CLR Garbage Collection

1. Memory architecture

2. Generations

• Generation 0 : Short lived objects (Collected frequently)
• Generation 1 : Medium lived objects (Collected less frequently)
• Generation 2 : Long lived objects (Variable size and expensive to collect)
• Generation 0 and 1 is known as the ephemeral segment (Fixed size)

SOS : !eeheap -gc

SOSEX : !gcgen <address> 

3. Roots

• GC uses roots to find which objects are alive or dead
• Any object with an existing reference to it has a root and is thus considered alive
• Roots are determined using the following components:JIT compoler, Stack walker, Handle table, Finalize Queue

SOS : !gcroot <address>

4. Finalization

• GC only knows about managed objects
• Objects that wrap native types need a cleanup mechanism
• Objects that wrap a native types must:
  • Implement a Finalizer
  • Implement IDisposable
  • Both methods should use same private helper method 

Finalization Best Practices

• Whenever possible do not rely on finalization rather always explicitly Dispose finalizable objects
• If you implement a finalizer you should also implement IDisposable (Dispose suppresses the object finalization)
• In C#, the using {} pattern automatically invokes the Dispose method

5. Large object heap

• Objects greater than 85,000 bytes
• Key difference is that LOH is not compacted (Very common cause of memory fragmentation)
• Introduced to avoid the cost of compaction

6. Pinning problems

• As part of compaction the GC may move an object around
• Problem for objects passed to native code (For example, a buffer to async native operation)
• Pinning tells the GC that it is not allowed to move the object
• Excessive pinning common cause of memory fragmentation