Fragmentation | Hexainclude

Fragmentation is the primary problem faced while allocating memory. In this article we will focus on Fragmentation and steps to minimize it. Fragmentation can be of two types:

External fragmentation arises when free memory areas existing in a system are too small to be allocated to processes.
When enough total memory space exists to allocate a process but it is not contiguous, storage space is fragmented into a large number of small blocks. Such unused small free blocks which are outside to partitions are referred to as external fragmentation.
Statistical analysis shows that with allocation of N blocks, another 0.5 blocks may be lost due to fragmentation. That is, one-third of memory may be unusable! This property is known as the 50-percent rule.

Internal fragmentation exists when the memory allocated to a process is not fully utilized by it.
When a process is allocated a larger block than the size of the process, some memory area inside the partition becomes unusable. This type of memory wastage is considered as internal fragmentation.
In other words, internal fragmentation is the amount of unusable memory which is inside to a partition.

A possible solution to reduce external fragmentation is compaction.
A compaction is a process to merge different unused small free blocks in the memory to produce a large free block.
The kernel periodically can perform memory compaction to reduce total amount of external fragmentation.
It involves movement of processes (dynamic relocation) in the memory during their execution. It can be implemented by using relocation register.
Consider process C in the figure (a) that contains address 3M bytes when C is in execution. We have two free memory blocks available one is followed by process A and other free block is followed by process D.
During compaction, C is moved to the memory area with start address 2M bytes (figure b). Similarly process D is moved to process C’s memory area.
After compaction, we have one large free block at the bottom that is used to load process E (figure c).
Dynamic relocation is not practical if the computer system does not use relocation register.