Immix Garbage Collection: Fast Collection, Space Efficiency, and Mutator Locality

No current garbage collector combines space efficiency, low collector times, and mutator (application) locality for contemporaneously allocated objects. For example, free-list allocation coupled with mark-sweep collection achieves space efficiency, but poor mutator locality. On the other hand, contiguous (bump-pointer) allocation with copying collection provides mutator locality but poor space efficiency since it requires reserving half the heap for copying in case all objects survive. This paper presents an algorithm that obtains space efficiency, low collection times, and mutator locality in a single regime. We call this garbage collector immix, which means to intimately commingle. The immix heap is organized hierarchically into blocks of contiguous memory, each consisting of N fixed size lines. Immix tracks memory use at a line granularity. Immix bump-allocates objects into one or more contiguous free lines to attain mutator locality. To attain space efficiency, immix uses mark-sweep collection to identify free lines. When needed to eliminate fragmentation, immix mixes copying and mark-sweep collection in a unique way to implement opportunistic single pass compaction. We demonstrate immix as a whole heap collector, as a component of a generational collector, and as a mostly non-moving collector. Our experimental results demonstrate that immix comprehensively and uniformly outperforms state-of-the-art whole heap mark-sweep, mark-compact, and semi-space collectors on a range of heap sizes on multiple architectures. Immix is the first collector to attain simultaneously space efficiency, low garbage collection times, and mutator locality.