🚀 go-pugleaf

Cray users often like to boast that very large main memories such as the
Cray 2 obviate the need for virtual memory.  J. Giles stated recently that
Cyber 205 users "turn off" virtual memory.  I work at a site which has a
Cray X-MP, a Cyber 205, and a Cray-2 (Ames Research Center).  I would like
to state for the record that there are considerable advantages to having a
machine have memory mapping virtual memory architecture, even though there
is no apparent single job performance advantage.  

Picture a very large (almost all of real memory) batch job running on a Cray.
Suppose that there is some interactive debugging going on (The Cyber 205 VSOS
operating system and the Cray CTSS operating system are both interactive and
have very nice interactive debuggers.)  The large batch job will have to be
written out to disk in entirety on the Cray before the small interactive job
can be rolled in.  That might take tens of seconds on a Cray 2.  What if only
a few pages had to be paged out as on the Cyber 205?  The batch job would
proceed with only a few pages missing, and the total swap time in any case
would be only a few tenths of a second.  There are many hypothetical examples
which can be imagined which are confirmed, in my experience, in real life:

All other things being equal, virtual memory systems have better throughput
than non virtual memory systems of equivalent memory size and CPU speed.

Virtual memory systems are better suited to mixed large batch and interactive
loads, and are capable of better response time at an equivalent overhead than
non virtual memory systems.

Anyone who has had to deal with setting policies for user memory allocations
on a Seymour Cray machine (from CDC 6600 days to the Cray 2) will know what
kind of trade offs are necessary and why often these machines have been run
batch only with users limited to half of the overall available memory.

Now, a reasonable question to ask on a fast machine is, how much CPU real
estate is going to be consumed by dynamic address translation hardware,
because added logic is going to slow a very fast machine down.   Cray has
kept his machines very simple architecturally, and has a much smaller 
number of gates than other comparable machines (e.g. Cray 1's and 2's
run between 600000 to 700000 gates per CPU, while the Cyber 205 has about
1.3 million, roughly twice as many), which may help explain why Cray has
always had the fastest clock in the supercomputer business (another factor
is that Cray is obviously a packaging genius).  

Conclusion:  An computer architect always has to trade off design features
to build a real machine, and in some cases virtual memory may have to be
traded off, but in the real world of operating a machine, virtual memory
is a significant advantage, other things being equal.

   Hugh LaMaster, m/s 233-9,   UUCP:  {seismo,hplabs}!nike!pioneer!lamaster 
   NASA Ames Research Center   ARPA:  lamaster@ames-pioneer.arpa
   Moffett Field, CA 94035     ARPA:  lamaster%pioneer@ames.arpa
   Phone:  (415)694-6117       ARPA:  lamaster@ames.arc.nasa.gov

"The reasonable man adapts himself to the world, the unreasonable man
adapts the world to himself, therefore, all progress depends on the
unreasonable man." -- George Bernard Shaw

("Any opinions expressed herein are solely the responsibility of the
author and do not represent the opinions of NASA or the U.S. Government")