And you can do it because someone found that it was necessary to do.
It's crazy I can't tell Java and NodeJS "use the memory you need". Instead I have to specify max memory sizes (and then watch as they inevitably consume all of it).
Of course they will consume all of it. That's how GCs typically work. They won't invoke a collection until there's no space left to allocate. Just because they "use all of it" doesn't mean all of that memory is actually live. It just hasn't done a collection yet.
Tracing garbage collectors have to have some metric for when to trigger a stop-the-world collection. IIRC, many GCs track heap usage and trigger stop-the-world when it gets above a threshold. GC'd language runtimes which don't have a tracing/compacting collector (e.g. CPython with refcounting) don't need to configure a heap size because the behavior doesn't vary as you use up your heap.
I wonder, could we just tell each JVM instance that it may use all of the memory on the system, and then let the OS kill the first VM that allocates more than the system has to offer? Would this get us the same semantics as those of a native application? Or does the JVM preallocate all of the memory that it is allowed to use?
The JVM allocates at start a portion, takes what it needs whenever it needs until the max, but never releases memory back to the OS. So in a given moment a 6GB vm is a 5GB process but internally is using just 3GB.
As of one of the most recent Java releases, I believe the G1 GC does this on Windows. G1 is not the default but can be selected with a single command line flag.
It's crazy I can't tell Java and NodeJS "use the memory you need". Instead I have to specify max memory sizes (and then watch as they inevitably consume all of it).