External Fragmentation occurs when free memory is broken into small, noncontiguous holes between allocated blocks. Even if the total amount of free memory is sufficient to satisfy a request, none of the individual holes is large enough, so the allocation fails.

1. How It Happens

1. Fixed-size partitions (or variable-sized allocations) are handed out to processes as they arrive.
2. When a process terminates (or releases memory), its block becomes a free hole.
3. Over time, as processes of different sizes are loaded and removed, these holes become scattered.
4. A new process requiring a contiguous chunk larger than any single hole can’t be placed, even if the sum of holes is adequate.

2. Example

Suppose we have 1 000 KB of RAM, and we allocate and free blocks in this sequence:

At step 8, external fragmentation prevents fulfilling P₅’s request because the largest contiguous hole is only 400 KB, and after placing P₄ and P₃ there’s a 400 KB hole—but if P₅ were, say, 450 KB, even though 650 KB is free in total, no single chunk is large enough.

3. Mitigation Techniques

• Compaction: periodically move allocated blocks together to combine holes into one large free block (costly in CPU/time).
• Paging/Segmentation: break processes into fixed-size pages or variable-size segments so they can occupy noncontiguous frames, eliminating the need for large contiguous holes.

Key Point: External fragmentation is fragmentation between allocations due to dynamic allocation/deallocation, leading to unusable scattered free space.