Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!husc6!mit-eddie!genrad!decvax!ucbvax!hplabs!pyramid!prls!mips!mash From: mash@mips.UUCP (John Mashey) Newsgroups: comp.arch,comp.unix.wizards,comp.os.minix Subject: Re: Free Software Foundation (was: Re: Mach, the new standard?) Message-ID: <712@winchester.UUCP> Date: Sat, 26-Sep-87 10:35:51 EDT Article-I.D.: winchest.712 Posted: Sat Sep 26 10:35:51 1987 Date-Received: Sun, 27-Sep-87 11:02:34 EDT References: <8490@think.UUCP> <1745@ncr-sd.SanDiego.NCR.COM> <819@sugar.UUCP> Reply-To: mash@winchester.UUCP (John Mashey) Organization: MIPS Computer Systems, Sunnyvale, CA Lines: 32 Xref: mnetor comp.arch:2379 comp.unix.wizards:4520 comp.os.minix:1728 In article <819@sugar.UUCP> peter@sugar.UUCP (Peter da Silva) writes: >In article <1745@ncr-sd>, greg@ncr-sd (Greg Noel) writes: >> At the risk of re-opening an old debate, the PDP-11 \does/ have virtual >> memory. It's just that, for various technical reasons, the original Unix >> implementation for it chose to use swapping instead of paging as its virtual >> memory technique. >And neither did any other operating system for the PDP-11 (RSX, RSTS, RT-11), >probably because it didn't in fact have the capability of supporting VM. >Why do you think DEC developed the Virtual Address Extension (VAX) in the >first place? Greg was right in the first place. As a good example, somebody at Naval Postgraduate school did a thesis where they modified UNIX to run demand-paged on an 11/55, and did various performance measurements. they found the paged version was superior in only a small domain. I recall the thesis included an honest, but somewhat chagrined comment like "Thompson was right; simple wins in this case". Why should this be? The technical reasons Greg alludes to are simple: If programs are relatively small [64K I + 64K D max] and if pages are relatively large (compared to size) [8K], then programs are likely to touch every or almost every page very quickly. In this case, you get better disk performance, and have simpler, denser kernel data structures, by swapping instead of paging. I.e., if working set = size of program, you might as well swap. -- -john mashey DISCLAIMER: UUCP: {decvax,ucbvax,ihnp4}!decwrl!mips!mash OR mash@mips.com DDD: 408-991-0253 or 408-720-1700, x253 USPS: MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086