virtual memory 4476

Anne & Lynn Wheeler

As I understand it, one key issue is what happens after the page falls out of the working set.

had each page make one trip through a fifo list. This was later enhanced to use a reference bit and a make two or more trips if the page was referenced (due to performance analysis by Belady and others).

In the Denning model, when a page was pushed out of the working set, if it was modified it was written back to file. In any case after that the frame went onto the free list and *** note *** all connection between it and its process was lost even though its contents were still valid and even if there were lots of free pages. If the page was touched again it was brought in from disk.

This meant there was a high cost to evicting a working set page so it would be avoided by having larger working sets, yada, yada, yada.

Unlike Denning, VMS retains a link to the evicted page so that if it is touched again before the frame is buttigned to someone else, that it can be pulled back into the working set quickly. (They just reset the PTE valid bit but leave the frame number valid. If the owner touches it again, it just flips the PTE to valid. If the page is grabbed for someone else, then the PFN gets zapped.)

This allows VMS to toss pages out of its working sets at low cost, which allows them to roll over working sets more often. In theory this mitigates the bad effects of the Denning fifo model.

So the question I was curious about is to what extent Grenoble was like Denning .vs. like VMS.

Eric