[email protected] (Staller, Allan) writes: > z/VM solved the nested paging issue the late 80's. Usually by making > the guest image so large that paging never became an issue. > There was also the "preferred guest" feature.
The problem was worse than you can ever imagine. I pontificated a lot about this in the mid-70s I had done a lot of paging algorithm stuff as undergraduate in the 60s ... and got into tiff with the POK crowd when they started adding paging to MVT (something they thought was benefit ... was actually picking shared, high-use linkpac pages for replacement before lower-use private data pages ... which didn't get fixed until well into the MVS release cycle). The other problem was running under vm370 ... most page replacement algorithms assume some flavor of LRU ... least recently used ... the page that has been least recently used in the past is assumed to have the lowest probability of use in the future. When MVS is running some flavor of LRU ... it is looking for the page with very low usage to use its real storage location. when running under vm370, with both vm370 and mvs running some flavor of LRU ... the apparent page use behavior of MVS running in vm370 virtual machine ... is the least recently used page (from vm370 point-of-view) is the most likely to be used next (not least likely) ... because MVS is out looking for the leased recently used for its next likely to be used aka a LRU algorithm running under a LRU algorithm starts to look a lot like an MRU (the least recently used page is the most likely to be used). This also comes up with large DBMS record caches which also tend to be managed with least-recently-used replacement strategies ... and why large DBMS record caches also tend to have to be PIN'ed (because the operating system will have a tendency to replace the least recently used page ... which has high probability of being needed by the DBMS). misc. past posts about page replacement algorithms http://www.garlic.com/~lynn/subtopic.html#clock -- virtualization experience starting Jan1968, online at home since Mar1970 ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
