第5章 Available GC
The Java HotSpot VM includes three different types of collectors, each with different performance characteristics.
Java虚拟机有三种不同类型的gc，他们有不同的特性。
顺序gc
The serial collector uses a single thread to perform all garbage collection work, which makes it relatively efficient because there is no communication overhead between threads. It is best-suited to single processor machines, because it cannot take advantage of multiprocessor hardware, although it can be useful on multiprocessors for applications with small data sets (up to approximately 100 MB). The serial collector is selected by default on certain hardware and operating system configurations, or can be explicitly enabled with the option -XX:+UseSerialGC.
顺序gc使用一个单独的线程来进行所有的垃圾回收工作。由于不需要进行线程间通信，所以相对高效。顺序gc适用于单处理器及有多核处理器的小规模程序。使用命令行参数-XX:+UseSerialGC来明确指定使用顺序gc。
并行gc
The parallel collector (also known as the throughput collector) performs minor collections in parallel, which can significantly reduce garbage collection overhead. It is intended for applications with medium-sized to large-sized data sets that are run on multiprocessor or multithreaded hardware. The parallel collector is selected by default on certain hardware and operating system configurations, or can be explicitly enabled with the option -XX:+UseParallelGC.
并行gc对于minor gc使用并行方式执行，可以大大提高效率。并行gc特别适合于，运行在多核多线程机器上的，中到大型规模的应用程序。使用命令行参数-XX:+UseParallelGC来明确指定使用并行gc。
Parallel compaction is a feature that enables the parallel collector to perform major collections in parallel. Without parallel compaction, major collections are performed using a single thread, which can significantly limit scalability. Parallel compaction is enabled by default if the option -XX:+UseParallelGC has been specified. The option to turn it off is -XX:-UseParallelOldGC
并行gc在进行major gc的时候使用并行压缩技术。如果不使用并行压缩，那么并行gc在进行major gc的时候只能使用单一线程进行回收，这会极大的影响major gc的效能。当指定命令行参数-XX:+UseParallelGC时并行压缩模式默认是打开的，可是使用参数-XX:-UseParallelOldGC来指定并行gc时关闭并行压缩模式。
并发gc
The mostly concurrent collector performs most of its work concurrently (for example, while the application is still running) to keep garbage collection pauses short. It is designed for applications with medium-sized to large-sized data sets in which response time is more important than overall throughput because the techniques used to minimize pauses can reduce application performance. The Java HotSpot VM offers a choice between two mostly concurrent collectors; see The Mostly Concurrent Collectors. Use the option -XX:+UseConcMarkSweepGC to enable the CMS collector or -XX:+UseG1GC to enable the G1 collector.
并发gc是指在程序运行的时间里同时并发进行垃圾回收，这样做的好处在于可以尽可能的缩短程序暂停时间。并发gc适合于多响应时间要求比较高的应用程序。Java虚拟机提供了两种并发gc，使用参数-XX:+UseConcMarkSweepGC来明确指定使用CMS收集器，使用参数-XX:+UseG1GC来明确指定使用G1收集器。
汇总
选择gc
Unless your application has rather strict pause time requirements, first run your application and allow the VM to select a collector. If necessary, adjust the heap size to improve performance. If the performance still does not meet your goals, then use the following guidelines as a starting point for selecting a collector.
除非你的程序有相当严格的暂停时间要求，否则你应当让虚拟机自己决定使用gc的类型。提升性能的首选应该是调整堆的大小。如果调整堆的大小不能带来显著的性能提升，那么根据如下原则来进行gc的选择：
If the application has a small data set (up to approximately 100 MB), then
select the serial collector with the option -XX:+UseSerialGC.
如果程序使用的数据集合在100MB左右，那么选择-XX:+UseSerialGC顺序gc即可。
If the application will be run on a single processor and there are no pause time requirements, then let the VM select the collector, or select the serial collector with the option -XX:+UseSerialGC.
如果应用程序运行在单核处理器系统上，并且没有暂停时间的需求，应当让虚拟机自动选择gc，或者使用顺序收集器
If (a) peak application performance is the first priority and (b) there are no pause time requirements or pauses of 1 second or longer are acceptable, then let the VM select the collector, or select the parallel collector with -XX:+UseParallelGC.
如果程序性能是首要需求，并且程序的暂停时间稍长可以被接受，那么应当让虚拟机选择gc类型，或者使用并行gc。
If response time is more important than overall throughput and garbage collection pauses must be kept shorter than approximately 1 second, then select the concurrent collector with -XX:+UseConcMarkSweepGC or -XX:+UseG1GC.
如果程序的响应时间是关键因素，并且要求尽可能短的暂停，那么应当选择并发gc，使用-XX:+UseConcMarkSweepGC或者-XX:+UseG1GC指令。