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JVM performance optimization, Part 3: Garbage collection

Choose the right garbage collector for your application needs

By Eva Andreasson, JavaWorld.com, 10/10/12

Page 4 of 8

Tracing collector algorithms

Copying and mark-and-sweep garbage collection are not new, but they're still the two most common algorithms that implement tracing garbage collection today.

Copying collectors

Traditional copying collectors use a from-space and a to-space -- that is, two separately defined address spaces of the heap. At the point of garbage collection, the live objects within the area defined as from-space are copied into the next available space within the area defined as to-space. When all the live objects within the from-space are moved out, the entire from-space can be reclaimed. When allocation begins again it starts from the first free location in the to-space.

In older implementations of this algorithm the from-space and to-space switch places, meaning that when the to-space is full, garbage collection is triggered again and the to-space becomes the from-space, as shown in Figure 1.

Figure 1. A traditional copying garbage collection sequence (click to enlarge)

More modern implementations of the copying algorithm allow for arbitrary address spaces within the heap to be assigned as to-space and from-space. In these cases they do not necessarily have to switch location with each other; rather, each becomes another address space within the heap.

One advantage of copying collectors is that objects are allocated together tightly in the to-space, completely eliminating fragmentation. Fragmentation is a common issue that other garbage collection algorithms struggle with; something I'll discuss later in this article.

Downsides of copying collectors

Copying collectors are usually stop-the-world collectors, meaning that no application work can be executed for as long as the garbage collection is in cycle. In a stop-the-world implementation, the larger the area you need to copy, the higher the impact on your application performance will be. This is a disadvantage for applications that are sensitive to response time. With a copying collector you also need to consider the worst-case scenario, when everything is live in the from-space. You always have to leave enough headroom for live objects to be moved, which means the to-space must be large enough to host everything in the from-space. The copying algorithm is slightly memory inefficient due to this constraint.

Mark-and-sweep collectors

Most commercial JVMs deployed in enterprise production environments run mark-and-sweep (or marking) collectors, which do not have the performance impact that copying collectors do. Some of the most famous marking collectors are CMS, G1, GenPar, and DeterministicGC (see Resources).

A mark-and-sweep collector traces references and marks each found object with a "live" bit. Usually a set bit corresponds to an address or in some cases a set of addresses on the heap. The live bit can, for instance, be stored as a bit in the object header, a bit vector, or a bit map.

After everything has been marked live, the sweep phase will kick in. If a collector has a sweep phase it basically includes some mechanism for traversing the heap again (not just the live set but the entire heap length) to locate all the non-marked chunks of consecutive memory address spaces. Unmarked memory is free and reclaimable. The collector then links together these unmarked chunks into organized free lists. There can be various free lists in a garbage collector -- usually organized by chunk sizes. Some JVMs (such as JRockit Real Time) implement collectors with heuristics that dynamically size-range lists based on application profiling data and object-size statistics.