Garbage Collection

Garbage Collection

Garbage Collection
Garbage collection (GC) is a form of automatic memory management. The garbage collector, or just collector, attempts to reclaim garbage, or memory occupied by objects that are no longer in use by the program. Garbage collection is often portrayed as the opposite of manual memory management, which requires the programmer to specify which objects to reallocate and return to the memory system. Garbage collection, like other memory management techniques, may take a significant proportion of total processing time in a program and can thus have significant influence on performance.
Garbage Collection:

Mark and Sweep Method:
 Naive Mark and Sweep in action on a heap containing eight objects. Arrows represent object references. Circles represent the objects themselves. Objects #1, #2, #3, #4, and #6 are strongly referenced from the root set. On the other hand, objects #5, #7, and #8 are not strongly referenced either directly or indirectly from the root set; therefore, they are garbage.
A Mark and Sweep collector tries to find the live heap links and mark those blocks that are reachable. Then it makes a pass over the heap and returns unmarked free blocks to the free pool.
The mark-and-sweep method is the first known strategy for garbage collection. In this method, the garbage collector scans the program, starting from a point called root set (a set of objects easily identified in the program) and tries to reach other objects from then. The collector repeats this scanning until it cannot find any more reachable objects. Every time an object is found, its "being used" bit is set. After the collector finishes marking objects, it goes through the whole heap freeing those objects that do not have the "being used" bit set.
This method has several disadvantages, the most notable being that the entire system must be suspended during collection. This will cause programs to 'freeze' periodically (and generally unpredictably), making real-time and time-critical applications impossible.

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