ZGC is a new garbage collector that can handle massive heap sizes, up to 16 terabytes in size, without an increase in pause time. For big data applications that may be processing massive amounts of data in-memory, keeping garbage collection pauses down below 10ms means that processing speed on large data sets won’t be stalled out by limitations in the language. 

Stdlib support for a global Trace trait that everyone derives would be awesome.

A lot of people seem to think that heap allocation is expensive and stack allocation is cheap. They are actually about the same, typically. It’s the deallocation costs – the marking and sweeping and compacting and moving memory from generation to generation – that are massive for heap memory compared to stack memory.

This sketch is complicated by the fact that there are actually three such arenas; the CLR collector is generational. Objects start off in the “short lived” heap. If they survive they eventually move to the “medium lived” heap, and if they survive there long enough, they move to the “long lived” heap. The GC runs very often on the short lived heap and very seldom on the long lived heap; the idea is that we do not want to have the expense of constantly re-checking a long-lived object to see if it is still alive. But we also want short-lived objects to be reclaimed swiftly.

When a garbage collection is performed there are three phases: mark, sweep and compact. In the “mark” phase, we assume that everything in the heap is “dead”. The CLR knows what objects were “guaranteed alive” when the collection started, so those guys are marked as alive. Everything they refer to is marked as alive, and so on, until the transitive closure of live objects are all marked. In the “sweep” phase, all the dead objects are turned into holes. In the “compact” phase, the block is reorganized so that it is one contiguous block of live memory, free of holes.