Wonder why GC has only 2 generations

The CLR Garbage Collector is a generational, mark-and-compact collector. It follows several principles that allow it to achieve excellent performance. First, there is the notion that objects that are short-lived tend to be smaller and are accessed often. The GC divides the allocation graph into several sub-graphs, called generations, which allow it to spend as little time collecting as possible. Gen 0 contains young, frequently used objects. This also tends to be the smallest, and takes about 10 milliseconds to collect. Since the GC can ignore the other generations during this collection, it provides much higher performance. G1 and G2 are for larger, older objects and are collected less frequently. When a G1 collection occurs, G0 is also collected. A G2 collection is a full collection, and is the only time the GC traverses the entire graph. It also makes intelligent use of the CPU caches, which can tune the memory subsystem for the specific processor on which it runs. This is an optimization not easily available in native allocation, and can help your application improve performance.

What Happens When a Collection Occurs?

Let's walk through the steps a garbage collector takes during a collection. The GC maintains a list of roots, which point into the GC heap. If an object is live, there is a root to its location in the heap. Objects in the heap can also point to each other. This graph of pointers is what the GC must search through to free up space. The order of events is as follows:

1. The managed heap keeps all of its allocation space in a contiguous block, and when this block is smaller than the amount requested, the GC is called.
2. The GC follows each root and all the pointers that follow, maintaining a list of the objects that are not reachable.
3. Every object not reachable from any root is considered collectable, and is marked for collection.

4. Removing objects from the reachability graph makes most objects collectable. However, some resources need to be handled specially. When you define an object, you have the option of writing a Dispose() method or a Finalize() method (or both). I'll talk about the differences between the two, and when to use them later.
5. The final step in a collection is the compaction phase. All the objects that are in use are moved into a contiguous block, and all pointers and roots are updated.
6. By compacting the live objects and updating the start address of the free space, the GC maintains that all free space is contiguous. If there is enough space to allocate the object, the GC returns control to the program. If not, it raises an OutOfmemoryException


With Best Regards,
Mitesh Mehta
Email : miteshvmehta@gmail.com
http://cc.1asphost.com/miteshvmehta/