Program restructuring for virtual memory

Our general experience so far has shown the display of a virtual memory use pattern to be a good diagnostic tool. The automatic sector reordering technique brings noticeable improvements in paging performance where there is room for improvement, reducing the necessary working space (for a given number of exceptions) by as much as one-third to one-half. We have worked entirely in the environment of a simple hierarchy of main memory and a uniform speed replacement memory, with all replacement blocks having the same size. We have found that the reordering process, assuming a page size of 4K bytes, also produces improvements on pages of 8K and 2K bytes with the improvements favoring the doubled page size over the halved. This is consistent with a tendency we have noticed in the programs we have examined for better packed memory to favor larger page sizes. But the page size for clustering bears a direct relationship to the program sectors themselves. It has not proved effective to cluster at the physical page size when the average sector size is half as large or more. The optimal page size for a program depends on (besides physical I/O timings) complicated patterns in the use of virtual memory, about which very little is known.