Cyclic Garbage Research Articles

Collecting Cyclic Garbage across Foreign Function Interfaces: Who Takes the Last Piece of Cake?

A growing number of libraries written in managed languages, such as Python and JavaScript, are bringing about new demand for a foreign language interface (FFI) between two managed languages. Such an FFI allows a host-language program to seamlessly call a library function written in a foreign language and exchange objects. It is often implemented by a user-level library but such implementation cannot reclaim cyclic garbage, or a group of objects with circular references, across the language boundary. This paper proposes Refgraph GC , which enables FFI implementation that can reclaim cyclic garbage. Refgraph GC coordinates the garbage collectors of two languages and it needs to modify the managed runtime of one language only. It does not modify that of the other language. This paper discusses the soundness and completeness of the proposed algorithm and also shows the results of the experiments with our implementation of FFI with Refgraph GC. This FFI allows a Ruby program to access a JavaScript library.

A simple and efficient algorithm for cycle collection

The lack of collecting cyclic garbage is generally considered the major weakness of reference counting. Reference counted systems handle this problem by incorporating either a global tracing collector, or a "partial" tracing collector that considers only the cycle candidates but needs several traces on them. In particular, the latter has become a preferred one as it has better scalability and locality (no need to scan the entire heap). This paper presents a new "lightweight" cyclic reference counting algorithm, which is based on partial tracing and deals with the cycle problem in a simpler and more efficient way. By exploiting the lightweight hypothesis that considers a single sub-graph, instead of individual cycles, as the basic unit of cycle collection, the algorithm can detect garbage cycles in a single trace. In addition, we propose a technique for eliminating redundant scans over garbage objects, thus improving the efficiency of the algorithm. The pseudocode and its correctness proof are also presented. Finally, an implementation based on Jikes Research Virtual Machine is provided to demonstrate the effectiveness of the new algorithm.

Complete distributed garbage collection using DGC-consistent cuts and .NET AOP-support

The memory management of distributed objects, when done manually, is an error-prone task. It leads to memory leaks and dangling references, causing applications to fail. Avoiding such errors requires automatic memory management, called distributed garbage collection (DGC). Current DGC solutions are either not safe, not complete or not portable to widely used platforms such as .NET. As a matter of fact, most solutions either run on specialised environments or require modifications of the underlying virtual machine (e.g. rotor, common language runtime (CLR)), hin- dering its immediate and widespread utilisation. This study describes the design, architecture, implementation and performance measurements of a DGC algorithm for .NET that: (i) is complete, that is, capable of reclaiming both acyclic and cyclic garbage, while (ii) being portable in the sense that it neither requires the underlying virtual machine to be modified, nor source or byte-code modi- fication. The distributed garbage collector was implemented on top of two implementations of the common language infrastructure (.NET virtual machine specification): CLR and shared source CLI, commonly known as Rotor. The implementation requires no modification of the environment, it makes use of the provided aspect-oriented functionalities, and the performance results are encouraging.

Garbage collection in object-oriented databases using transactional cyclic reference counting

Garbage collection is important in object-oriented databases to free the programmer from explicitly deallocating memory. In this paper, we present a garbage collection algorithm, called Transactional Cyclic Reference Counting (TCRC), for object-oriented databases. The algorithm is based on a variant of a reference-counting algorithm proposed for functional programming languages The algorithm keeps track of auxiliary reference count information to detect and collect cyclic garbage. The algorithm works correctly in the presence of concurrently running transactions, and system failures. It does not obtain any long-term locks, thereby minimizing interference with transaction processing. It uses recovery subsystem logs to detect pointer updates; thus, existing code need not be rewritten. Finally, it exploits schema information, if available, to reduce costs. We have implemented the TCRC algorithm and present results of a performance study of the implementation.

An implementation of complete, asynchronous, distributed garbage collection

Most existing reference-based distributed object systems include some kind of acyclic garbage collection, but fail to provide acceptable collection of cyclic garbage. Those that do provide such GC currently suffer from one or more problems: synchronous operation, the need for expensive global consensus or termination algorithms, susceptibility to communication problems, or an algorithm that does not scale. We present a simple, complete, fault-tolerant, asynchronous extension to the (acyclic) cleanup protocol of the SSP Chains system. This extension is scalable, consumes few resources, and could easily be adapted to work in other reference-based distributed object systems---rendering them usable for very large-scale applications.

Formal models of distributed memory management

We develop am abstract model of memory management in distributed systems. The model is low-level enough so that we can express communication, allocation and garbage collection, but otherwise hides many of the lower-level details of an actual implementation.Recently, such formal models have been developed for memory management in a functional, sequential setting [10]. The models are rewriting systems whose terms are programs. Programs have both the "code" (control string) and the "store" syntactically apparent. Evaluation is expressed as conditional rewriting and includes store operations. Garbage collection becomes a rewriting relation that removes part of the store without affecting the behavior of the program.Distribution adds another dimension to an already complex problem. By using techniques developed for communicating and concurrent systems [9], we extend their work to a distributed environment. Sending and receiving messages is also made apparent at the syntactic level. A very general garbage collection rule based on reachability is introduced and proved correct. Now proving correct a specific collection strategy is reduced to showing that the relation between programs defined by the strategy is a sub-relation of the general relation. Any actual implementation which is capable of providing the transitions (including their atomicity constraints) specified by the strategy is therefore correct.This model allows us to specify and prove correct in a compact manner two garbage collectors; the first one does a simple garbage collection local to a node. The second garbage collector uses migration of data in order to be able to reclaim inter-node cyclic garbage.

Partitioned garbage collection of a large object store

We present new techniques for efficient garbage collection in a large persistent object store. The store is divided into partitions that are collected independently using information about inter-partition references. This information is maintained on disk so that it can be recovered after a crash. We use new techniques to organize and update this information while avoiding disk accesses. We also present a new global marking scheme to collect cyclic garbage across partitions. Global marking is piggybacked on partitioned collection; the result is an efficient scheme that preserves the localized nature of partitioned collection, yet is able to collect all garbage. We have implemented the part of garbage collection responsible for maintaining information about inter-partition references. We present a performance study to evaluate this work; the results show that our techniques result in substantial savings in the usage of disk and memory.

Collecting cyclic distributed garbage by controlled migration

Distributed systems with a large number of nodes use internode reference counting for timely and fault-tolerant garbage collection. However, this fails to collect cyclic garbage distributed across nodes. One fix is to migrate all objects on a garbage cycle to a single node, where they can be collected by the tracing-based local collector. Existing proposals based on this technique have practical problems due to unnecessary migration of objects. We propose a scheme that avoids migration of live objects, batches objects to avoid a cascade of migration messages, and short-cuts the migration path to avoid multiple migrations. We use simple estimates to detect objects that are highly likely to be cyclic garbage and to select a node to which such objects are migrated. The scheme collects all distributed cyclic garbage, has low overhead, and preserves the decentralized and fault-tolerant nature of distributed reference counting and migration.