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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.