Improving program locality in the GC using hotness.

The hierarchical memory system with increasingly small and increasingly fast memory closer to the CPU has for long been at the heart of hiding, or mitigating the performance gap between memories and processors. To utilise this hardware, programs must be written to exhibit good object locality. In languages like C/C++, programmers can carefully plan how objects should be laid out (albeit time consuming and error-prone); for managed languages, especially ones with moving garbage collectors, a manually created optimal layout may be destroyed in the process of object relocation. For managed languages that present an abstract view of memory, the solution lies in making the garbage collector aware of object locality, and strive to achieve and maintain good locality, even in the face of multi-phased programs that exhibit different behaviour across different phases. This paper presents a GC design that dynamically reorganises objects in the order mutators access them, and additionally strives to separate frequently and infrequently used objects in memory. This improves locality and the efficiency of hardware prefetching. Identifying frequently used objects is done at run-time, with small overhead. HCSGC also offers tunability, for shifting relocation work towards mutators, or for more or less aggressive object relocation. The ideas are evaluated in the context of the ZGC collector on OpenJDK and yields performance improvements of 5% (tradebeans), 9% (h2) and an impressive 25–45% (JGraphT), all with 95% confidence. For SPECjbb, results are inconclusive due to a fluctuating baseline.