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: Why Virtual Memory Message-ID: <483@unc.unc.UUCP> Date: Fri, 1-Nov-85 13:54:12 EST Article-I.D.: unc.483 Posted: Fri Nov 1 13:54:12 1985 Date-Received: Wed, 6-Nov-85 04:24:50 EST References: <480@seismo.CSS.GOV>, <384@unc.unc.UUCP> <6086@utzoo.UUCP> <240@l5.uucp> Organization: CS Dept, U. of N. Carolina, Chapel Hill Lines: 19 > One thing you vmunix users might try is running "top" (show top N processes > and assorted stats about the system) to answer this question. > > Almost all the processes have about a third of their address space paged in. > Is an MMU cheaper than three times as much memory? Sure, especially if > you are using it for memory protection anyway. > -- > What is the sound of one disk paging? An interesting question is one of how cheap an MMU is. The address translation hardware on the Cyber 203 & 205, for example, is rela- tively fancy. Similarly the occasional attempt to deal with inter- nal fragmentation by having 3 or 4 page sizes means the translation hardware is not going to be particularly simple, particularly when software rather than hardware is used to load the registers. Machine designers have also been reluctant to provide more than 8 or 16 registers for translation on the grounds that the increase in perfoma- ance does not justify the cost.