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

Asked 2010-07-28T23:20:49.927
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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.

The default is 1/4 of your physical RAM (with upper & lower limit); as a consequence, if you have a memory-hungry application you have to manually change the limit (parameter -Xmx), or your app will perform poorly, possible even crash with an OutOfMemoryError.

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

This would solve a whole class of common problems with Java software (just Google to see how many hints there are on the net on solving problems by setting -Xmx).

Edit:

Some answers point out that this will protect the rest of the system from a Java program with a run-away memory leak; without the limit this would bring the whole system down by exhausting all memory. This is true. However, it is equally true for any other program, and modern OSes already let you limit the maximum memory for a programm (Linux ulimit, Windows "Job Objects"). So this does not really answer the question, which is "Why does the JVM do it differently from most other programs / runtime environments?".

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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.

answered 2010-07-28T23:30:08.330

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