Thanks for the answer. But is this actual behaviour for the GCs of the JDK? I was certain that at the very least Hotspot makes use of stack allocation as much as possible.

But perhaps the JDK GCs don't care so much about the stack because that is already dealt by the JVM a step prior? In any case, there will likely still be young objects allocated in the heap and this new algorithm might prove useful.

But you can tell I am far from an expert here.

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masklinn 10 months ago | parent [-]

> Thanks for the answer. But is this actual behaviour for the GCs of the JDK? I was certain that at the very least Hotspot makes use of stack allocation as much as possible.

Not really, java has some escape analysis but it's very limited in its ability to stack allocate as it can't put entire structures (objects) on the stack, it only works if the compiler manages to scalar-replace the object (https://shipilev.net/jvm/anatomy-quarks/18-scalar-replacemen...) and that has somewhat restricted applicability (https://pkolaczk.github.io/overhead-of-optional/). The behaviour I'm talking about is mostly that of Go, as it is much more capable of stack allocating, and specifically has a non-generational GC because in testing they found generational GCs had a very variable impact depending on workload (rather than a universally or near universally positive impact).

	▲	jerven 10 months ago \| parent \| next [-]
		There was a microsoft prototype for more stack allocation in OpenJDK (https://archive.fosdem.org/2020/schedule/event/reducing_gc_t...). I recall that being put on hold because of how it would interact with project Loom fast stack copying. But I don't know the current status. GO has a non moving GC and I understand, that the cost of introducing safe moving GC is considered high. If one has a moving GC which the serious java one's are read/write barriers are already required, especially if they are concurrent like ZGC, C4 or Shenadoah. ZGc, C4 and Shenadoah all started out as non generational GC implementations, which gained them later, because in most cases they do increase performance/reduce overhead. Valhalla makes objects denser, and reduces overhead of identity which is great. Reducing the difference in memory layout between java objects and nested go structs. Go with arena's reduce the GC de-allocation costs. Something that the ZGC team is looking at in relation to loom/virtual threads. (but I can't find the reference for that right now)
	▲	DarkNova6 10 months ago \| parent \| prev [-]
		Thank you for these excellent sources!