Serial vs Parallel
With serial collection, only one thing happens at a time. For example, even when multiple CPUs are available, only one is utilized to perform the collection.
The simultaneous operation enables the collection to be done more quickly, at the expense of some additional complexity and potential fragmentation.
Concurrent versus Stop-the-world
When stop-the-world garbage collection is performed, execution of the application is completely suspended during the collection. Alternatively, one or more garbage collection tasks can be executed concurrently, that is, simultaneously, with the application.
Typically, a concurrent garbage collector does most of its work concurrently, but may also occasionally have to do a few short stop-the-world pauses.
Stop-the-world garbage collection is simpler than concurrent collection, since the heap is frozen and objects are not changing during the collection. Its disadvantage is that it may be undesirable for some applications to be paused.
Correspondingly, the pause times are shorter when garbage collection is done concurrently, but the collector must take extra care, as it is operating over objects that might be updated at the same time by the application.
This adds some overhead to concurrent collectors that affects performance and requires a larger heap size.
Compacting versus Non-compacting versus Copying
After a garbage collector has determined which objects in memory are live and which are garbage, it can compact the memory, moving all the live objects together and completely reclaiming the remaining memory.
After compaction, it is easy and fast to allocate a new object at the first free location. A simple pointer can be utilized to keep track of the next location available for object allocation.
In contrast with a compacting collector, a non-compacting collector releases the space utilized by garbage objects in-place, i.e., it does not move all live objects to create a large reclaimed region in the same way a compacting collector does.
The benefit is faster completion of garbage collection, but the drawback is potential fragmentation. In general, it is more expensive to allocate from a heap with in-place deallocation than from a compacted heap.
It may be necessary to search the heap for a contiguous area of memory sufficiently large to accommodate the new object.
A third alternative is a copying collector, which copies (or evacuates) live objects to a different memory area. The benefit is that the source area can then be considered empty and available for fast and easy subsequent allocations, but the drawback is the additional time required for copying and the extra space that may be required.
Several metrics are utilized to evaluate garbage collector performance, including:
- Throughput—the percentage of total time not spent in garbage collection, considered over long periods of time.
- Garbage collection overhead—the inverse of throughput, that is, the percentage of total time spent in garbage collection.
- Pause time—the length of time during which application execution is stopped while garbage collection is occurring.
- Frequency of collection—how often collection occurs, relative to application execution.
- Footprint—a measure of size, such as heap size.
- Promptness—the time between when an object becomes garbage and when the memory becomes available.
- Practical Garbage Collection – Part 1: Introduction (worldmodscode.wordpress.com)
- Reducing memory churn when processing large data set (stackoverflow.com)
- The Top Java Memory Problems – Part 2 (dynatrace.com)
- imabonehead: Performance Tuning the JVM for Running Apache Tomcat | TomcatExpert (tomcatexpert.com)
- When Does the Garbage Collector Run in JVM ? (javacircles.wordpress.com)
- Why Garbage Collection Paranoia is Still (sometimes) Justified (prog21.dadgum.com)
- Adventures in Java Garbage Collection Tuning (rapleaf.com)