Path: utzoo!utgpu!water!watmath!clyde!rutgers!cmcl2!husc6!mailrus!umix!uunet!steinmetz!sunset!oconnor
From: oconnor@sunset.steinmetz (Dennis M. O'Connor)
Newsgroups: comp.arch
Subject: How much is enough? (was Re: More than 32 bits needed where?)
Message-ID: <9629@steinmetz.steinmetz.UUCP>
Date: 20 Feb 88 15:40:41 GMT
References: <15781@beta.UUCP>
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Organization: GE Corporate R&D Center
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In re the how-big-an-address-space-do-you-need question :

  Well, for PHYSICAL space, I am reminded of yet another "Amdahl's Law":
    X "MIPS" == X Megabytes == X Mbits/sec I/O.
  The opinion is that this indicates a computer system well-balanced
  for general-purpose computation. It assumes virtual memory space
  in excess of the physical memory space, and that the paging device
  is part of that high-speed I/O and does NOT have a high latency
  ( like paging over Ethernet appears to sometimes have ).

  Since what a "MIPS" is is fuzzy, obviously the "identity" above
  will be fuzzy as well. But it seems to be within a factor of 4.

  So, SUN-4s should have (@7"MIPS") 8 to 32 MBytes of RAM.
  A Cray XMP (@800"MIPS"?) should have maybe 800 to 3200 MBytes.

  But what about Symbolics LISP Machines ? These claim to be
  1 MIP workstations ( the 3600s anyway ) but are NOT very
  happy with only 1-4 MBytes of RAM. Well, the "cop-out" is
  that they are NOT "general purpose" computing. Is a Cray ?

  Signal Processors are also NOT general purpose : they are
  happy with a good bit less than 1MByte/MIPS.

  My feeling on the matter is that as CPU performance
  climbs, RAM needs will climb as well. We'll need more
  than 32-bits of address space for 1000MIPS machines.

  If I was designing a microprocessor architecture today,
  ( I WAS designing one last year, but that's finished )
  I wouldn't let it worry me. Micros have already hit 40MIPS,
  and will hit 100MIPS in the next five years, but (fearless
  prediction ahead) MICRO-processors wont hit 1000MIPS until
  the year 2005 or so, if even then. How many microprocessor
  architectures from 1965 are still around today? :-)

  As for virtual address size : wellb beats me. Seems to me
  virtual memory is more like closet space than physical
  memory is. If you've got it, you'll use it.
--
	Dennis O'Connor 	oconnor@sunset.steinmetz.UUCP ??
				ARPA: OCONNORDM@ge-crd.arpa
    "Nuclear War is NOT the worst thing people can do to this planet."