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Date: Wed, 16 May 2012 15:04:56 +0200
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Subject: Re: two instances of Tomcat in same machine
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Miguel Gonzalez wrote:
> there wasn't a limit of 32 bits OS for running JVM for more than 1.5 Gb?

Not exactly.  I'm not sure that my knowledge is totally accurate here, but here is my take 
on it :

- a "32 bit OS" generally means that the total address space available to *a process* 
running at any point in time under that OS is 2^32 = 4,294,967,296 bytes, which is 4 GB 
(give or take :-)).
- this includes the memory accupied by the resident part of the OS, so that in the 
practice nowadays, a process can use only a fraction of that, say 2-3 GB.
- that available addressable space is shared between the code of the process, and the 
in-memory data needed by that process
- the JVM (which runs Tomcat) is one such process.  The code of the JVM itself occupies 
some memory. For the JVM, the Tomcat code and your application code is data, which uses a 
part of the addressable space available to the JVM process. The JVM also allocates some 
space for the data that Tomcat and your applications use, dynamically, as needed and 
requested by Tomcat and your applications.  In Java, most of that "work memory" is 
allocated in a memory structure called the Heap.
- in the practice thus, if you are running a Java application under a Java JVM under a 
32-bit OS, the maximum size of the Heap that your application will be able to use is 
likely to limited to around 2-2.5 GB.  OS-specific characteristics may further reduce this.

On hardware that can use 64-bit addresses, and with an OS that can use a 64-bit address 
space, these limits are much higher (2^64 = 18,446,744,073,709,551,616 bytes), so that in 
the practice you would run out of money long before your run out of Heap space.
(And you would also run out of time before you fill up all that memory).


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