The JVM -Xmx argument lets one set the max heap size for the JVM to some value. But, is there a way to make that value dynamic? In other words, I want to tell the JVM “look, if you need it, just keep taking RAM from the system until the system is out.”
Two-part reason for asking:
First, the app in question can use a really wide range of ram depending on what the user is doing, so the conceptual min and max values are pretty far apart. Second, it would seem that the JVM reserves the max heap space from virtual memory at boot time. This particular app is run on a pretty wide variety of hardware, so picking a “one-size-fits-all” max heap space is hard since it has to be low enough to run on low-end hardware, but we’d really like to be able to take advantage of really beefy machines if they’re available.
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.