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From: jlg@lanl.ARPA (Jim Giles)
Newsgroups: net.arch
Subject: Re: VERY LARGE main memories
Message-ID: <7148@lanl.ARPA>
Date: Fri, 5-Sep-86 17:49:18 EDT
Article-I.D.: lanl.7148
Posted: Fri Sep  5 17:49:18 1986
Date-Received: Fri, 5-Sep-86 22:11:05 EDT
References: <1130@bu-cs.bu-cs.BU.EDU> <7144@lanl.ARPA>
Reply-To: jlg@a.UUCP (Jim Giles)
Organization: Los Alamos National Laboratory
Lines: 47

In article <7144@lanl.ARPA> jlg@a.UUCP (Jim Giles) writes:
>The task of zeroing memory on the Cray 2 with 256MW (=2GB) should take
>less than 5-10 seconds.  You must remember that a one MIP machine is
>pathetically slow compared to a Cray (at least for vector operations
>like zeroing memory, searching, sorting, and many scientific
>applications).
>

I made a slight error (by an order of magnitude) - The Cray 2 should
take about .5 to 1.0 seconds to clear all its memory.

It seems that large memory machines are obviously desireable.  In
lattice guage theory a single lattice could easily use all the Cray 2
memory. 3-d hydro codes also use enormous ammounts of memory.
Furthermore, applications like these make frequent references to the
entire data structure (ie. the whole array is updated every single
time-step).

The desirability of paging for such machines is not so obvious.
Consider a code which updates a large array on each step through a
loop (each time-step).  If the central memory is too small to hold the
entire array and you have a virtual memory scheme, some part of the
array will get swapped out on each time step.  Most likely, it will be
the least recently used page that gets swapped - the very one that you
will need first on the subsequent time step!  You are now in a
situation of chasing you tail around memory - losing time all the
while.

Without virtual memory though, your code can anticipate the problem by
initializing asynchronous I/O long before it needs to use the data.
And, since it's not driven by page faults, you can select only a
particular part of the array to be swapped - thus minimizing I/O.
This kind of programming effort is somewhat unfasionable these days,
but it's exactly the sort of thing that most programmers that use
these big machines immediately do.  They bought the machine because
the critical issue was SPEED - and anything that reduces this speed
(like virtual memory) is to be shunned.  (Cyber 205 users usually
turn off the virtual memory when they need speed, Crays don't even
have virtual memory.)

On the other hand, a small memory - single user machine (like my SUN
workstation) should never be built without virtual memory.  The
desirability of a feature should always be driven entirely be the
purpose of the machine.

J. Giles
Los Alamos