Why does a GraalVM (SubstrateVM) native image uses so much less memory at runtime than a corresponding JIT build?

Question

I'm wondering why a GraalVM (SubstrateVM) native image of a Java application makes it run where the runtime behavior will consume much less memory, yet if run normally, it will consume a lot more memory?

And why can't the normal JIT be made to similarly consume a small amount of memory?

Answer 1

GraalVM native images don't include the JIT compiler or the related infrastructure -- so there's no need to allocate memory for JIT, for the internal representation of the program to JIT it (for example a control flow graph), no need to store some of the class metadata, etc.

So it's unlikely that a JIT which actually does useful work can be implemented with the same zero overhead.

It could be possible to create an economic implementation of the virtual machine that will perhaps use less memory than HotSpot. Especially if you only want to measure the default configuration without comparing the setups where you control the amounts of memory the JVM is allowed to use. However, one needs to realize that it'll either be an incremental improvement on the existing implementations or picking a different option for some trade-off, because the existing JVM implementations are actually really-really good.