Garbage Collection

Benefits

• 개발자가 Memory 해제를 하지 않아도 됨(unmanaged resource 사용하지 않을 때)
• managed heap에 효율적으로 객체 할당
• 사용하지 않는 객체를 회수하고 메모리 정리 (data field initialization)
• 다른 객체를 사용 중인 메모리에 할당하지 않음

Garbage Collection Algorithm

• The list of active roots is maintained by the just-in-time (JIT) compiler and common language runtime (CLR)
• Garbage Collector running → 참조 그래프 생성 building graph of all objects reachable from the roots (any object not in the graph are not accessible by the application) → looking for blocks of garbage objects(free space) → shifting the non-garbage objects down in memory(memcpy) / removing all of the gaps in the heap → modifying the application’s root for pointer to point to the object’s new locations (all pointer to the objects are invalid)

Roots : storage locations e.g. global, static object pointers, local variable/parameter object pointers on thread’s stack(on the managed heap or to null)

Every Application has a set of roots.

Occurrance of GC

• Heap is full
• GC Thread (working threads stop)
• Generates a performance hit
• but managed heap is much faster than C-runtime heap

GC Features

Unmanaged resources

https://docs.microsoft.com/en-us/dotnet/standard/garbage-collection/fundamentals?redirectedfrom=MSDN

• e.g. object that wraps an operating system resources (file, window handle, network connection)

• provide code to clean up (Dispose, safe handle, finalize method)

  https://docs.microsoft.com/en-us/dotnet/standard/garbage-collection/unmanaged

Finalization

• developers explicitly handle resource management and cleanup by implementing Finalize, Close / Dispose
• you create an object and when the object is collected, the object’s Finalize method is called gracefully clean up after itself (properly) when it is being collected.
• GC calls Finalize method (if it exists) and reclaims memory when the garbage object is detected.
• When an application creates a new object, the new operator allocates the memory from the heap. If the object’s type contains a Finalize method, then a pointer to the object is placed on the finalization queue.

public class BaseObj {
 public BaseObj() {
 }
 protected override void Finalize() {
 // Perform resource cleanup code here...
 // Example: Close file/Close network connection
 Console.WriteLine("In Finalize.");
 }
}

Now you can create an instance of this object by calling:

BaseObj bo = new BaseObj();

Finalize internals

1. Create New object
2. The new operator allocates the memory from the heap
3. If the object’s type contains a Finalize method, then a pointer to the object is placed on the finalization queue.
4. The finalization queue is an internal data structure controlled by the garbage collector.

Resurrection

If the object requires finalization, the object(no longer accessing a live object) is considered live again until it is actually finalized

Generations

• performance improvement
• the newer an object is, the shorter lifetime is expected to be.

• collecting compacting generation 0 object (reclaim a significant amount of space)

  survivors are compacted and considered to be in generation 1(promotion)

• then, collect objects from order generations(1, 2-highest)

Generation 0 : youngest generation and contains short-lived objects

Generation 1 : contains short-lived objects and a buffer

Generation 2 : contains long-lived objects

Reference

https://docs.microsoft.com/en-us/archive/msdn-magazine/2000/december/garbage-collection-part-2-automatic-memory-management-in-the-microsoft-net-framework

GCinNET.pdf