Thursday, May 12, 2005

How GC works on .NET?

Two excellent articles on how Garbage Collection works in .NET.
A "MUST READ" for all .NET developers..

Excerpt from the article:

How does the garbage collector know if the application is using an object or not?

As you might imagine, this isn't a simple question to answer.Every application has a set of roots. Roots identify storage locations, which refer to objects on the managed heap or to objects that are set to null. For example, all the global and static object pointers in an application are considered part of the application's roots. In addition, any local variable/parameter object pointers on a thread's stack are considered part of the application's roots. Finally, any CPU registers containing pointers to objects in the managed heap are also considered part of the application's roots. The list of active roots is maintained by the just-in-time (JIT) compiler and common language runtime, and is made accessible to the garbage collector's algorithm.

Now, the garbage collector starts walking the roots and building a graph of all objects reachable from the roots. For example, the garbage collector may locate a global variable that points to an object in the heap.

Once this part of the graph is complete, the garbage collector checks the next root and walks the objects again. As the garbage collector walks from object to object, if it attempts to add an object to the graph that it previously added, then the garbage collector can stop walking down that path. This serves two purposes. First, it helps performance significantly since it doesn't walk through a set of objects more than once. Second, it prevents infinite loops should you have any circular linked lists of objects.Once all the roots have been checked, the garbage collector's graph contains the set of all objects that are somehow reachable from the application's roots; any objects that are not in the graph are not accessible by the application, and are therefore considered garbage. The garbage collector now walks through the heap linearly, looking for contiguous blocks of garbage objects (now considered free space). The garbage collector then shifts the non-garbage objects down in memory (using the standard memcpy function that you've known for years), removing all of the gaps in the heap. Of course, moving the objects in memory invalidates all pointers to the objects. So the garbage collector must modify the application's roots so that the pointers point to the objects' new locations. In addition, if any object contains a pointer to another object, the garbage collector is responsible for correcting these pointers as well.