() : ------------EEEE a(Q) : EEQQ----------------QQQQQQ------EE ------------------- d(Q,V): BBEEEEQQVVEE c(V) : BBBBBB----------EEVVVVEE b() : BBBBBB------------------EEEE a(Q) : EEQQQQQQQQ----------------------EE

23. Real-Time Java public classPeriodic extendsRealTimeThread{ public Periodic(PriorityParameters pp, PeriodicParameters p) { ... } publicvoid run(){ for (;;){ ... waitForNextPeriod(); } } }

24. PeriodicParameters public classPeriodicparameters ... { public PeriodicParameters( HighResolutionTime start, RelativeTime period, RelativeTime cost, RelativeTime deadline, AsyncEventHandler overrunhandler, AsyncEventHandler misshandler){ ... } }

25. And more http://www.rtj.org