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From: lamaster@nike.uucp (Hugh LaMaster)
Newsgroups: net.arch
Subject: Re: VERY LARGE main memories
Message-ID: <627@nike.UUCP>
Date: Wed, 24-Sep-86 14:39:28 EDT
Article-I.D.: nike.627
Posted: Wed Sep 24 14:39:28 1986
Date-Received: Wed, 24-Sep-86 22:15:47 EDT
References: <1130@bu-cs.bu-cs.BU.EDU> <7144@lanl.ARPA> <7148@lanl.ARPA> <609@nike.UUCP> <7839@lanl.ARPA>
Sender: usenet@nike.UUCP
Reply-To: lamaster@pioneer.UUCP (Hugh LaMaster)
Organization: NASA Ames Research Center, Moffett Field, Calif.
Lines: 35
Keywords: virtual memory

One important point that I forgot to mention in previous posting:
On a machine with memory mapping (Cyber-205 for example) there is only a very
small penalty for reclaiming a small amount of memory for another task.  On a
very large main memory system, this is an important point.  A 256 MW Cray-2
(with approx. 4 ns clock) would take 1 second to completely copy main memory.
Unfortunately, this is exactly what is required to reclaim memory on the Cray-2
or any other machine which requires memory to be contiguous.  This is an 
important contribution to system overhead, even in batch mode, but can become
a major bottleneck.  This combines with the already mentioned problems to
produce effectively even poorer memory utilization, because there is a limit
on how frequently memory can be packed in order to make available space
usable.  That there is no such problem on a virtual memory machine is an
additional benefit to virtual memory.  J. Giles is correct in stating
that if single job STEP (or single program) speed is the most important
criterion, there is probably no advantage to a virtual memory machine, as
long as the individual job steps take hundreds of seconds or more.  And, as
stated earlier, there is a price to pay for virtual memory:  the extra 
cpu logic it takes to implement it, which undoubtedly slows the cpu by
some amount.  It may be interesting to note that CDC gave the future 
existence of very large main memories as one of the reasons for designing
the virtual memory architecture of the new Cyber 800 and 900 series (NOS/VE)
machines as they did.  They stated that memory management of main memory 
would be too inefficient without it.

   Hugh LaMaster, m/s 233-9,   UUCP:  {seismo,hplabs}!nike!pioneer!lamaster 
   NASA Ames Research Center   ARPA:  lamaster@ames-pioneer.arpa
   Moffett Field, CA 94035     ARPA:  lamaster%pioneer@ames.arpa
   Phone:  (415)694-6117       ARPA:  lamaster@ames.arc.nasa.gov

"The reasonable man adapts himself to the world, the unreasonable man
adapts the world to himself, therefore, all progress depends on the
unreasonable man." -- George Bernard Shaw

("Any opinions expressed herein are solely the responsibility of the
author and do not represent the opinions of NASA or the U.S. Government")