Short-Term Memory

Abstract

We propose a new memory model called short-term memory for managing short-living objects on the heap. In contrast to the traditional persistent memory model for heap management, objects in short-term memory expire after a finite amount of time, which makes deallocation unnecessary. Instead, expiration of objects may be extended, if necessary, by refreshing. We have developed a concurrent, incremental, and non-moving implementation of short-term memory for explicit refreshing called self-collecting mutators that is based on programmer-controlled time and integrated into state-of-the-art runtimes of three programming languages: C, Java, and Go. All memory management operations are lock-free and run in constant time modulo the underlying allocators. Our implementation does not require any additional heap management threads, hence the name. Expired objects may be collected anywhere between one at a time for maximal incrementality and all at once for maximal throughput and minimal memory consumption. The integrated systems are heap management hybrids with persistent memory as default and short-term memory as option. Our approach is fully backwards compatible. Legacy code runs without any modifications with negligible runtime overhead and constant per-object space overhead. Legacy code can be modified to take advantage of short-term memory by having some but not all objects allocated in short-term memory and managed by explicit refreshing. We study single- and multi-threaded use cases in all three languages macro-benchmarking C and Java and micro-benchmarking Go. Our results show that using short-term memory (1) simplifies heap management in a state-of-the-art H.264 encoder written in C without additional time and minor space overhead, and (2) improves, at the expense of safety, memory management throughput, latency, and space consumption by reducing the number of garbage collection runs, often even to zero, for a number of Java and Go programs. More information can be found in the technical report and the ISMM 2011 paper.


Figure 1 object lifetime in short-term memory: every object has an expiration date. The expiration date of an object can be extended by refreshing


Figure 2 memory consumption over time: The C-implementation of self-collecting mutators vs. the ptmalloc2 memory allocator executing the mpg123 mp3 converter. Memory consumption is measured before and after memory allocations. The vertical lines indicate time advance


Figure 3 total runtime benchmark: The Java-implementation of self-collecting mutators vs. the mark-sweep garbage collector and the generational garbage collector of Jikes RVM




Figure 4 and 5 latency and memory consumption: The Java- and Go-implementation of self-collecting mutators vs. the mark-sweep garbage collector and the generational garbage collector of Jikes RVM, and the mark-sweep garbage collector of Go

Publications

Contact: Andreas Haas

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