Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site unc.unc.UUCP Path: utzoo!watmath!clyde!bonnie!akgua!mcnc!unc!omondi From: omondi@unc.UUCP (Amos Omondi) Newsgroups: net.arch Subject: Re: Page size and the meaning of life Message-ID: <406@unc.unc.UUCP> Date: Sat, 26-Oct-85 21:13:47 EST Article-I.D.: unc.406 Posted: Sat Oct 26 21:13:47 1985 Date-Received: Mon, 28-Oct-85 03:29:38 EST References: <926@decwrl.UUCP> <931@lll-crg.ARpA> <7459@watdaisy.UUCP> <939@lll-crg.ARpA> Organization: CS Dept, U. of N. Carolina, Chapel Hill Lines: 38 > >Perhaps the question to ask is do we need disk paging? > >With large memories becoming available rolling pages out to disk may become > >unneccessary, but the concept of virtual memory and its associated attributes > >is probably still useful. > I'm sorry I was not precise enough. The question was meant to be do we need > disk paging? The much needed firewall protection and address space shareing > for programs in a multiprocessor can be provided by a simple {base,limit} > segmentation scheme. One or course needs several sets of such registers > to establish the several segments, code, static data, stack, shared static > data, ... that one needs in a program. Do we really need the page oriented > virtual memory systems that occur in todays micros and mini computers? If > we have more than enough physical memory, do we need the overhead associated > with the page mapping hardware? It is difficult to make such hardware operate > at supercomputer speeds and poses severe difficulties for non bus oriented > architectures (large N multiprocessors). One answer, and probably the only reasonable one, appeared in an earlier article i.e. the need to deal with storage allocation; specifically, the need to deal with external fragmentation. I'm not sure i agree with the speed argument. If you have base-limit registers then you still have to some checks on the validity of the virtual address; this takes no more time than on a segmented-paged system since in the latter it is usual to do checks on all the fields of the virtual address in parallel. As to the supercomputer bit, the Cyber 205, a supercomputer in every sense of the word, implements virtual store and so far its users seem to be quite happy with its perfomance. Of course for disc transfers they have a very large "super-page" for efficiency ... I never heard anyone say they had "enough" physical memory ! Everyone always seems to want more. I really don't think you'll get a "satisfying" answer. Inspite of the fact that paging has been around for a while, it is still not clear that it is the best thing to have and there is no doubt that more research needs to be done.