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From: hes@ecsvax.UUCP (Henry Schaffer)
Newsgroups: net.arch
Subject: Re:  Memory Rules of Thumb
Message-ID: <794@ecsvax.UUCP>
Date: Fri, 22-Nov-85 22:48:39 EST
Article-I.D.: ecsvax.794
Posted: Fri Nov 22 22:48:39 1985
Date-Received: Sun, 24-Nov-85 06:19:50 EST
Distribution: net
Organization: NC State Univ.
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<>I was intrigued by this topic, and asked a colleague for his comments.
He is John Stephenson at TUCC who is an expert on large IBM systems
using the OS/MVS operating system.  

>I think that if anyone can invent a rule of thumb for memory size as
>discussed by 
>Shein in net.arch that it will apply only to a certain operating system
>and type of work.  With mvs the rule is simply that if the wait light
>in not out and the i/o system is running hard, especially paging hard,
>then memory is the number one candidate for addition.  Of course, since
>the paging system is simulating memory, faster paging is a possible
>alternative but there is an additional, largely unknown, cpu cost to
>paging.  Here relative costs enter the picture and since they change
>all the time one's thumb is likely to get sore.

  Note that IBM mainframes have a "wait light" which goes on when the
cpu can't do anything because e.g., it is waiting for the completion of
I/O.
--henry schaffer