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From: joel@peora.UUCP
Newsgroups: net.arch
Subject: Re: Very large memories
Message-ID: <2449@peora.UUCP>
Date: Mon, 22-Sep-86 19:39:15 EDT
Article-I.D.: peora.2449
Posted: Mon Sep 22 19:39:15 1986
Date-Received: Tue, 23-Sep-86 18:51:44 EDT
References: <1164@ncr-sd.UUCP> <2383@peora.UUCP> <683@ur-tut.UUCP>
Organization: Concurrent Computer Corporation, Orlando, Fl
Lines: 63

>--> Jon Krueger
>Virtual memory is a series of tradeoffs best described as "getting
>more out of less".  There will continue to be a need to do this
>indefinitely.  Yes, memory gets cheaper, but programs get larger even
>faster.  The demands of applications that benefit from or actually
>require more space will outpace the semiconductor industry's ability
>to provide physical memory at a given cost.  It will always be
>convenient or necessary for some applications on some hardware to
>permit code to execute in spaces larger than current or available
>physical memory.  For instance, my concept of a high-performance

	I don't agree that applications are growing faster than
	memory is getting cheaper. I might agree that the applications
	are growing faster than the amount of memory usually installed
	in most computers.

	Back in 78 it cost me $200 dollars to add 16KB to my Apple II.
	Last month I paid $401 to put 2 more megabytes in PC clone.
	That is a cost decrease of 64 and probably close to 100 if you
	allow for inflation. Are typical applications 100 times bigger
	than they were 8 years ago? I don't think so. 10 times maybe.

	Naturally computer makers expand the memory systems based on
	requirements, rather than memory cost, so the typical memory
	on a computer system has only increased from 64KB to the
	640KB-1MB range over the same period. This has the result of
	making memory a much less significant part of the system
	cost.

	This trend has been less reflected in mini and mainframe
	computers with their longer product lifecycles. One rather
	amusing result of this is that I have 3MB on my home computer,
	and only 4MB on my super-minicomputer at work.

	I agree that virtual memory is a tradeoff, but it my contention
	that tradeoffs that made sense back in the 60s make a lot
	less sense today and will make even less sense tomorrow.

	The cost of disk storage hasn't gone down nearly as fast as
	the cost of memory. Especially the kind of high performance
	drives that make good paging devices. You can have virtual
	memory without fast disks, but then you need more memory
	to maintain equivalent performance levels.

	I will agree that for any given memory N, there will always
	be applications that won't run in N. But as time goes on these
	applications will more and more be oddballs, rather than the
	mainstream. There will niche computers to run them, but the
	common run of applications won't need this kind of support.

	This is not to say that future processors won't support
	virtual memory. Address translation/ memory protection
	support is useful on any multitasking system, with or
	without virtual memory and the incremental cost for
	hardware to generate page faults isn't much. A lot
	of people seem to be excited about the 80386, but not
	because of virtual memory, but because of better support
	for multitasking.
-- 
     Joel Upchurch @ CONCURRENT Computer Corporation (A Perkin-Elmer Company)
     Southern Development Center
     2486 Sand Lake Road/ Orlando, Florida 32809/ (305)850-1031
     {decvax!ucf-cs, ihnp4!pesnta, vax135!petsd, akgua!codas}!peora!joel