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Path: utzoo!watmath!clyde!cbosgd!ihnp4!nsc!amdahl!rtech!daveb
From: daveb@rtech.UUCP (Dave Brower)
Newsgroups: net.arch
Subject: Re: Memory Law
Message-ID: <746@rtech.UUCP>
Date: Sat, 16-Nov-85 04:54:17 EST
Article-I.D.: rtech.746
Posted: Sat Nov 16 04:54:17 1985
Date-Received: Mon, 18-Nov-85 05:13:09 EST
References: <764@bu-cs.UUCP>
Organization: Relational Technology, Alameda CA
Lines: 34

> Sooooo....I have been trying to come up with a reasonable rule of thumb
> for how much memory is too much (?!) It will go something like this:
> 
> 	Don't buy more memory than your CPU can zero out in N seconds.

There's handwaving here that comfy values are between 5 and 8 Meg/mip.
Any less and you're plainly memory starved, and above that you may get
diminishing returns.

A maxi-memory opinion is put forth by Steve Wallach in this month's
Unix Review.   He's talking about 100M systems:

   "Do you really have to have all that memory?  Yes, even for UNIX.
   ...  With all this physical memory, we can make the disk cache as
   big as we like, so when we run up against I/O benchmarks, we just
   define a disk cache large enough to keep us from ever having to
   go to disk.  ... Some people cry, 'Foul!  That's not a fair 
   benchmark because I can't do that on my VAX'--to which, of course,
   we respond, "Right."  Then we smile and don't say anything more."

The problem with this is that it doesn't represent very many real job
mixes.   On a 750 with 8 meg you're probably running out of gas in the
CPU, which is where Barry's N sec to clear comes in.  

You probably also need to take into account the I/O bandwidth.  370-ish
systems seem to be able to handle more memeory than the CPU speed would
indicate because of the bandwidth available with the channel I/O.

Remember when 1k S100 cards were $400?

-- 
{amdahl|dual|sun|zehntel}\		|
{ucbvax|decvax}!mtxinu---->!rtech!daveb | "Something can always go wrong"
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