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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.