External Fragmentation
Indicating that the memory that is external to all partitions becomes increasingly fragmented.
External fragmentation happens when there’s a sufficient quantity of area within the memory to satisfy the memory request of a method. However, the process’s memory request cannot be fulfilled because the memory offered is in a non-contiguous manner.
Whether you apply a first-fit or best-fit memory allocation strategy it’ll cause external fragmentation.
To counter this, we can apply compaction. However downside of compaction is time consuming procedure and wasteful processor time.
Note
Compaction implies the need for a dynamic relocation capability. That is, it must be possible to move a program from one region to another in main memory without invalidating the memory references in the program.