Setting JVM heap size at runtime

Question

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

Answer 1

java -Xmx500m on startup only

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

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 5

If I understand your question correctly, you're trying to change the heap size at runtime. I don't see any reason why this should be possible. Set the heap size at startup using the -Xmx JVM option. I also advise you to set the -Xms option only if you absolutely need to. This option sets the initial amount of head memory that is allocated for the JVM.

You should know how your application behaves in terms of memory. Set the the value of -Xmx wisely. If your app is some kind of a server app you can set a higher value, otherwise compromise your choice with other possible apps running in client machines and of course available memory.

Answer 6

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 7

I asked same question to myself. And unlike answers above there is something I can do about my app increasing max heap JVM size. If app is a web server in cluster mode, I could start a new instance with changed min/max heap size and than shutdown initial instance. That shall be especially simple in GlassFish where you have management instance separated from nodeAgent(app server clustered instance) JVM.

Since many JVM applications are web apps, I think it worth to preserve in this blog.

Answer 8

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.