Setting Jvm Heap Size at Runtime

Setting JVM heap size at runtime

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.

Can the JVM max heap size be dynamic?

But, is there a way to make that value dynamic?

Literally, no. The max heap size is set at JVM launch time and cannot be increased.

In practice, you could just set the max heap size to as large as your platform will allow, and let the JVM grow the heap as it needs. There is an obvious risk in doing this; i.e. that your application will use all of the memory and cause the user's machine to grind to a halt. But that risk is implicit in your question.

EDIT

It is worth noting that there are various -XX... GC tuning options that allow you to tweak the way that the JVM expands the heap (up to the maximum).

Another possibility is to split your application into 2 parts. The first part of the application does all of the preparation necessary to determine the "size" of the problem. Then it works out an appropriate max heap size, and launches the memory hungry second part of the application in a new JVM.

• This only works if the application can sensibly be partitioned as above.

• This only works if it is possible to compute the problem size. In some cases, computing the problem size is tantamount to computing the result.

• It is not clear that you will get better overall performance than if you just let the heap grow up to a maximum size.

Increase heap size in Java

You can increase to 2GB on a 32 bit system. If you're on a 64 bit system you can go higher. No need to worry if you've chosen incorrectly, if you ask for 5g on a 32 bit system java will complain about an invalid value and quit.

As others have posted, use the cmd-line flags - e.g.

java -Xmx6g myprogram

You can get a full list (or a nearly full list, anyway) by typing java -X.

How to increase application heap size in Eclipse?

In the run configuration you want to customize (just click on it) open the tab Arguments and add -Xmx2048min the VM arguments section.
You might want to set the -Xms as well (small heap size).

Runtime heap size set in Eclipse

Your program doesn't run in Eclipse's heap space. Eclipse spawns off a separate JVM for your program.

Runtime.totalMemory() does indeed return the current heap size.

The -Xms argument specifies the initial heap size. Java will expand if it cannot free up enough memory through garbage collection until it reaches the maximum, as set by -Xmx. At this point, the JVM will exit with an OutOfMemoryError.

Java memory management is a complex topic, involving garbage collection, moving objects from nursery to tenured space, etc.

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