我已经写过 Sieve of Eratosthenes,它应该并行工作,但事实并非如此。当我增加线程数时,计算时间并没有变短。任何想法为什么?
主班
import java.util.Date;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class ConcurrentTest {
public static void main(String[] args) throws InterruptedException {
Sieve task = new Sieve();
int x = 1000000;
int threads = 4;
task.setArray(x);
Long beg = new Date().getTime();
ExecutorService exec = Executors.newCachedThreadPool();
for (int i = 0; i < threads; i++) {
exec.execute(task);
}
exec.shutdown();
Long time = 0L;
// Main thread is waiting until all threads are terminated
// ( it means that computing is done)
while (true)
if (exec.isTerminated()) {
time = new Date().getTime() - beg;
break;
}
System.out.println("Time is " + time);
}
}
筛类
import java.util.concurrent.ConcurrentHashMap;
public class Sieve implements Runnable {
private ConcurrentHashMap<Integer, Boolean> array =
new ConcurrentHashMap<Integer, Boolean>();
private int x;
public void run() {
while(true){
// I am getting synchronized number to check if it's prime
int n = getCounter();
// If no more numbers to check, stop loop
if( n == -1)
break;
// If HashMap contains number, we can further
if(!array.containsKey(n))continue;
for (int i = 2 * n; i <= x; i += n) {
// Compound numbers are removed from HashMap, Eg. 6, 12 and much more.
array.remove(i);
}
}
}
private synchronized int getCounter(){
if( counter < x)
return counter++;
else return -1;
}
public void setArray(int x) {
this.x = x;
for (int i = 2; i <= x; i++)
array.put(i, false);
}
}
我用不同数量的线程做了一些测试。这些是结果:
Nr of threads 1 Time is 1850, 1795, 1825
Nr of threads 2 Time is 1845, 1836, 1814
Nr of threads 3 Time is 1767, 1820, 1756
Nr of threads 4 Time is 1732, 1840, 2083
Nr of threads 5 Time is 1791, 1795, 1803
Nr of threads 6 Time is 1825, 1728, 1707
Nr of threads 7 Time is 1754, 1729, 1686
Nr of threads 8 Time is 1760, 1717, 1817
Nr of threads 9 Time is 1721, 1699, 1673
Nr of threads 10 Time is 1661, 1722, 1718