how can JVM use more than 4gb of memory

I have a request to install a Linux server (preferably Ubuntu 64bit server), and Java (64 bit) on the following machine:

• Intel Core2Quad Q8200 - 2.33 GHz
• 8gb DDR2 ram
• 2x 320GB SATA HDD in soft RAID1 mirror (mirror)

The problem is how to configure system and Java because I need JVM to use more than 4gb of memory.

It cannot be distributed on many virtual machines. There is data more than 4GB large and it has to be in memory because HDD is slow and performance is critical.

This is a configuration and performance question and I am interested in comments if anyone has experie开发者_Go百科nce?

thank you very much for helping me on this...

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A 64 bit JVM should have no problem at all with giant heaps, certainly much larger than your available RAM. Just increase the heap size when you start the JVM, for example:

java -Xmx6g

You used to have to specify the 64bit flag (with -d64), but I don't think this is necessary any more.

32bit JVMs can manage something like 3GB of heap.

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skaffman's answer which provides the required flag for allocating 6GB of memory is correct. If the 'g' does not do the trick you might want to try 6000m (check the link below for details on flags/platform)

For other options you can find useful information on all available options for the Java HotSpot VM here.

http://java.sun.com/javase/technologies/hotspot/vmoptions.jsp

(Behavioral and performance options are available. Platform specific links also available on this page)

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JVM (especially 64bit) does not hesitate taking all the memory, and 4Gb is not a problem. Just install 64-bit Ubuntu and default-jre package will also be 64 bit.

Also take special care about how your data is stored in memory. Again, 64-bit JDK is very hungry for memory due to higher overhead for pointers etc. so if you distribute these 4Gb in small chunks in some data structure, 8Gb will not be enough.

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If you install a 64-bit Ubuntu, I believe that

sudo apt-get install sun-java6-jdk

gives you a 64-bit Java.

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EDIT: The 64 bit Java can give you as much memory as you need with the appropriate switches. The limit is with the 32-bit JVM's which cannot go over 2-4 Gb depending on operating system.