I want memory allocation to run in constant time, so I can achieve low latency. That's the point of concurrent GCs - they run in the background to free up memory.

Releasing memory is definitely useful for mobile devices or browsers. Even in some server use cases it's useful, e.g. when you pay for memory usage. I guess that's why the JVM has this -XX:SoftMaxHeapSize option.

I guess it depends whether GCs are always scheduled in an allocation or can be triggered another way. Either way that should be easy to disable.

I read somewhere that D doesn't have write barriers, so I would assume they have a hard time implementing more advanced GC features like generational collection or concurrent marking. It's not suprising that the GCs in the JVM achieve much better pause time.

In Java, everything is heap allocated, whereas in D you have a choice.