JVM学习笔记-分代收集器(Generational Collectors)
Generational Collectors
One disadvantage of simple stop and copy collectors is that all live objects must be copied at every collection. This facet of copying algorithms can be improved upon by taking into account two facts that have been empirically observed in most programs in a variety of languages:
简单的停止并拷贝收集器的缺点是，每一次收集时，所有的活动对象都必须被拷贝。大部分语言的大多数程序都有如下特点，如果我们认真考虑这些，拷贝算法的这个缺点是可以被改进的。
- Most objects created by most programs have very short lives.
- Most programs create some objects that have very long lifetimes.
A major source of inefficiency in simple copying collectors is that they spend much of their time copying the same long-lived objects again and again.
简单的拷贝收集器浪费效率的一个主要原因就是，它们每次都把这些生命周期很长的对象来回拷贝，消耗大量的时间。
Generational collectors address this inefficiency by grouping objects by age and garbage collecting younger objects more often than older objects. In this approach, the heap is divided into two or more sub-heaps, each of which serves one "generation" of objects. The youngest generation is garbage collected most often. As most objects are short-lived, only a small percentage of young objects are likely to survive their first collection. Once an object has survived a few garbage collections as a member of the youngest generation, the object is promoted to the next generation: it is moved to another sub-heap. Each progressively older generation is garbage collected less often than the next younger generation. As objects "mature" (survive multiple garbage collections) in their current generation, they are moved to the next older generation.
按代收集的收集器通过把对象按照寿命来分组解决这个效率低下的问题，更多的收集那些短暂出现的年幼对象，而非寿命较长的对象。在这种方法里，堆被划分为两个或者更多的子堆，每一个子堆为一“代”对象服务。最年幼的那一代进行最频繁的垃圾收集。因为大多数对象都是短促出现的，只有很小部分的年幼对象可以在它们经历第一次收集后存活。如果一个最年幼的对象经历了好几次垃圾收集后仍然存活，那么这个对象就成长为寿命更高的一代；它被转移到另外一个子堆中去。年龄更高的每一代的收集都没有年轻的那一些来的频繁。每当对象在它所属的年龄层(代)中变得成熟(逃过了多次垃圾收集)之后，它们就被转移到更高的年龄层中去。
The generational collection technique can be applied to mark and sweep algorithms as well as copying algorithms. In either case, dividing the heap into generations of objects can help improve the efficiency of the basic underlying garbage collection algorithm.
按代进行的垃圾收集除了可以应用于拷贝算法，也可以应用于标记并清楚算法。不管在哪种情况下，把堆按照对象年龄层分解都可以提高最基本的垃圾收集算法性能