So I chose to close the books and write something that made sense of all the big promises .
Simple Idea: Say if my process needs to add a billion numbers ,can i make it faster ,by dividing its workload into "N" Threads.
Sample code and o/p below to demonstrate the benefits of multi threading .
import java.util.concurrent.CountDownLatch;
public class AddBillionNumbers
{
static long count =0;
public static long addSequentially(){
for(long i=1;i<=1000000000;i++){
count += i;
}
return count;
}
public static void main( String args[]){
//Sequential Single Threaded Approach
long start = System.currentTimeMillis();
System.out.println("Sum of a billion numbers is : "+addSequentially());
long end = System.currentTimeMillis();
System.out.println("Single Threaded Computation performed in : "+ (end - start)+ " Ms\n");
//Multithreaded Approach
long start2 = System.currentTimeMillis();
CountDownLatch doneSignal = new CountDownLatch(6);
/*
* I am trying to break up the computation process into 6 threads .
* Thread1 will add Numbers 1,7,13,19...
* Thread2 will add Numbers 2,8,14,20....
* ...
* Thread6 will add Numbers 6,12,18,24.....
* Note the diffrence between the start and jump indexes are chosen to
* make sure no 2 threads end up adding the same number.
* I am using a CountDownLatch to know immediately when all my threads have finished as
* opposed to checking isAlive individually on each thread.
*/
//Executor e = Executors.newFixedThreadPool(6);
for (int i = 1; i <= 6; ++i) // create and start threads
{
new Thread((new HelperThread(doneSignal,i,6))).start();
}
try {
doneSignal.await();
} catch (InterruptedException ie) {
ie.printStackTrace();
}
System.out.println("Sum of a billion numbers is : "+ HelperThread.computationCount);
long end2 = System.currentTimeMillis();
System.out.println("MultiThreaded Computation performed in : "+ (end2 - start2)+ " Ms");
}
}
class HelperThread extends Thread{
int startIndex,jumpIndex;
static long computationCount = 0;
String threadName;
private final CountDownLatch doneSignal;
public HelperThread(CountDownLatch doneSignal,int startIndex, int jumpIndex)
{
this.doneSignal = doneSignal;
this.startIndex = startIndex;
this.jumpIndex = jumpIndex;
}
public void run() {
count(startIndex, jumpIndex);
doneSignal.countDown();
}
public void count(int startIndex, int jumpIndex){
long count = 0;
for(int i=startIndex;i<=1000000000;i+=jumpIndex){
count += i;
}
synchronized(HelperThread.class){
computationCount += count;
}
}
}
O/P below on 32 bit 4Processor 3GB RAM Windows
Sum of a billion numbers is : 500000000500000000
Single Threaded Computation performed in : 2901 Ms
Sum of a billion numbers is : 500000005000000000
MultiThreaded Computation performed in : 485 Ms
