virtual memory 4478
the other thing that was starting to happend by the second half of the 70s was things were starting to...
another characteristic of the real memory resource vis-a-vis disk resource trade-off was the original os-360 design using CKD DASD...
so, I've told the story of how MVS decided to bias the LRU replacement algorithm in favor of non-changed pages ... because there was lower overhead and less latency ... than compared to when a changed page had to be first written out.
however, if you are just doing straight page replacement selection, somewhat syncronized with the page fault ... then the latency for servicing a page fault when a changed page has been selected is possibly twice the elapsed time of servicing a page fault when a non-changed page has been selected (since replacing a page that has been changed first requires writing the current contents out before the replaced contents can be read in).
to cut down on this latency, you run the selecting of pages for replacement slightly out of sync and ahead of servicing a page fault. you have a small (fifo) pool of pages selected for replacement. a page fault results in selecting a replacement page from the pool ... probably dropping the pool below threshold, which then requires invoking the replacement algorithm to replenish the pool. changed pages that are selected for replacement are written out as they are added to the replacement pool. by the time, a page fault requires the page, any changed page has completed its write and the real storage location is immediately available to bring in the replacement page ... cutting down on the latency to service a page fault.
you might also check carr's global clock thesis ... i was able to find it online.
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