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

Why does this fixed limit even exist? Why does the JVM not allocate memory as needed, like native programs do on most operating systems?

The reason is NOT that the GC needs to know before hand what the maximum heap size can be. The JVM is clearly capable of expanding its heap ... up to the maximum ... and I'm sure it would be a relatively small change to remove that maximum. (After all, other Java implementations do this.) And it would equally be possible to have a simple way to say "use as much memory as you like" to the JVM.

I'm sure that the real reason is to protect the host operating system against the effects of faulty Java applications using all available memory. Running with an unbounded heap is potentially dangerous.

Basically, many operating systems (e.g. Windows, Linux) suffer serious performance degradation if some application tries to use all available memory. On Linux for example, the system may thrash badly, resulting in everything on the system running incredibly slowly. In the worst case, the system won't be able to start new processes, and existing processes may start crashing when the operating system refuses their (legitimate) requests for more memory. Often, the only option is to reboot.

If the JVM ran with an unbounded heap by default, any time someone ran a Java program with a storage leak ... or that simply tried to use too much memory ... they would risk bringing down the entire operating system.

In summary, having a default heap bound is a good thing because:

• it protects the health of your system,
• it encourages developers / users to think about memory usage by "hungry" applications, and
• it potentially allows GC optimizations. (As suggested by other answers: it is plausible, but I cannot confirm this.)

EDIT

In response to the comments:

• It doesn't really matter why Sun's JVMs live within a bounded heap, where other applications don't. They do, and advantages of doing so are (IMO) clear. Perhaps a more interesting question is why other managed languages don't put a bound on their heaps by default.

• The -Xmx and ulimit approaches are qualitatively different. In the former case, the JVM has full knowledge of the limits it is running under and gets a chance to manage its memory usage accordingly. In the latter case, the first thing a typical C application knows about it is when a malloc call fails. The typical response is to exit with an error code (if the program checks the malloc result), or die with a segmentation fault. OK, a C application could in theory keep track of how much memory it has used, and try to respond to an impending memory crisis. But it would be hard work.

• The other thing that is different about Java and C/C++ applications is that the former tend to be both more complicated and longer running. In practice, this means that Java applications are more likely to suffer from slow leaks. In the C/C++ case, the fact that memory management is harder means that developers don't attempt to build single applications of that complexity. Rather, they are more likely to build (say) a complex service by having a listener process fork of child processes to do stuff ... and then exit. This naturally mitigates the effect of memory leaks in the child process.

• The idea of a JVM responding "adaptively" to requests from the OS to give memory back is interesting. But there is a BIG problem. In order to give a segment of memory back, the JVM first has to clear out any reachable objects in the segment. Typically that means running the garbage collector. But running the garbage collector is the last thing you want to do if the system is in a memory crisis ... because it is pretty much guaranteed to generate a burst of virtual memory paging.

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.

I think part of it has to do with the implementation of the Garbage Collector (GC). The GC is typically lazy, meaning it will only start really trying to reclaim memory internally when the heap is at its maximum size. If you didn't set an upper limit, the runtime would happily continue to inflate until it used every available bit of memory on your system.

That's because from the application's perspective, it's more performant to take more resources than exert effort to use the resources you already have to full utilization. This tends to make sense for a lot of (if not most) uses of Java, which is a server setting where the application is literally the only thing that matters on the server. It tends to be slightly less ideal when you're trying to implement a client in Java, which will run amongst dozens of other applications at the same time.

Remember that with native programs, the programmer typically requests but also explicitly cleans up resources. That isn't typically true with environments who do automatic memory management.