Why does the (Oracle) JVM have a fixed upper limit for memory usage (-Xmx)?

Question

In the spirit of question Java: Why does MaxPermSize exist?, I\'d like to ask why the Oracle JVM uses a fixed upper limit for the size of its memory allocation pool.

Answer 1

Hm, I'll try summarizing the answers so far.

There is no technical reason why the JVM needs to have a hard limit for its heap size. It could have been implemented without one, and in fact many other dynamic languages do not have this.

Therefore, giving the JVM a heap size limit was simply a design decision by the implementors. Second-guessing why this was done is a bit difficult, and there may not be a single reason. The most likely reason is that it helps protect a system from a Java program with a memory leak, which might otherwise exhaust all RAM and cause other apps to crash or the system to thrash.

Sun could have omitted the feature and simply told people to use the OS-native resource limiting mechanisms, but they probably wanted to always have a limit, so they implemented it themselves. At any rate, the JVM needs to be aware of any such limit (to adapt its GC strategy), so using an OS-native mechanism would not have saved much programming effort.

Also, there is one reason why such a built-in limit is more important for the JVM than for a "normal" program without GC (such as a C/C++ program):

Unlike a program with manual memory management, a program using GC does not really have a well-defined memory requirement, even with fixed input data. It only has a minimum requirement, i.e. the sum of the sizes of all objects that are actually live (reachable) at a given point in time. However, in practice a program will need additional memory to hold dead, but not yet GCed objects, because the GC cannot collect every object right away, as that would cause too much GC overhead. So GC only kicks in from time to time, and therefore some "breathing room" is required on the heap, where dead objects can await the GC.

This means that the memory required for a program using GC is really a compromise between saving memory and having good througput (by letting the GC run less often). So in some cases it may make sense to set the heap limit lower than what the JVM would use if it could, so save RAM at the expense of performance. To do this, there needs to be a way to set a heap limit.

Answer 2

It is due to the design of the JVM. Other JVM's (like the one from Microsoft and some IBM ones) can use all the memory available in the system if needed, without an arbitrary limit.

I believe it allows for GC-optimizations.