Summary

You have now seen a top-to-bottom review of two virtual memory systems. Hopefully, most of the details were easy to follow, as you should have already had a good understanding of the basic mechanisms and policies. More detail on VAX/VMS is available in the excellent (and short) paper by Levy and Lipman“Virtual Memory Management in the VAX/VMS Operating System” by H. Levy, P. Lipman. IEEE Computer, Volume 15:3, March 1982. Read the original source of most of this material. Particularly important if you wish to go to graduate school, where all you do is read papers, work, read some more papers, work more, eventually write a paper, and then work some more.. We encourage you to read it, as it is a great way to see what the source material behind these chapters is like.

You have also learned a bit about Linux. While a large and complex system, it inherits many good ideas from the past, many of which we have not had room to discuss in detail. For example, Linux performs lazy copy-on-write copying of pages upon fork(), thus lowering overheads by avoiding unnecessary copying. Linux also demands zeroes pages (using memory-mapping of the /dev/zero device), and has a background swap daemon (swapd) that swaps pages to disk to reduce memory pressure. Indeed, the VM is filled with good ideas taken from the past, and also includes many of its own innovations.