> E.g. Haskell has an awesome concurrent GC that’d work like crap for Java, because it assumes tons of really short-lived, really small garbage and almost no mutation. The other way around is also bound to be problematic—I don’t know how the Scala people do it
I don't know a ton about Haskell's GC, but at surface level it seems very similar to several of the JVM GC implementations - a generational GC with a concept of a nursery. Java GC is very heavily designed around the weak generational hypothesis (ie, most objects don't live long) and very much optimizes for short-lived object lifecycles, so most GC implementations have at least a few nursery-type areas before anything gets to the main heap where GC is incredibly cheap, plus some stuff ends up getting allocated on the stack in some cases.
The only big difference is that in Haskell there are probably some optimizations you can do if most of your structures are immutable since nothing in an older generation can refer to something in the nursery. But it isn't super clear to me that alone makes a big enough difference?
One major simplification you can make is that due to purity, older values _never_ point to newer values. This means when doing generational GC, you don’t have to check for pointers from older generations into newer generations.
This feels wrong. Specifically, doesn't laziness bite you in this scenario? If I make a stream that is realized over GC runs, I would expect that a node in an old generation could point to a realized data element from a newer generation. Why not?
> E.g. Haskell has an awesome concurrent GC that’d work like crap for Java, because it assumes tons of really short-lived, really small garbage and almost no mutation. The other way around is also bound to be problematic—I don’t know how the Scala people do it
I don't know a ton about Haskell's GC, but at surface level it seems very similar to several of the JVM GC implementations - a generational GC with a concept of a nursery. Java GC is very heavily designed around the weak generational hypothesis (ie, most objects don't live long) and very much optimizes for short-lived object lifecycles, so most GC implementations have at least a few nursery-type areas before anything gets to the main heap where GC is incredibly cheap, plus some stuff ends up getting allocated on the stack in some cases.
The only big difference is that in Haskell there are probably some optimizations you can do if most of your structures are immutable since nothing in an older generation can refer to something in the nursery. But it isn't super clear to me that alone makes a big enough difference?