Fragmentation

Fragmentation is a phenomenon in which storage space is used inefficiently. Fragmentation occurs when main memory is available but space is not sufficient to load any other process.  Due to swapping and dynamic memory allocation, user process swap in and swap out of the main memory. As a result there are free space but we cannot load new process because available size is smaller than size of the process.

Types of Fragmentation:

Internal Fragmentation occurs when memory allocated to a process is slightly larger than the actual memory requirement of the process. The difference is known as internal fragmentation. Internal fragmentation is actually unused memory internal to a process partition.

Example: Suppose, four process arrives and P1 requires 35 KB, P2 requires 70 KB, nd P4 requires 28 KB. The new process P5 requires 54KB. Memory requirement of process P5 is not fulfilled because internal fragmentation occurs in this case.

REASON: Total Free Memory = 50 + 2 + 5= 57 KB. But we cannot allocate to process P5 because free memory is within allocated process memory area.

2   External Fragmentation occurs when there is a sufficient area within main memory to satisfy current process memory requirement. BUT memory cannot be allocated because available memory is not contiguous.

Example: Suppose a new process arrives and needs 100 KB. In this case, external fragmentation occurs and memory is not allocated to new process.

REASON: The total free memory available is 30+20+60+10=120KB. But we cannot allocate memory to new process because available free memory is not contiguous.

 

The solution for external fragmentation is compaction and non contiguous memory allocation

Compaction is a process to shuffle memory content so that all free memory forms one large block. Practically this is not possible every time.

Another way to solve external fragmentation is allowing logical address space to be non contiguous. The two methods used are Paging and Segmentation.