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To Test the max heap size and max permgen space, As per reply suggested by Gilli at Max amount of memory per java process in Windows?, i wrote a simple java class with main method just printing system out

when i ran the class from Console with below param it ran fine but with 2048 it gave the error could not reserve Could not reserve enough space for object heap. It means for my system max heap size is allowed somewhere between 1536 to 2048

 java -Xms1536m -Xmx1536m  TestJVMParam

As per link at http://publib.boulder.ibm.com/infocenter/javasdk/tools/index.jsp?topic=/com.ibm.java.doc.igaa/_1vg00014884d287-11c3fb28dae-7ff6_1001.html, it is said

When running 64-bit Java, the memory space provided by the operating system to the Java process is very large. You can therefore assume that no limit is imposed on the maximum size of the Java heap because of the contention of memory resource between the Java heap and the native heap.

So i have 8 GB of memory why VM Could not reserve enough space for object heap?

Now second thing i want is to find max permgen size for my system. So i ran the same program with below paramters

 java -Xms1536m -Xmx1536m -XX:PermSize=128m -XX:MaxPermSize=128m TestJVMParam

but again it says Could not reserve enough space for object heap. How is that possible as i have 8GB RAM with windows7 64 bit os. And its not allowing even 128m for permgen? i am sure i am missing something here but not able to figure it out?

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M Sach
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  • Mat Its 32 bit JVM. Oh its dependent on jvm bit too alongwith os bits. – M Sach Apr 22 '12 at 09:57
  • Then you've got essentially the same limitations as if you were on a 32bit OS with a 32bit JVM. Switch to a 64bit JVM. – Mat Apr 22 '12 at 09:58
  • Thanks Mat. But what about XX:MaxPermSize. Why programm is not running with 128m ? – M Sach Apr 22 '12 at 09:59
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    If you want to go beyond 32bit limitations, you need both a 64bit OS and a 64bit JVM. You could have 3TB of RAM on that machine, a 32bit process is a 32bit process and has limited heap space. – Mat Apr 22 '12 at 10:02
  • Mat one last question.I almost the logic you explained. So for 32 bit jvm, max memory setting as per this programm seems to be 1536 for heap.But if thats true, when i mention catalina_opt(as advanced variable) value as -Xms1536m -Xmx1536m -XX:PermSize=128m -XX:MxPermSize=128m, My tomcat start and does not complain. Idealy it shoild not have started. Right(as program complained)? – M Sach Apr 22 '12 at 10:16
  • It is possible that Tomcat found a separate java installation on your machine and is using that. Write a webpage that prints value of this property to easily see which version of java Tomcat is running. `System.getProperty("java.version"));` – Dunes Apr 22 '12 at 10:25

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