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From: bzs@bu-cs.BU.EDU (Barry Shein)
Newsgroups: net.arch
Subject: Re: VERY LARGE main memories
Message-ID: <1130@bu-cs.bu-cs.BU.EDU>
Date: Wed, 27-Aug-86 23:01:03 EDT
Article-I.D.: bu-cs.1130
Posted: Wed Aug 27 23:01:03 1986
Date-Received: Thu, 28-Aug-86 06:25:19 EDT
Organization: Boston U. Comp. Sci.
Lines: 78


>From: mc68020@gilbbs.UUCP (Thomas J Keller)
>   Someone please correct me if I am wrong, but as I have been lead to 
>understand the situation, it will prove somewhat difficult to successfully
>implement large physical memory systems on the order of 1Gb.  The primary
>impediment seems to be the delays caused by propagation delays in the
>decoding trees.   Anyone care to enlight me (us)?

>From: johnson@uiucdcsp.CS.UIUC.EDU
>I believe that the large memory computers are designed for database
>applications.

I'm not sure you two are wrong, but I'm not sure you're right either.

The Cray-2 (certainly a number cruncher) comes with around 2GB of
main memory. The recently announced ELXSI (more like a $200K [entry]
machine if I read the article right) boasts a maximum 1GB configuration
(I figure you can buy the ELXSI on the volume discount on the memory
to fill it [~$1M list], but I wander.) Again, a number cruncher I
believe.

So, for what it's worth these are essentially counter-examples of
some value.

As I brought up once before, I still think there may be some constant
N which completes the sentence "never buy more memory than you can
zero out in N seconds" [I call it Shein's law of memory but some have
claimed that Amdahl may have said something similar, great minds run
in the same gutters :-]

The reasoning is if you can't touch it in N seconds you probably can't
use it very effectively either. For some more thoughts on this you
might want to pick up Danny Hillis' "The Connection Machine" where he
paints an interesting vision of modern computers as vast seas of
inactive silicon (the memory) with this (typically) one poor little
CPU touching one or two spots per cycle.

Of course, if you spend most of your time waiting for disk vast
memories may help, but so would (and does) clever memory/disk
scheduling (within limits.)

The point is it is not clear that increasing memories unbounded
produces unbounded performance gains, in fact, it almost certainly
doesn't.

You need a CPU (or more than one) to do something with all this
wonderful stuff you have in memory.

Before you all jump down my throat because you are sure that
if you had 16MB rather than 8MB on your machine and hence more
*must* be better consider:

A one MIP machine zeroing memory in a loop:

		CLRL	R1
	LOOP:
		CLRL	(R1)+
		CMPL	R1,HIMEM
		BNE	LOOP

would (theoretical machine) take 3 * (1G/1M) or 3000 seconds or
a little less than one hour to complete. It's hard to believe
such a machine could make -effective- use of that much memory.

I know, it's debateable, but anyone arguing against that statement
is probably ignoring any rational concern for cost/benefit trade-offs
(eg. spend $1M on the memory and $200K on the processor, or $1M on
the processor and $200K on the memory? or some similar variation.)

Of course, assuming the memory were free and reasonably random behavior
I agree that a huge memory would have some value to a database application
that filled the memory, but I doubt it would be a reasonable thing to
do unless memory prices dropped drastically. You'd probably be better
off putting your money into the processor (the context of the argument
seemed to imply smaller processors, obviously Cray is already up against
that limit.)

	-Barry Shein, Boston University