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From: phil@osiris.UUCP (Philip Kos)
Newsgroups: net.arch
Subject: Re: VERY LARGE main memories
Message-ID: <873@osiris.UUCP>
Date: Thu, 28-Aug-86 17:51:59 EDT
Article-I.D.: osiris.873
Posted: Thu Aug 28 17:51:59 1986
Date-Received: Thu, 28-Aug-86 23:42:34 EDT
References: <1130@bu-cs.bu-cs.BU.EDU>
Organization: Johns Hopkins Hospital
Lines: 39
Summary: other factors

A similar discussion came up a few months ago, in the context of
"how much memory is it reasonable to try to hang off my type X
machine?"  I expressed some confusion over conclusions drawn by
someone else (it may have been Barry Shein, whose article I am
replying to), who maintained steadfastly that there was a limit
beyond which it was not useful to go.  The argument really only
applies to machines with virtual memory, and it goes like this:

Your memory is organized in pages which are mapped from various
process's virtual spaces into the physical memory addressing.  It
takes a certain number of MM table entries to map a certain amount
of memory (considering a single VM architecture).  You want to
improve performance by reducing paging; it seems logical to do
this by increasing main memory.

Now.  By adding more main memory, you *do* decrease paging, which
is obviously a good thing.  However, you also make memory mapping
more time-consuming (more page table entries to maintain), so
there's a tradeoff here.  You might be able to expand the memory
to, say, 16M without taking any hits - the upper limit depends,
of course, on your VM archi- tecture.  Beyond this limit, the
page table has to be expanded, and maintaining it becomes more
complicated.  Eventually you reach a point where the time saved
by not paging is less than the extra time spent maintaining the
mapping table.  This is the point where you should just give up
trying to speed up your system by adding more memory.

Is this right?  It's sort of off the top of my head, and I may
have gotten some of the details wrong (what details, I hear you
ask?) but it seems to convey the gist of the argument...


Phil Kos			  ...!decvax!decuac
The Johns Hopkins Hospital                         > !aplcen!osiris!phil
Baltimore, MD			...!allegra!umcp-cs

"In the end there's still that song,
Comes crying like the wind
Down every lonely street that's ever been." - Robert Hunter