Tuning Garbage Collection

Tuning Garbage Collection with the 5.0 Java[tm] Virtual Machine
The Java TM 2 Platform Standard Edition (J2SE TM platform) is used for a wide variety of applications from small applets on desktopsto web services on large servers. In the J2SE platform version 1.4.2 there were four garbage collectors from which to choose but without an explicit choice by the user the serial garbage collector wasalways chosen. In version 5.0 the choice of the collector is based on the class of the machine on which the application is started.
This “smarter choice” of the garbage collector is generally betterbut is not always the best. For the user who wants to make their own choice of garbage collectors, this document will provide information on which to base that choice. This will first include thegeneral features of the garbage collections and tuning options to take the best advantage of those features. The examples are given in the context of the serial, stop-the-world collector. Then specificfeatures of the other collectors will be discussed along with factors that should considered when choosing one of the other collectors.
When does the choice of a garbage collector matter to the user?For many applications it doesn't. That is, the application can perform within its specifications in the presence of garbage collection with pauses of modest frequency and duration. An example wherethis is not the case (when the serial collector is used) would be a large application that scales well to large number of threads, processors, sockets, and a large amount of memory.
Amdahl observed thatmost workloads cannot be perfectly parallelized; some portion is always sequential and does not benefit from parallelism. This is also true for the J2SE platform. In particular, virtual machines forthe Java TM platform up to and including version 1.3.1 do not have parallel garbage collection, so the impact of garbage collection on a multiprocessor system grows relative to an otherwise parallel...