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From: earl@BRL-VAT.ARPA (VLD/ATB)
Newsgroups: net.unix
Subject: Re:  Convex C-1 computer?
Message-ID: <10188@brl-tgr.ARPA>
Date: Wed, 24-Apr-85 13:34:19 EST
Article-I.D.: brl-tgr.10188
Posted: Wed Apr 24 13:34:19 1985
Date-Received: Fri, 26-Apr-85 22:18:19 EST
Sender: news@brl-tgr.ARPA
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You'd best check with somebody at Convex & run your benchmarks there to
ascertain those numbers.  But here is some info based upon our
initial findings:

1.  A vectorizing C compilier does not exist for this machine, thus
you will not be able to take advantage of the vector hardware using C
unless you can kludge up some kind of calls to Fortran vector subroutines.
Convex says that a vectorizing C compiler is in the works and should appear
in about a year.

2. The vectorizing Fortran compiler is still in development.  The optimizing
option still has some bugs; sometimes simple code compiled with the
highest optimization causes error reports, but moving down one optimization
level works OK.  These bugs are being fixed when encountered.

3. The non-vector operational speed is not too impressive.  We've seen
only 3.5 to 5.5 speedups over our VAX 750.  Therefore, if your code is
either written in C or non-vectorizable Fortran, you will not get a
10 Mip machine.

4.  However, if you're starting a program from scratch, (assume Fortran
for the time being...) and you can structure it to take advantage of the
vector architecture (using arrays that are long & not
wide instead of wide & long), do not call any subroutines in the loop,
do not jump out of the loop (terminate prematurely) [for example, set
a flag and when the loop completes check the flag...], and other practices
appropriate to vector machines, the machine will supposedly get up to
40 Mips.  Of course, that's a lot of "ifs".

5. Convex also has plans for improving performance, but I don't know
details or schedules.

6. Convex's Fortran compiler is generally pretty decent in that we
made a tar tape of a program & subroutines (about 130 modules in
all) with our 4.2/System V Vax 750 and were able to read it into the
Convex, get it to compile and run with relatively little difficulty
in a few hours.

7.  The machine itself is nice and compact and requires
no special site considerations (other than clean air and free air movement).


I have heard (from Convex, of course) that they have taken some Fortran
programs (not written with vectorization in mind originally) and by making
minor changes to exploit the compiler vectorization (such as redimensioning
arrays, etc) have gotten 17-19 Mips from the machine. Some of our
heavy Fortan users (read that "not pro-unix number crunchers") estimate
the Convex to be 1/8 of a Cray.

The best approach, of course, is to take (or send) your favorite applications
codes to Convex & benchmark it.  If you're used to running on a Cyber 205
or Cray, then you'd probably be very happy with the performance considering
the price.  But if you're a typical Unix C user, then you might not
be happy.