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From: ddr@cs.ed.ac.uk (Doug Rogers)
Newsgroups: comp.arch
Subject: Re: Amdahl's Rule
Message-ID: <11299@skye.cs.ed.ac.uk>
Date: 23 May 91 09:31:49 GMT
References: <48qO01yL75GS00@amdahl.uts.amdahl.com> <1991May19.025126.24526@netcom.COM>
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Organization: Department of Computer Science, University of Edinburgh
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Amdahls law on memory must be thought of as pragmatic. There is no simple causal relationship that would suggest this ratio of between 1 and 2 Megabytes of
data per MIP. Two effects are at present anyway pulling in opposite directions.
The use of RISC machines demands more memory and the use of shared libraries reduces
memory needs. I think the DEC machine does not use shared libraries as is the case
for the HP offerings at present. Both these machines therefore require an excessive
amount of memory compared to the SUN machines. The size of memory is anyway dominated by the memory access time versus swap time. Speed up all the parts of the machine simultaneously and the machine will run proportionaly faster. Here I think the introduction of file servers and/or the use of discs which are not significantly faster than 5 years ago has forced the holding of all the cde in memory. ie page fualting is now very expensive.

The argument of cost also does not work as the relative cost of the system components has been changed dramatically. I wonder if the reason the ratio is being kept is because it is being designed for?

-- 
Douglas Rogers                     JANET: ddr@uk.ac.ed.lfcs
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