Summary

Segmentation solves a number of problems and helps us build a more effective virtualization of memory. Beyond just dynamic relocation, segmentation can better support sparse address spaces, by avoiding the huge potential waste of memory between logical segments of the address space. It is also fast, as doing the arithmetic segmentation is easy and well-suited to hardware; the overheads of translation are minimal. A fringe benefit arises too: code sharing. If code is placed within a separate segment, such a segment could potentially be shared across multiple running programs.

However, as we learned, allocating variable-sized segments in memory leads to some problems that we’d like to overcome. The first, as discussed above, is external fragmentation. Because segments are variable-sized, free memory gets chopped up into odd-sized pieces, and thus satisfying​ a memory-allocation request can be difficult. One can try to use smart algorithms“Dynamic Storage Allocation: A Survey and Critical Review” by Paul R. Wilson, Mark S. Johnstone, Michael Neely, David Boles. International Workshop on Memory Management, Scotland, UK, September 1995. A great survey paper on memory allocators. or periodically compact memory, but the problem is fundamental and hard to avoid.

The second and perhaps more important problem is that segmentation still isn’t flexible enough to support our fully generalized, sparse address space. For example, if we have a large but sparsely-used heap all in one logical segment, the entire heap must still reside in memory in order to be accessed. In other words, if our model of how the address space is being used doesn’t exactly match how the underlying segmentation has been designed to support it, segmentation doesn’t work very well. We thus need to find some new solutions. Ready to find them?