Setting JVM heap size at runtime

Question

Is there a way to set heap size from a running Java program?

Answer 1

No.

What you can do with an app that has very variable heap requirements is to set your max heap size very high with -Xmx and tune -XX:MaxHeapFreeRatio and -XX:MinHeapFreeRatio so that the app will not hang on to a lot of memory when the heap shrinks (it does that with default settings).

But note that this may cause performance problems when the memory actually used by the app varies both strongly and quickly - in that case you're better off having it hang on to all the memory rather than give it back to the OS only to claim it again a second later. You might also want to fiddle with the GC options to ensure that the GC doesn't leave too much unclaimed objects lying around, which it tends to do when there's a lot of room for the heap to grow, and which would defeat the goal of wanting the heap size to adjust to the app's needs.

Answer 2

According to http://www.dreamincode.net/forums/showtopic96263.htm, you can't do this at runtime, but you can spawn another process with a different heap size.

Answer 3

You can tweak those settings when you start your application but once the JVM is up and running those values cannot be changed. Something like this:

java -Xms32m -Xmx512m FooBar

will set the minimum heap size to 32MB and the maximum heap size to 512MB. Once these are set, you cannot change them within the running program.

Answer 4

The consensus may indeed be that this is not possible, but we should be looking at the JVM source to see how it can be ergonomically controlled. It would be very nice to be able to have a JVMTI agent be able to adjust the heap/perm/tenured/new/&c sizing online/at runtime.

What would this do? it would allow agents to infer sizing adjustments based upon performance or footprint goals which will be important when moving JVMs into the Cloud.

Answer 5

You can use the -mx option on startup (also known as -Xmx) This is maximum size you should ever need in which cause you shouldn't need to set it to more than the maximum size you will ever need.

However, a work around is to have the main() check the maximum size and restart the java if the maximum size is not as desired. i.e. start another java program and die.