Speaking of the OCaml GC in general, wouldn't it make sense to replace the current generational collector with a collector framework that requires less copying in the common case. For example, dividing the heap into two parts, "large object space" and "small object space", where LOS is managed by mark-sweep/mark-compact (could use the current major heap) and SOS is managed by a recent mark-copy/mark-region collector (Immix [1] comes to mind here). That way objects would no longer need to be copied between generations, and using Immix may also reduce cache misses and improve locality of small/medium sized objects (with proper evacuation heuristics). This should be doable with a moderate amount of work, and should require no incompatible changes, while opening the door for concurrent/parallel garbage collection (this would require incompatible changes then, replacing caml_young_ptr/caml_young_limit with TLS vars, etc.).

Benedikt

[1] http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.144.3640