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From: rjn@duke.UUCP (R. James Nusbaum)
Newsgroups: net.arch
Subject: Reasons For Large Main Memories
Message-ID: <8494@duke.duke.UUCP>
Date: Sat, 30-Aug-86 12:40:33 EDT
Article-I.D.: duke.8494
Posted: Sat Aug 30 12:40:33 1986
Date-Received: Sun, 31-Aug-86 19:52:16 EDT
Reply-To: rjn@duke.UUCP (R. James Nusbaum)
Distribution: na
Organization: Duke University, Durham NC
Lines: 22


Some very good reasons for large main memories seem to have been ignored
in the recent discussion.  For a multiprocess machine large main memories
obviously eliminate paging, which is a significant system overhead.  There
are also certain single user systems which can easily use 10 meg or more
of address space.  These systems are Lisp systems.  It is easy and in fact
desirable to have a very large main memory so that the entire system can
reside in silicon and not on disk.  This very problem crops up in HP's new
line of AI workstations based on 68020s with 8 meg of main memory.  A simple
application can easily build a core image of well over 8 meg.  When it comes
time to garbage collect, the garbage collection process must swap pages in
and out of main memory.  Since the disks in these machines are fairly slow
garbage collection can take 30 or 40 seconds.  Paging can also cause problems
when traversing a large data structure (data structures of over 2 meg are
easy to build) which resides on multiple pages.  While it is true that
sophisticated software can overcome some of the paging problems, in many
cases the cost of more memory is less than the total cost of developing
this software.  In todays world silicon is often cheaper than programmer time.

Jim Nusbaum
Duke University CS Dept.
Durham,  NC