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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.arch
Subject: Re: Page size and the meaning of life
Message-ID: <6085@utzoo.UUCP>
Date: Sat, 26-Oct-85 21:16:36 EDT
Article-I.D.: utzoo.6085
Posted: Sat Oct 26 21:16:36 1985
Date-Received: Sat, 26-Oct-85 21:16:36 EDT
References: <926@decwrl.UUCP> <931@lll-crg.ARpA> <7459@watdaisy.UUCP>, <939@lll-crg.ARpA>
Organization: U of Toronto Zoology
Lines: 19

> ...  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...

There are a couple of very useful tricks one can pull with paged systems
that cannot be done with base-limit schemes.  For one thing, it is possible
to enlarge a process's stack without having to move the whole thing around
in memory (scatter allocation).  For another, it is possible to do a much
more efficient implementation of fork() using copy-on-write techniques.
Neither of these matters too much for small processes, but they start to
be major considerations for really big ones.

Note that it is possible to overlap page-translation time with memory-
access time, as on the Celerity C1200, so that very little speed penalty is
incurred.  Generalizing this to supercomputers with 50-ns memory is not
so straightforward, admittedly.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry