committed heap vs max heap

Question

Hi JVM configuration is :

• Xmx = 4096
• Xms = 1024

In my monitoring tools : I have always a committed heap at 2g and my server goes to OutOfMemory errors .

I don't understand why my JVM committed heap is limited to 2g and didn't grow up to 4g (Max heap)

Why the free heap is not used and my server goes to OutOfMemory exception ?

NB:

• My application server is websphere 8.5.
• My server is Linux 64 Bits .

Thanks in advance

Answer 1

While setting preferredHeapBase may resolve the problem, there could be a number of other reasons for OOM:

• A very large memory request could not be satisfied - check the verboseGC output just before OOM timestamp.
• Inadequate User Limits Insufficient ulimit -u (NPROC) Value Contributes to Native OutOfMemory. Check the user limits.
• The requirement from the application for more than 4gb native memory. In that case, -Xnocompressedrefs will resolve the problem (but with a larger java memory footprint and performance impact).

There are other reasons but I find those to be the top hits when diagnosing OOM when there is plenty of java heap space free.