Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.3 4.3bsd-beta 6/6/85; site l5.uucp Path: utzoo!linus!philabs!cmcl2!harvard!seismo!lll-crg!well!ptsfa!l5!gnu From: gnu@l5.uucp (John Gilmore) Newsgroups: net.arch Subject: Re: Why Virtual Memory Message-ID: <239@l5.uucp> Date: Wed, 30-Oct-85 17:43:43 EST Article-I.D.: l5.239 Posted: Wed Oct 30 17:43:43 1985 Date-Received: Sun, 3-Nov-85 07:51:08 EST References: <480@seismo.CSS.GOV>, <384@unc.unc.UUCP> <6086@utzoo.UUCP> Organization: Nebula Consultants in San Francisco Lines: 20 Summary: I. P. Sharp no longer runs their Amdahl without paging In article <6086@utzoo.UUCP>, henry@utzoo.UUCP (Henry Spencer) writes: > Virtual memory has always meant some speed penalty, although clever design > can minimize it. Even 10-year-old big machines run more quickly with > address translation switched off, as witness IP Sharp [big APL timesharing > firm] which runs its monster Amdahl unmapped and sees about a 15% speed > improvement as a result. (They can get away with this because they run no > directly-executable user code.) Any machine has some overhead due to updating page table entries -- in hardware, or in software, or both. You get what you pay for. Sharp used to run DOS/360 (heavily hacked) on their Amdahl, since their APL time sharing system was written to timeshare efficiently on the 360 which did not have address translation or paging (remember those days??). They ported their APL to MVS a few years ago because a lot of customers wanted to run it that way, and eventually converted their data centre to MVS too. I think it cost them about 40% in overhead, but they could stop maintaining DOS/360 (which IBM dropped 10 years ago and Amdahl had no interest in). It got to be a pain writing new disk drivers and machine check handlers to keep up with the latest in IBM and 3rd party mainframe fashions.