Memory Management Tutorial
Section 2 Dynamic Storage Allocation
For the purpose of dynamic storage allocation, we view memory as a single array broken into a series of variable-size blocks, where some of the blocks are free and some are reserved or already allocated. The free blocks are linked together to form a freelist used for servicing future memory requests. This figure illustrates the situation that can arise after a series of memory allocations and deallocations.
Figure 2.1: The results from a series of memory allocations and deallocations. Memory is made up of a series of variable-size blocks, some allocated and some free. In this example, shaded areas represent memory currently allocated and unshaded areas represent unused memory available for future allocation.
When a memory request is received by the memory manager, some block on the freelist must be found that is large enough to service the request. If no such block is found, then the memory manager must resort to a failure policy such as discussed in Section 3 - Failure Policies and Garbage Collection.
If there is a request for m words, and no block exists of exactly size m, then a larger block must be used instead. One possibility in this case is that the entire block is given away to the memory allocation request. This might be desirable when the size of the block is only slightly larger than the request. This is because saving a tiny block that is too small to be useful for a future memory request might not be worthwhile. Alternatively, for a free block of size k, with k > m, up to k - m space may be retained by the memory manager to form a new free block, while the rest is used to service the request.
Memory managers can suffer from two types of fragmentation. External fragmentation occurs when a series of memory requests result in lots of small free blocks, no one of which is useful for servicing typical requests. Internal fragmentation occurs when more than m words are allocated to a request for m words, wasting free storage. The difference between internal and external fragmentation is illustrated by this figure. The small white block labeled "External fragmentation" is too small to satisfy typical memory requests. The small grey block labeled "Internal fragmentation" was allocated as part of the grey block to its left, but it does not actually store information.
Figure 2.2: An illustration of internal and external fragmentation.
Some memory management schemes sacrifice space to internal fragmentation to make memory management easier (and perhaps reduce external fragmentation). For example, external fragmentation does not happen in file management systems that allocate file space in clusters. Another example of sacrificing space to internal fragmentation so as to simplify memory management is the buddy method described later in this section.
The process of searching the memory pool for a block large enough to service the request, possibly reserving the remaining space as a free block, is referred to as a sequential fit method.