On Windows 32-bit JVMs are often limited to less than 1.3GB of heap due
to fragmentation of the 32-bit address space. Sometimes one cannot even
get 1GB of heap!
For any hefty server process on Windows you really want a 64-bit JVM for
this reason.
On other OS's one can often allocate much more heap under 32-bit
addressing. How much depends on the OS, etc, but generally is much more
like 3GB.
--
Jess Holle
On 5/16/2012 8:04 AM, André Warnier wrote:
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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