Best GC options for minimum memory usage?

Question

It seems that the more -Xmx memory I set, the more memory my app is using. Alas, the garbage collector is controlling the memory by itself and even System.gc() does not garbage collect all unused objects.
There are lots of parameters at http://java.sun.com/docs/hotspot/VMOptions.html, but what's the most aggressive one(s) to free unused memory as soon as possible?

Answer 1

Hi Martin,
please, let me tell you a few basic things first:
- under good circumstances the GC runs only when some heap space is full and more
memory is needed.
- even if the GC frees memory it will usually not be returned to the OS immediately but only
when some other process needs it. This means you will not see processes shrink
immediately when the JVM reduces its heap sizes. This is certainly true for Solaris.
- it is bad practice to call System.gc() in your Java code.
- the smaller the configured heap the more time (in percent) the JVM will use for GC. This is
because the memory in use has to be searched each time the GC runs
- the heap actually used by your JVM adds to the real memory size of the process
- the max values (-Xmx + -XX:MaxPermSize) add to the virtual memory size of the process
even if these max values are not actually used
Now, knowing these, to minimize your actual heap usage, I recommend you the following settings:
-Xms20m # set this to the minimum you will need, omiting it only slows down JVM startup
-Xmx200m # set this to the maximum you might need or can spare
-XX:MinHeapFreeRatio=20 # minimum heap percentage free after a Full GC, default=40
-XX:MaxHeapFreeRatio=40 # maximum heap percentage free after a Full GC, default=70
-XX:NewSize=10m # don't make it too small because this will make GC less efficient, the
# actual value depends on the nature of the app
-XX:MaxNewSize=10m # ditto, you can only make it larger than NewSize
-XX:SurvivorRatio=6 # try to compensate for small NewSize
-XX:TargetSurvivorRatio=80 # don't waste survivor space memory
I hope this helps. Please, keep in mind that a GC-based system is not really meant to
minimize memory usage!