Wednesday, April 20, 2016

What is Garbage collection .Net & Interview Questions Related to garbage collector

Garbage collection is automatic process performs by Garbage collector (which lives in CLR) to free up memory of objects which are no longer in used.

Every time you create a new object, the common language runtime allocates memory to store that object. But at some point that object may be no longer needed by your application. So that garbage collector deallocates memory of this type of objects, so that application can store new objects into memory. If memory is not deallocated then sooner your application will run out of memory.


Interview Questions related to Garbage Collector


A. How does Garbage collection works?

Every time a garbage collection is performed, the garbage collector basically looks at the memory and frees up memory which is no longer needed, that is, memory which is occupied by ‘dead objects’.


B. How does it know when an object is ‘dead’?

An object is dead if it is unreachable by your code. The obvious example is a local variable inside a method. There is no way for your code to access that variable once the method has returned, so it becomes ‘dead’.

C. How often does the garbage collector perform a garbage collection?

There are three ways a garbage collection can be triggered.

Firstly, if your system has low physical memory, this can trigger a garbage collection.

Secondly, a threshold is defined which indicates an acceptable level of memory on the heap which can be used by allocated objects. If this threshold is surpassed, then a garbage collection is triggered.

Finally, a garbage collection can explicitly be triggered by calling the GC.Collect method. Only under very rare circumstances is this ever required.

D. What are generations in garbage collector?

           Generation define the age of the object.


      E. Generations of Garbage collections? / How many generations are there in Garbage collector?

     The heap is organized into generations so it can handle long-lived and short-lived objects. Garbage collection primarily occurs with the reclamation of short-lived objects that typically occupy only a small part of the heap. There are three generations of objects on the heap:

Generation 0.  This is the youngest generation and contains short-lived objects. An example of a short-lived object is a temporary variable. Garbage collection occurs most frequently in this generation.Newly allocated objects form a new generation of objects and are implicitly generation 0 collections, unless they are large objects, in which case they go on the large object heap in a generation 2 collection.Most objects are reclaimed for garbage collection in generation 0 and do not survive to the next generation.

Generation 1. This generation contains short-lived objects and serves as a buffer between short-lived objects and long-lived objects.

Generation 2. This generation contains long-lived objects. An example of a long-lived object is an object in a server application that contains static data that is live for the duration of the process.

     
      Objects that are not de-allocated in a garbage collection are known as survivors, and are  promoted to the next generation. Objects that survive a generation 0 garbage collections  are promoted to generation 1; objects that survive a generation 1 garbage collection are  promoted to generation 2; and objects that survive a generation 2 garbage collection  remain in generation 2.

       F. Advantages of garbage collector
           
           1. No need to take care of free-up memory while developing applications
           2. Allocates memory of objects efficiently on managed heap.
           3. Release memory of objects which are no longer in used in applications.

       G. Phases of garbage collector
  1. Marking Phase - In this phase garbage collector finds and creates list of all live objects.
  2. Relocating Phase - In this phase garbage collector updates references of objects that will be compacted.
  3. Compacting Phase - In this phase frees-up memory of objects which are no longer in used & compact surviving objects.

       H. Does garbage collector clean unmanaged objects?
            No.
  
 I. When we define clean up destructor, how does it affect garbage collector?
      If you define clean up in destructor garbage collector will take more time to clean up the objects and more and more objects are created in Gen 2 

 J. How can we overcome the destructor problem?
     By implementing "Idisposable" interface.

 K. Where do we normally put unmanaged code clean up?
      In finalize destructor.


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