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From: bzs@bu-cs.BU.EDU (Barry Shein)
Newsgroups: net.arch
Subject: Re: VERY LARGE main memories
Message-ID: <1132@bu-cs.bu-cs.BU.EDU>
Date: Fri, 29-Aug-86 17:45:42 EDT
Article-I.D.: bu-cs.1132
Posted: Fri Aug 29 17:45:42 1986
Date-Received: Fri, 29-Aug-86 21:43:16 EDT
Organization: Boston U. Comp. Sci.
Lines: 40


>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.

T'was I. This is a slightly different topic (virtual memory systems)
but I am quite sure that you can hit this limit easily on a 750 (eg.),
I've suspected my 8MB VAX750 (4.2bsd) of having this problem (too much
time spent in the kernel due to virtual memory management) tho I've
never had the time to investigate (anyone?)

Of course, there's the bridging argument that no matter how much
memory you have it's just a matter of time before you wish it had
virtual memory management. I won't argue this either way but I believe
there is an amusing anecdote told around a famous institution about a
certain famous individual who proclaimed some years back, upon the
arrival of their 256KW [36-bit] memory, that now they have more memory
than anyone can possibly use.

I already know of people in two applications areas that believe they
could easily fill 1GB main memories with their current needs (a
database and a graphics animation shop.)

	-Barry Shein, Boston University