![Monitor Solution to Dining Philosophers
monitor DiningPhilosophers
{
enum { THINKING; HUNGRY, EATING) state [5] ;
condition self [5];
void pickup (int i) {
state[i] = HUNGRY;
test(i);
if (state[i] != EATING) self[i].wait;
}
void putdown (int i) {
state[i] = THINKING;
// test left and right neighbors
test((i + 4) % 5);
test((i + 1) % 5);
}](https://image.slidesharecdn.com/operatingsystem26monitors-210603194202/85/Operating-system-26-monitors-7-320.jpg)
![Solution to Dining Philosophers
(Cont.)
void test (int i) {
if ((state[(i + 4) % 5] != EATING) &&
(state[i] == HUNGRY) &&
(state[(i + 1) % 5] != EATING) ) {
state[i] = EATING ;
self[i].signal () ;
}
}
initialization_code() {
for (int i = 0; i < 5; i++)
state[i] = THINKING;
}
}](https://image.slidesharecdn.com/operatingsystem26monitors-210603194202/85/Operating-system-26-monitors-8-320.jpg)
Operating system 26 monitors
- 1. Operating System 26 Monitors Prof Neeraj Bhargava Vaibhav Khanna Department of Computer Science School of Engineering and Systems Sciences Maharshi Dayanand Saraswati University Ajmer
- 2. Monitors • A high-level abstraction that provides a convenient and effective mechanism for process synchronization • Abstract data type, internal variables only accessible by code within the procedure • Only one process may be active within the monitor at a time • But not powerful enough to model some synchronization schemes monitor monitor-name { // shared variable declarations procedure P1 (…) { …. } procedure Pn (…) {……} Initialization code (…) { … } } }
- 3. Schematic view of a Monitor
- 4. Condition Variables • condition x, y; • Two operations are allowed on a condition variable: – x.wait() – a process that invokes the operation is suspended until x.signal() – x.signal() – resumes one of processes (if any) that invoked x.wait() • If no x.wait() on the variable, then it has no effect on the variable
- 5. Monitor with Condition Variables
- 6. Condition Variables Choices • If process P invokes x.signal(), andprocess Q is suspended in x.wait(), what should happen next? – Both Q and P cannot execute in paralel. If Q is resumed, then P must wait • Options include – Signal and wait – P waits until Q either leaves the monitor or it waits for another condition – Signal and continue – Q waits until P either leaves the monitor or it waits for another condition – Both have pros and cons – language implementer can decide – Monitors implemented in Concurrent Pascal compromise • P executing signal immediately leaves the monitor, Q is resumed – Implemented in other languages including Mesa, C#, Java
- 7. Monitor Solution to Dining Philosophers monitor DiningPhilosophers { enum { THINKING; HUNGRY, EATING) state [5] ; condition self [5]; void pickup (int i) { state[i] = HUNGRY; test(i); if (state[i] != EATING) self[i].wait; } void putdown (int i) { state[i] = THINKING; // test left and right neighbors test((i + 4) % 5); test((i + 1) % 5); }
- 8. Solution to Dining Philosophers (Cont.) void test (int i) { if ((state[(i + 4) % 5] != EATING) && (state[i] == HUNGRY) && (state[(i + 1) % 5] != EATING) ) { state[i] = EATING ; self[i].signal () ; } } initialization_code() { for (int i = 0; i < 5; i++) state[i] = THINKING; } }
- 9. • Each philosopher i invokes the operations pickup() and putdown() in the following sequence: DiningPhilosophers.pickup(i); EAT DiningPhilosophers.putdown(i); • No deadlock, but starvation is possible Solution to Dining Philosophers (Cont.)
- 10. Monitor Implementation Using Semaphores • Variables semaphore mutex; // (initially = 1) semaphore next; // (initially = 0) int next_count = 0; • Each procedure F will be replaced by wait(mutex); … body of F; … if (next_count > 0) signal(next) else signal(mutex); • Mutual exclusion within a monitor is ensured
- 11. Monitor Implementation – Condition Variables • For each condition variable x, we have: semaphore x_sem; // (initially = 0) int x_count = 0; • The operation x.wait can be implemented as: x_count++; if (next_count > 0) signal(next); else signal(mutex); wait(x_sem); x_count--;
- 12. Monitor Implementation (Cont.) • The operation x.signal can be implemented as: if (x_count > 0) { next_count++; signal(x_sem); wait(next); next_count--; }
- 13. Resuming Processes within a Monitor • If several processes queued on condition x, and x.signal() executed, which should be resumed? • FCFS frequently not adequate • conditional-wait construct of the form x.wait(c) – Where c is priority number – Process with lowest number (highest priority) is scheduled next
- 14. • Allocate a single resource among competing processes using priority numbers that specify the maximum time a process plans to use the resource R.acquire(t); ... access the resurce; ... R.release; • Where R is an instance of type ResourceAllocator Single Resource allocation
- 15. A Monitor to Allocate Single Resource monitor ResourceAllocator { boolean busy; condition x; void acquire(int time) { if (busy) x.wait(time); busy = TRUE; } void release() { busy = FALSE; x.signal(); } initialization code() { busy = FALSE; } }