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Distributed Garbage Collection Essay

3065 words - 13 pages

Distributed Garbage Collection
A study and comparison of Mark-Sweep and Reference Counting

Tanmay Mogra Y7471 Shashwat Mishra Y7416

Table of Contents
History and Motivation 1 ------------------------------------------------

Work Completed Mark and Sweep ------------------------------------------------2 Basic Algorithm ------------------------------------------------2 Strengths and Weaknesses ------------------------------------------------2 Varations of Mark and Sweep -------------------------------------------------2 Reference Counting Basic Algorithm Strengths and Weaknesses 3 3 Mark and Sweep vs. Reference Counting Design/Implementation A Simulation of Reference Counting using ...view middle of the document...

As modern software development involves large, complex artefacts of software, often developed by a large team of developers, no one person knows the implementation details of the entire system. It is thus very difficult for any developer to manage memory manually. Manual allocation and de-allocation is prone to errors that cause difficult-to-debug problems. This issue is made more prominent in distributed systems where there are many of threads of control, many processes interacting and often components developed by many different parties. Explicit management is out of the question in such situations. This calls for automatic memory management schemes. In distributed systems, garbage collection becomes more difficult because of the very concurrent nature of a distributed system. Since each node has one or more processes, there are very many mutators constantly changing the reference graph of objects in memory. For an object to be able to explicitly collect another object’s memory, it would have to know that the particular object was not going to be referred to by all the processes in the system. In context of the distributed garbage collection, the part which receives request from the application program and handles allocation of free cells of the heap memory is known as Allocator. The part which handles reclaimation of memory is known as the Collector. The application program itself is known as the Mutator because it changes the connectivity of the graph of the heap data structures by requesting creation/deletion of references. Some locations in the heap are known as root. A root is a location in the memory which is always accessible to the program. For example in a java program class variables can play the role of root. Any location in the memory is termed as reachable if it is reachable from root through one or more references. The heart of the problem lies in reclaiming the nonreachable memory. Classically, there are three broad algorithms for handling distributed garbage collection. Reference counting ( which works by maintaing a count of references to a particular object ), Mark and Sweep (which works in phases to reclaim memory) and Copy Collection (like Mark and Sweep with the advantage that it incorporates defragmentation into reclaimation).

Mark and Sweep
Basic Algorithm :Presented in [McCarthy, 1960] for the LISP language, this was the first algorithm for automatic heap management. Also known as tracing collector it works in two phases. All the objects have a mark-bit which is initially set to 0. Phases :1. Garbage Detection: This is done by tracing, starting at the root set and traversing the graph of pointer relationships using BFS or DFS. The mark-bits in the objects that are reached are set to 1. 2. Garbage Reclamation: Once all the live objects have been made distinguishable from the garbage, memory is swept. It is exhaustively examined (all cells) to find all the unmarked objects and reclaim them. All the marked objects...

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