Monitors
- Supported by the compiler. (Java, C#)
- See example code. (Producer-Consumer in Java)
Presenter Notes
public class ProducerConsumer{
static final int N = 10;
static int current_item = 0;
static producer p = new producer();
static consumer c = new consumer();
static our_monitor mon = new our_monitor();
public static void main (String args[]){
p.start();
c.start();
}
static class producer extends Thread{
public void run(){
int item;
while(true){
item = produce_item();
mon.insert(item);
}
}
private int produce_item(){
current_item++;
waste_time();
return current_item;
}
private static void waste_time(){
try {
sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
static class consumer extends Thread{
public void run(){
int item;
while(true){
item = mon.remove();
consume_item(item);
}
}
private void consume_item(int item){
waste_time();
}
private static void waste_time(){
try {
sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
static class our_monitor{
private int buffer[] = new int[N];
private int count = 0,lo = 0, hi = 0;
public synchronized void insert(int val){
if(count == N) {
System.out.println("Buffer full, producer sleeping...");
go_to_sleep();
}
buffer[hi] = val;
hi = (hi+1)%N;
count = count + 1;
System.out.println("Item " + current_item + " is produced.");
System.out.print("BUFFER: ");
for(int i=0;i<N;i++){
System.out.print(buffer[i]+" ");
}
System.out.println();
if (count == 1) {
System.out.println("Waking up the consumer...");
notify();
}
}
public synchronized int remove() {
int val;
if (count == 0) {
System.out.println("Buffer empty, consumer sleeping...");
go_to_sleep();
}
val = buffer[lo];
buffer[lo] = 0;
lo = (lo+1)%N;
count = count -1;
System.out.println("Item " + val + " is consumed.");
System.out.print("BUFFER: ");
for(int i=0;i<N;i++){
System.out.print(buffer[i]+" ");
}
System.out.println();
if (count == N-1) {
System.out.println("Waking up the producer...");
notify();
}
return val;
}
private void go_to_sleep(){
try{
wait();
} catch(InterruptedException e) {
e.printStackTrace();
}
}
}
}
Message passing
Presenter Notes
Barriers
(a) Processes approaching a barrier.
(b) All processes but one blocked at the barrier.
(c) When the last process arrives at the barrier, all of them are let through.