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POMulti threaded object creation slower then in a single thread
 

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    <p>I have what probably is a basic question. When I create 100 million Hashtables it takes approximately 6 seconds (runtime = 6 seconds per core) on my machine if I do it on a single core. If I do this multi-threaded on 12 cores (my machine has 6 cores that allow hyperthreading) it takes around 10 seconds (runtime = 112 seconds per core).</p> <p>This is the code I use:</p> <p><strong>Main</strong></p> <pre><code>public class Tests { public static void main(String args[]) { double start = System.currentTimeMillis(); int nThreads = 12; double[] runTime = new double[nThreads]; TestsThread[] threads = new TestsThread[nThreads]; int totalJob = 100000000; int jobsize = totalJob/nThreads; for(int i = 0; i &lt; threads.length; i++) { threads[i] = new TestsThread(jobsize,runTime, i); threads[i].start(); } waitThreads(threads); for(int i = 0; i &lt; runTime.length; i++) { System.out.println("Runtime thread:" + i + " = " + (runTime[i]/1000000) + "ms"); } double end = System.currentTimeMillis(); System.out.println("Total runtime = " + (end-start) + " ms"); } private static void waitThreads(TestsThread[] threads) { for(int i = 0; i &lt; threads.length; i++) { while(threads[i].finished == false)//keep waiting untill the thread is done { //System.out.println("waiting on thread:" + i); try { Thread.sleep(1); } catch (InterruptedException e) { e.printStackTrace(); } } } } } </code></pre> <p><strong>Thread</strong></p> <pre><code>import java.util.HashMap; import java.util.Map; public class TestsThread extends Thread { int jobSize = 0; double[] runTime; boolean finished; int threadNumber; TestsThread(int job, double[] runTime, int threadNumber) { this.finished = false; this.jobSize = job; this.runTime = runTime; this.threadNumber = threadNumber; } public void run() { double start = System.nanoTime(); for(int l = 0; l &lt; jobSize ; l++) { double[] test = new double[65]; } double end = System.nanoTime(); double difference = end-start; runTime[threadNumber] += difference; this.finished = true; } } </code></pre> <p>I do not understand why creating the object simultaneously in multiple threads takes longer per thread then doing it in serial in only 1 thread. If I remove the line where I create the Hashtable this problem disappears. If anyone could help me with this I would be greatly thankful.</p>
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    1. USSipko
    1. USSipko
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    1. COI'd say creating 100,000,000 hashtables is the problem.
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      1. USBathsheba
    2. COIs the application actually using multiple cores? I could see it taking longer if it was not spanning cores and had to flop out the threads because then you would have that overhead.
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      1. USCodeChimp
    3. COAnd when you remove the hashtable creation your threads are doing nothing, so there is nothing to compare ;0
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      1. USBartoszKP
 

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