.net垃圾回收和CLR 4.0对垃圾回收所做的改进之一

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上面我们了解了CLR如何管理托管资源. 下面我们来看垃圾收集器如何寻找不再使用的托管对象，并释放其占用的内存. In previous paragraphs, we learn how CLR manages managed resources. In following paragraphs, we will see how garbage collector find objects no longer needed, and release the memory.

垃圾收集器如何寻找不再使用的托管对象，并释放其占用的内存How garbage collector find objects no longer needed and release memory
前面我们了解了CLR如何管理托管栈上的对象.按照先进后出原则即可比较容易地管理托管栈的内存. 托管堆的管理比托管栈的管理复杂多了.下面所谈都是针对托管堆的管理. In previous paragraphs, we learn how CLR manages the objects on managed stack. It is easy to manage managed stack as long as you utilize the rule “first in last out”. The management of managed heap is much more complicated than the management of managed stack. The following is all about the management of managed heap.

根The root
垃圾收集器寻找不再使用的托管对象时, 其判断依据是当一个对象不再有引用指向它, 就说明此对象是可以释放了. 一些复杂的情况下可以出现一个对象指向第二个对象,第二个对象指向第三个对象,…就象一个链表. 那么, 垃圾收集器从哪里开始查找不再使用的托管对象呢? 以刚才所说的链表为例, 显然是应该从链表的开头开始查找. 那么，在链表开头的是些什么东东呢? The criteria garbage collector uses to judge whether an object is no longer needed is that an object can be released when the object does have any reference. In some complicated cases, it happends that the first object refers to the second object, and the second object points to the third object, etc. It is looking like a chain of single linked nodes. Then the question is : where does the garbage collector begins to find objects no longer needed? For the example of the single linked node chain, we can say it is obvious garbage collector starts from the beginning of the chain. Then the next question is: what are the stuff at the beginning of the chain.

是局部变量, 全局变量, 静态变量, 指向托管堆的CPU寄存器. 在CLR中，它们被称之为根. The answer is : local variables, global variables, static variables, the CPU registers pointing to managed heap. In CLR, they are called “the roots”.

有了开始点, 垃圾收集器接下来怎么做呢? Got the roots, what will garbage collector do next?

创建一个图, 一个描述对象间引用关系的图. Build a graph, which shows the reference relationship among objects.
垃圾收集器首先假定所有在托管堆里面的对象都是不可到达的(或者说没有被引用的,不再需要的), 然后从根上的那些变量开始, 针对每一个根上的变量, 找出其引用的托管堆上的对象, 将找到的对象加入这个图, 然后再沿着这个对象往下找，看看它有没有引用另外一个对象, 有的话，继续将找到的对象加入图中,如果没有的话, 就说明这条链已经找到尾部了. 垃圾收集器就去从根上的另外一个变量开始找, 直到根上的所有变量都找过了, 然后垃圾收集器才停止查找. 值得一提的是, 在查找过程中, 垃圾收集器有些小的优化, 如: 由于对象间的引用关系可能是比较复杂的, 所以有可能找到一个对象, 而此对象已经加入图了, 那么垃圾收集器就不再在此条链上继续查找, 转去其他的链上继续找. 这样对垃圾收集器的性能有所改善.

First garbage collector supposes all the objects in managed heap are not reachable( do not have reference, or no longer needed). Then start from the variables in the roots. For each of the variable in the roots, search the object the variable refers to, and add the found object into the graph, and search again after the found object for next refered object, etc. Check whether the found object has next reference. If has, continue to add the next found object into the graph. If not, it means this is the end of the chain, then stop searching on the chain, continue on next variable in the roots, keep searching on roots, until all the searching are finished. In the searching process, garbage collector has some optimization to improve the performance. Like: Because the reference relationship could be complicated among objects, it is possible to find an object that has been added into the graph, then garbage collector stops searching on the chain, continue to search next chain. This way helps on performance of garbage collection.

垃圾收集器建好这个图之后, 剩下那些没有在这个图中的对象就是不再需要的. 垃圾收集器就可以回收它们占用的空间.After buidling the reference graph among objects, the objects not in the graph are no longer needed objects. Garbage collector could release the memory space occupied by the no longer needed objects.

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