death by swap

Posted Nov 7, 2009 17:06 UTC (Sat) by giraffedata (subscriber, #1954) [Link]

but back in the 70's they realized that most of the time most programs don't use all their memory at any one time. so the odds are pretty good that the page of ram that you swap out will not be needed right away.

I think you didn't follow the scenario. We're specifically talking about a page that is likely to be needed right away. It's a page that the normal page replacement policy would have left alone because it expected it to be needed soon -- primarily because it was accessed recently.

But the proposed policy would steal it anyway, because the process that is expected to need it is over its quota and the policy doesn't want to harm other processes that aren't.

What was known in the 70s was that at any one time, a program has a subset of memory it accesses a lot, which was dubbed its working set. We knew that if we couldn't keep a process' working set in memory, it was wasteful to run it at all. It would page thrash and make virutally no progress. Methods abounded for calculating the working set size, but the basic idea of keeping the working set in memory or nothing was constant.