Mark-Sweep Approach

In Reference-count approach, we can free the memory whenever object is eligible for garbage collection (Reference count is reached to 0). But in mark-sweep approach, memory is not freed whenever the object is eligible for garbage collection.

This approach runs in two phases.

1. Mark Phase
2. Sweep phase

Mark Phase

Whenever gc (Garbage Collector) is called for garbage collection, It enumerates all the roots and then starts visiting the objects referenced by them recursively. Whenever it visits the object, it sets a flag to make sure that this object is reachable. In the Mark phase we are identifying all the reachable objects or nodes.

Sweep Phase

Sweep starts by iterating through the entire memory and frees memory used by unreachable objects. Unreachable objects are identified by using the flag setting , which we are done in Mark Phase.

Advantages

1. Easy to Implement

2. It easily reclaims the cyclic structures, (Cyclic structures is a problem in Reference Counting Approach).

Disadvantages

1. Entire heap should be visited in Sweep phase, which impacts performance of your application.

2. Even if you identify the reachable object in Mark phase, you are revisiting them in sweep phase again, Since in sweep phase we are traversing the entire heap memory.

3. Mark-sweep collectors tend to leave the heap fragmented.

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