Java: Commandline parameter to release unused memory

Question

In Bash, I use the commmand java -Xmx8192m -Xms512m -jar jarfile to start a Java process with an initial heap space of 512MB and maximum heap space of 8GB.

I like how the heap space increases based on demand, but once the heap space has been increased, it doesn't release although the process doesn't need the memory. How can I release the memory that isn't being used by the process?

Example: Process starts, and uses 600MB of memory. Heap space increases from 512MB to a little over 600MB. Process then drops down to 400MB RAM usage, but heap allocation stays at 600MB. How would I make the allocation stay near the RAM usage?

Answer 1

You cannot; it's simply not designed to work that way. Note that unused memory pages will simply be mapped out by your hardware, and so won't consume any real memory.

Answer 2

Generally you would not like JVM to return memory to the OS and later claim in back as both operations are not so cheap.

There are a couple XX parameters that may or may not work with your preferred garbage collector, namely

• -XX:MaxHeapFreeRatio=70 Maximum percentage of heap free after GC to avoid shrinking.
• -XX:MinHeapFreeRatio=40 Minimum percentage of heap free after GC to avoid expansion.

Source

I believe you'd need stop the world collector for them to be enforced. Other JVMs may have their own parameters.

I'd normally have not replied but the amount of negative/false info ain't cool.