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From: mash@mips.COM (John Mashey)
Newsgroups: comp.arch
Subject: Re: 64 bits
Message-ID: <40932@mips.mips.COM>
Date: 17 Aug 90 21:13:33 GMT
References: <5539@darkstar.ucsc.edu> <13285@yunexus.YorkU.CA> <30728@super.ORG> <9660@ganymede.inmos.co.uk> <224@csinc.UUCP> <GADBOIS.90Aug17183952@geier.philosophie.uni-stuttgart.de>
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In article <GADBOIS.90Aug17183952@geier.philosophie.uni-stuttgart.de> gadbois@cs.utexas.edu writes:
>In article <224@csinc.UUCP> rpeglar@csinc.UUCP (Rob Peglar) writes:
>
>   From: rpeglar@csinc.UUCP (Rob Peglar)
>   Date: 17 Aug 90 13:36:42 GMT
>
>   [...]
>
>   I know quite a few people who would be delighted to have a
>   microprocessor that had a 64-bit address (text) space.  They would
>   be even more delighted to have a 48-bit or 64-bit data space.  As
>   for "ya gotta have the disk to back this", sure, but the cost of
>   external storage (non-volatile) - french for "disk" - is falling
>   fast ($/GB).  Magnetic media are around $2x00/GB, shop around for
>   the best x.  Optical media is following the trend.  Even down and
>   dirty DRAM would be (probably) $70,000-80,000/GB; certainly
>   feasible for some people.
>
>I have noted this before, but it probably bears repeating:  While big
>address spaces are certainly desirable, don't forget that we are
>talking about powers of 2 here.  Assuming that media for backing store
>costs $1.00 per megabyte, enough to support a full 64-bit address
>space would set you back $17,592,186,044,416.00.  A 48-bit space at
>those rates would cost $268,435,456.00, and even a measly 40-bit (just
>one terabyte) one is over a million bucks.  Media costs are going to
>have to drop a lot for really big address spaces to be practical.

This is making an assumption that is untrue, i.e., that there is
rule that says you must have backing store >= virtual memory size.
1) For some classes of problems, you would never try to run them in
physical memory size low enough that you would actually page very much.
2) UNIXes are generally doing better about having less static allocation
of swap space.
3) Maybe somebody can cite the reference, but I recall a paper a fews back
from Gould talking about this exact issue, in some USENIX conference,
where people wanted to run big simulations without needing so much
swap space.
4) Finally, as pointed out before, there are problem classes for which
the practicality is determined by the ability to get more than 32 bits
of virtual address space, regardless of how much physical space is
needed behind it to make performance reasonable.
-- 
-john mashey	DISCLAIMER: <generic disclaimer, I speak for me only, etc>
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