JVM Anatomy Quarks: Compressed References

Re:JVM Anatomy Quarks: Compressed References by zamalek on Wednesday March 06, 2019 @01:00AM (#19316684)

> Once the heap size gets larger than the threshold under which compressed references are working, a surprising thing happens: references suddenly become uncompressed

Am I correct in assuming that this is made possible because Hotspot can re-JIT? So the GC/Allocator just warns the JITter, "hey, all of your Oops [flags] code is no longer valid," resulting in a re-JIT of everything? Or a prologue in every method that checks the Oops flags (and triggers a re-JIT)?

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Re:JVM Anatomy Quarks: Compressed References by deadc0de on Wednesday March 06, 2019 @01:02AM (#19316692)

No, compressed oops mode doesn't change at runtime. That's pretty hard to pull off. You'd need to change object layout dynamically and unless you want to reformat the whole heap you'd need tags or regions with different pointer rules. Any of these techniques would have a very substantial runtime overhead.

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Re:JVM Anatomy Quarks: Compressed References by xxs on Wednesday March 06, 2019 @04:51AM (#19317642)

nope, CompressedOps is an old technique that depends on the max heap. less than 32GB, 64bit -> compressedOps

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