What do you mean - if Java returns memory to the OS? Which one - Java heap of the malloc/free by the JVM?

Java is pretty greedy with the memory it claims. Especially historically it was pretty hard to get the JVM to release memory back to the OS.

To an outsider, that looks like the JVM heap just steadily growing, which is easy to mistake for a memory leak.

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k_roy 6 hours ago | parent | next [-]

> Especially historically it was pretty hard to get the JVM to release memory back to the OS.

This feels like a huge understatement. I still have some PTSD around when I did Java professionally between like 2005 and 2014.

The early part of that was particularly horrible.

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xxs 6 hours ago | parent | prev | next [-]

Java has a quite strict max heap setting, it's very uncommon to let it allocate up to 25% of the system memory (the default). It won't grow past that point, though.

Baring bugs/native leaks - Java has a very predictable memory allocation.

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adgjlsfhk1 6 hours ago | parent | prev [-]

This only really ends up being a problem on windows. On systems with proper virtual memory setups, the cost of unused memory is very low (since the the OS can just page it out)

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cogman10 3 hours ago | parent | next [-]

Unfortunately, the JVM and collectors like the JVM's plays really bad with virtual memory. (Actually, G1 might play better. Everything else does not).

The issue is that through the standard course of a JVM application running, every allocated page will ultimately be touched. The JVM fills up new gen, runs a minor collection, moves old objects to old gen, and continues until old gen gets filled. When old gen is filled, a major collection is triggered and all the live objects get moved around in memory.

This natural action of the JVM means you'll see a sawtooth of used memory in a properly running JVM where the peak of the sawtooth occasionally hits the memory maximum, which in turn causes the used memory to plummet.

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snackbroken 3 hours ago | parent | prev [-]

For video games it is pretty bad, because reading back a page from disk containing "freed" (from the application perspective, but not returned to the OS) junk you don't care about is significantly slower than the OS just handing you a fresh one. A 10-20ms delay is a noticeable stutter and even on an SSD that's only a handful of round-trips.

	▲	cogman10 3 hours ago \| parent [-]
		Games today should be using ZGC. There's a lot of bad tuning guides for minecraft that should be completely ignored and thrown in the trash. The only GC setting you need for it is `-XX:+UseZGC` For example, a number of the minecraft golden guides I've seen will suggest things like setting pause targets but also survivor space sizes. The thing is, the pause target is disabled when you start playing with survivor space sizes.