🚀 go-pugleaf

> Comments on Cyber 205, page faults, etc.

I suffered with a Cyber 205 for 6 or 7 years, and I know the machine
from a scientific user's viewpoint.  For just about all fluids 
codes, we found virtual memory to be essentially worthless.  For
out of core codes, we had to manage I/O by hand, and about all
virtual memory did for us was to enable us to create blocks of
virtual storage that we could move things in and out of "by hand"

As for it's architecture, the Cyber is a good case of why architectural
classifications aren't a good way to classify computers as "super"
vs. otherwise.  What made the Cyber a pain to program, and killed it
vs. the Cray, was that (1) it had a long pipeline, hence a big hit
for short vectors, which meant that codes had to be rewritten to
use very long vectors, (2) it couldn't execute vector instructions
unless data was contiguous in memory, so for non-unit strides, you
had to do scatter-gather.

The bottom line is that lots of codes never ran very efficiently
on our machine, because it was too hard to recode.  The IMSL
general eigenvalue finder ran about half the speed on the Cyber
205 as it does on my IBM RS/6000 model 730 (using tuned code).
The cache machines, like IBM, also have problems with non-unit
strides, but I'm finding that using strip-mining, it is relatively
easy to get around them, provided your algorithm has re-use of
data.

While we're on the subject of vector architectures, anybody
remember the Texas Instruments ASC?  It had its hardware problems,
but it had the nice feature that it had vector instructions for
multiply-dimensioned arrays, and the compiler (when it worked at
all, which wasn't often) used them intelligently.  This 
machine was ahead of its time, and went beyond what the real
technology could support, but I think its designers deserve to
get some credit.  Our old TI ASC at Princeton was junked and
went to Texas to be recycled, sharing a truck with some crates of
N.J. lettuce.  It's too bad the architecture was junked as well,
as it had a lot going for it.
.