Praveen Swamy

• Sep 2nd, 2015

 

External fragmentation happens when the memory allocator leaves sections of unused memory blocks between portions of allocated memory. For example, if several memory blocks are allocated in a continuous line but one of the middle blocks in the line is freed (perhaps because the process that was using that block of memory stopped running), the free block is fragmented. The block is still available for use by the allocator later if there’s a need for memory that fits in that block, but the block is now unusable for larger memory needs. It cannot be lumped back in with the total free memory available to the system, as total memory must be contiguous for it to be useable for larger tasks. In this way, entire sections of free memory can end up isolated from the whole that are often too small for significant use, which creates an overall reduction of free memory that over time can lead to a lack of available memory for key tasks.