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POWhy does this Java code not utilize all CPU cores?
 

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  1. POWhy does this Java code not utilize all CPU cores?
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    <p>The attached simple Java code should load all available cpu core when starting it with the right parameters. So for instance, you start it with </p> <blockquote> <p>java VMTest 8 int 0</p> </blockquote> <p>and it will start 8 threads that do nothing else than looping and adding 2 to an integer. Something that runs in registers and not even allocates new memory. </p> <p>The problem we are facing now is, that we do not get a 24 core machine loaded (AMD 2 sockets with 12 cores each), when running this simple program (with 24 threads of course). Similar things happen with 2 programs each 12 threads or smaller machines. </p> <p>So our suspicion is that the JVM (Sun JDK 6u20 on Linux x64) does not scale well. </p> <p>Did anyone see similar things or has the ability to run it and report whether or not it runs well on his/her machine (>= 8 cores only please)? Ideas?</p> <p>I tried that on Amazon EC2 with 8 cores too, but the virtual machine seems to run different from a real box, so the loading behaves totally strange.</p> <pre><code>package com.test; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Future; import java.util.concurrent.TimeUnit; public class VMTest { public class IntTask implements Runnable { @Override public void run() { int i = 0; while (true) { i = i + 2; } } } public class StringTask implements Runnable { @Override public void run() { int i = 0; String s; while (true) { i++; s = "s" + Integer.valueOf(i); } } } public class ArrayTask implements Runnable { private final int size; public ArrayTask(int size) { this.size = size; } @Override public void run() { int i = 0; String[] s; while (true) { i++; s = new String[size]; } } } public void doIt(String[] args) throws InterruptedException { final String command = args[1].trim(); ExecutorService executor = Executors.newFixedThreadPool(Integer.valueOf(args[0])); for (int i = 0; i &lt; Integer.valueOf(args[0]); i++) { Runnable runnable = null; if (command.equalsIgnoreCase("int")) { runnable = new IntTask(); } else if (command.equalsIgnoreCase("string")) { runnable = new StringTask(); } Future&lt;?&gt; submit = executor.submit(runnable); } executor.awaitTermination(1, TimeUnit.HOURS); } public static void main(String[] args) throws InterruptedException { if (args.length &lt; 3) { System.err.println("Usage: VMTest threadCount taskDef size"); System.err.println("threadCount: Number 1..n"); System.err.println("taskDef: int string array"); System.err.println("size: size of memory allocation for array, "); System.exit(-1); } new VMTest().doIt(args); } } </code></pre>
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    1. COAdditional information. Just found out that the 64bit version of the JDK loads the cores way better (about 90%) than the 32bit version (about 45%). Which is strange, because the OS and the AMD cpu support 32bit and I do not run any memory operations during that test.
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    2. COJust for understanding - why do you not use the invokeAll(..) method? And why don't you use callables, as far as i know, runnable is not part of the java.concurrent?
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    3. COYou should also watch for other running processes. Did you make a "clean" run, with no other progamms/processes taking cpu time?
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