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From: mfb@mbunix.mitre.org (Michael F Brenner)
Subject: Re: CPU & Memory opinions wanted...
Date: 1997/01/22
Message-ID: <5c5ja9$log@top.mitre.org>#1/1
X-Deja-AN: 211508142
references: <5c322v$ag2@faatcrl.faa.gov>
organization: The MITRE Corporation, Bedford Mass.
keywords: average reserve memory CPU realtime
newsgroups: comp.lang.ada
summary: if you dont need the reserve you can average them
Date: 1997-01-22T00:00:00+00:00
List-Id: <comp.lang.ada>


Mike Brenner of MITRE has the following opinion (the following 
sentence must be quoted in its entirety):
Adding the reserves up and dividing by two is mathematically valid under
two circumstances: (1) perfect independent sharing of work and resources
with no overhead; and (2) logically frivolous reserve requirements;
since you did not say you had any realtime deadlines to meet, we 
presume there are none, so the reserve requirements may be frivolous,
and your procedure  might work; however, it increases the risk
substantially that you are developing on an old, slow, small-RAM
system that will thus be more expensive to maintain (enhance) later.

When a reserve requirement is arbitrary it is more common to ask for
a waiver, so the customer knows they are getting a cheaper system,
a working system, a system with a higher risk that last minute error 
corrections will not fit, and a system for which they will have to budget
hardware additions in order to add substantially to the software. By
being up front about the pluses and minuses of not having enough reserve
on CPU A and plenty of reserve on CPU B, a more correct risk analysis
is applied than averaging numbers that are not related. If final
system testing results in a 15% increase in software size on CPU A,
it is the 14% reserve on that CPU that will kill the system, and the
30% average reserve across all boards cannot save you.