Virtual Memory Usage from Java under Linux, too mu

Question

I have a problem with a Java application running under Linux.

When I launch the application, using the default maximum heap size (64 MB), I see using the tops application that 240 MB of virtual Memory are allocated to the application. This creates some issues with some other software on the computer, which is relatively resource-limited.

The reserved virtual memory will not be used anyway, as far as I understand, because once we reach the heap limit an OutOfMemoryError is thrown. I ran the same application under windows and I see that the Virtual Memory size and the Heap size are similar.

Is there anyway that I can configure the Virtual Memory in use for a Java process under Linux?

Edit 1: The problem is not the Heap. The problem is that if I set a Heap of 128 MB, for example, still Linux allocates 210 MB of Virtual Memory, which is not needed, ever.**

Edit 2: Using ulimit -v allows limiting the amount of virtual memory. If the size set is below 204 MB, then the application won't run even though it doesn't need 204 MB, only 64 MB. So I want to understand why Java requires so much virtual memory. Can this be changed?

Edit 3: There are several other applications running in the system, which is embedded. And the system does have a virtual memory limit (from comments, important detail).

Answer 1

One way of reducing the heap sice of a system with limited resources may be to play around with the -XX:MaxHeapFreeRatio variable. This is usually set to 70, and is the maximum percentage of the heap that is free before the GC shrinks it. Setting it to a lower value, and you will see in eg the jvisualvm profiler that a smaller heap sice is usually used for your program.

EDIT: To set small values for -XX:MaxHeapFreeRatio you must also set -XX:MinHeapFreeRatio Eg

java -XX:MinHeapFreeRatio=10 -XX:MaxHeapFreeRatio=25 HelloWorld

EDIT2: Added an example for a real application that starts and does the same task, one with default parameters and one with 10 and 25 as parameters. I didn't notice any real speed difference, although java in theory should use more time to increase the heap in the latter example.

At the end, max heap is 905, used heap is 378

At the end, max heap is 722, used heap is 378

This actually have some inpact, as our application runs on a remote desktop server, and many users may run it at once.

Answer 2

The Sun JVM requires a lot of memory for HotSpot and it maps in the runtime libraries in shared memory.

If memory is an issue consider using another JVM suitable for embedding. IBM has j9, and there is the Open Source "jamvm" which uses GNU classpath libraries. Also Sun has the Squeak JVM running on the SunSPOTS so there are alternatives.