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    <p>There's nothing better than an empirical experiment to answer your question. I've wrote a Java program and run it while specifying the XMX flag (also used XMS=XMX to force the JVM pre-allocate all of the memory). To further protect against JVM optimizations, I've actively allocate X number of 10MB objects. I run a number of test on a number of JVMs increasing the XMX value together with increasing the number of MB allocated, on a different 32bit operating systems using both Sun and IBM JVMs, here's a summary of the results:</p> <p>OS:Windows XP SP2, JVM: Sun 1.6.0_02, Max heap size: 1470 MB<br> OS: Windows XP SP2, JVM: IBM 1.5, Max heap size: 1810 MB<br> OS: Windows Server 2003 SE, JVM: IBM 1.5, Max heap size: 1850 MB<br> OS: Linux 2.6, JVM: IBM 1.5, Max heap size: 2750 MB </p> <p>Here's the detailed run attempts together with the allocation class helper source code:</p> <p>WinXP SP2, SUN JVM:<pre> C:>java -version java version "1.6.0_02" Java(TM) SE Runtime Environment (build 1.6.0_02-b06) Java HotSpot(TM) Client VM (build 1.6.0_02-b06, mixed mode)</p> <p>java -Xms1470m -Xmx1470m Class1 142 ... about to create object 141 object 141 created</p> <p>C:>java -Xms1480m -Xmx1480m Class1 145 Error occurred during initialization of VM Could not reserve enough space for object heap Could not create the Java virtual machine. </pre> WinXP SP2, IBM JVM<pre> C:>c:\ibm\jdk\bin\java.exe -version java version "1.5.0" Java(TM) 2 Runtime Environment, Standard Edition (build pwi32devifx-20070323 (if ix 117674: SR4 + 116644 + 114941 + 116110 + 114881)) IBM J9 VM (build 2.3, J2RE 1.5.0 IBM J9 2.3 Windows XP x86-32 j9vmwi3223ifx-2007 0323 (JIT enabled) J9VM - 20070322_12058_lHdSMR JIT - 20070109_1805ifx3_r8 GC - WASIFIX_2007) JCL - 20070131</p> <p>c:\ibm\jdk\bin\java.exe -Xms1810m -Xmx1810m Class1 178 ... about to create object 177 object 177 created</p> <p>C:>c:\ibm\jdk\bin\java.exe -Xms1820m -Xmx1820m Class1 179 JVMJ9VM015W Initialization error for library j9gc23(2): Failed to instantiate he ap. 1820M requested Could not create the Java virtual machine. </pre>Win2003 SE, IBM JVM<pre> C:>"C:\IBM\java" -Xms1850m -Xmx1850m Class1 sleeping for 5 seconds. Done.</p> <p>C:>"C:\IBM\java" -Xms1880m -Xmx1880m Class1 JVMJ9VM015W Initialization error for library j9gc23(2): Failed to instantiate he ap. 1880M requested Could not create the Java virtual machine.</pre> Linux 2.6, IBM JVM<pre> [root@myMachine ~]# /opt/ibm/java2-i386-50/bin/java -version java version "1.5.0" Java(TM) 2 Runtime Environment, Standard Edition (build pxi32dev-20060511 (SR2)) IBM J9 VM (build 2.3, J2RE 1.5.0 IBM J9 2.3 Linux x86-32 j9vmxi3223-20060504 (JIT enabled) J9VM - 20060501_06428_lHdSMR JIT - 20060428_1800_r8 GC - 20060501_AA) JCL - 20060511a</p> <p>/opt/ibm/java2-i386-50/bin/java -Xms2750m -Xmx2750m Class1 270</p> <p>[root@myMachine ~]# /opt/ibm/java2-i386-50/bin/java -Xms2800m -Xmx2800m Class1 270 JVMJ9VM015W Initialization error for library j9gc23(2): Failed to instantiate heap. 2800M requested Could not create the Java virtual machine. </pre></p> <p>Here's the code:</p> <pre><code> import java.util.StringTokenizer; public class Class1 { public Class1() {} private class BigObject { byte _myArr[]; public BigObject() { _myArr = new byte[10000000]; } } public static void main(String[] args) { (new Class1()).perform(Integer.parseInt(args[0])); } public void perform(int numOfObjects) { System.out.println("creating 10 MB arrays."); BigObject arr[] = new BigObject[numOfObjects]; for (int i=0;i &lt;numOfObjects; i++) { System.out.println("about to create object "+i); arr[i] = new BigObject(); System.out.println("object "+i+" created"); } System.out.println("sleeping for 5 seconds."); try { Thread.sleep(5000); }catch (Exception e) {e.printStackTrace();} System.out.println("Done."); } } </code></pre>
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    1. POMaximum amount of memory per Java process on Windows?
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    1. COI'm assuming that you have the max amount of memory for the 32-bit OS for this test.
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      1. USJay R.
    2. COExactly what you said. For Windows 7 64-bit and Sun JDK 6 update 27 32-bit , maximum heap size is 1220 MB; although the computer has 6GB RAM.
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      1. USMuhammad El Nakeep
 

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