深入理解Java HotSpot虚拟机内存管理

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"《JVM内存管理白皮书》是由Sun Microsystems出版的一本关于Java HotSpot虚拟机内存管理的专业书籍，旨在帮助读者深入理解JVM的内存工作机制。本书涵盖了垃圾收集的基本概念、不同类型的垃圾收集器、自动内存管理的策略以及性能调优工具等多个方面，是Java开发者学习JVM内存管理的重要参考资料。" 详细内容: 1. **内存管理的两种形式**：书中介绍了显式内存管理和自动内存管理的区别。显式内存管理需要程序员手动分配和释放内存，而自动内存管理则由JVM的垃圾收集器自动进行，降低了内存泄露的风险。 2. **垃圾收集的概念**：讨论了垃圾收集的目标和特性，包括如何确定不再使用的对象、如何高效地回收这些对象以及如何避免性能影响。同时，书中探讨了设计选择和性能度量标准。 3. **代际收集理论**：强调了在J2SE 5.0 HotSpot JVM中，垃圾收集器采用的代际模型，即新生代、老年代和永久代的概念，以优化不同生命周期对象的处理。 4. **HotSpot JVM的垃圾收集器**：详细解析了多种垃圾收集器的工作原理和特性，如： - **串行收集器**：适用于低CPU核心数的环境，进行单线程的垃圾收集。 - **并行收集器**：利用多线程提高垃圾收集效率，适用于CPU核心数较多的情况。 - **并行压缩收集器**：在并行收集的基础上，进行内存压缩，减少碎片。 - **并发标记-清除(CMS)收集器**：主要特点是并发执行，降低停顿时间，适用于响应时间敏感的应用。 5. **自适应调整与行为调优**：JVM会根据系统条件自动选择合适的垃圾收集器、堆大小和虚拟机配置。并介绍了一些基于行为的并行收集器调优策略。 6. **推荐实践**：提供了何时选择不同垃圾收集器的建议，以及堆大小设置和针对并行收集器的调优策略。同时，对遇到内存溢出错误时应采取的措施进行了指导。 7. **评估垃圾收集性能的工具**：包括命令行选项（如`-XX:+PrintGCDetails`和`-XX:+PrintGCTimeStamps`）以及jmap、jstat、HPROF堆分析器和HAT堆分析工具等，用于监控和分析JVM的内存行为。 8. **关键选项与参数**：书中还列举了一些关键的JVM选项，以帮助用户更好地理解和调整JVM的行为。《JVM内存管理白皮书》是Java开发人员深入学习JVM内存管理、优化应用性能和排查问题的宝贵资源，对于提升Java应用程序的稳定性和性能具有重要的指导价值。

Design Choices

A number of choices must be made when designing or selecting a garbage collection algorithm:

• Serial versus 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. When

parallel collection is used, the task of

garbage collection is split into parts and those subparts are executed simultaneously, on different

CPUs. 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 bene

fit is that the source area can then be considered empty and available for fast and easy

subsequent alloca

tions

, but the dr

awbac

k is the additional time r

equired for copying and the extra

space that may be required.

Performance Metrics

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.

Sun Microsystems, Inc.

Garbage Collection

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