A Compacting Garbage Collector for Erlang

By utilising the fact that all data in the heap in an Erlang implementation may be arranged so that all pointers go towards earlier allocated objects, a compacting garbage collection algorithm has been designed. In contrast to a copying garbage collection algorithm, only one memory space is used. In fact, no extra memory is used at all, not even any reserved bits within the cells. The algorithm is quite easy to extend to a variant of generational garbage collection. Introduction The Erlang functional programming language [Arm96] is typically implemented using a copying garbage collection algorithm. In [Arm95] a completely new method was suggested utilising the fact that objects on the heap may be allocated so that each object only points to earlier allocated ones. In order to be able to deallocate data in the middle of the heap, without disturbing the order of objects, all objects were linked, each with a ‘history cell’ showing the time of creation. Here we propose an alternative method to utilise unidirectionality of the heap which uses a quite standard representation of the heap, not using linked lists or history cells. One main motivation to develop the algorithm was to be able to design a garbage collector for an Erlang engine that could run on systems with more limited memory resources. The algorithm presented has the following features: • It does not copy data to a new heap, thereby minimising memory fragmentation. • No extra memory is used, not even mark-bits or extra tags. • The order of the objects on the heap is preserved. • It is simple enough to make room for low level optimisations. • Execution time is linear proportional to the size of the data areas. • A version of generational garbage collection is simple to implement. • Overlapping objects are handled. The algorithm has two phases: 1. Marking, pointer updating and compaction, all combined. 2. Sliding the heap and updating external pointers. A Compacting Garbage Collector for Erlang Kent Boortz and Dan Sahlin