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dmichulke

Most useful info:

If you run the 64 bit JRE and have memory problems, use the flags

- Xcompressedrefs

- XX:+UseCompressedOops

for a decent 35-45% reduction in memory use.

The rest is well-known stuff (use primitives, use arrays with fixed size, ...)

Still, the numbers are quite interesting (and the overhead quite scary)

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On hotspot ergonomics automatically turn those options on as long as your max heap is < 32GiB.

    $ java -Xmx31G -XX:+PrintFlagsFinal 2>&1 | grep UseCompressed
         bool UseCompressedClassPointers               := true                                {lp64_product}
         bool UseCompressedOops                        := true                                {lp64_product}

They also can't be enabled with >32GiB heaps.

They can if you increase the object alignment, but that's generally not advisable and should only be done when measurements show a benefit.

I think it depends on the JVM. You can use compressed refs on JRokit with > 32G heaps. Its a bit of "bit twiddling." This article explains it pretty well:

https://blogs.oracle.com/jrockit/entry/understanding_compres...

This article is super old. Those options are now default, and the overhead is much smaller. (I think even in 2012, we already had 1-word headers.)

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