Can a large RS/6000 easily replace a Convex?

[John D. McCalpin]

>>>>> On 3 Jun 91 14:31:34 GMT, johan at dutnak2.tudelft.nl (Johan Haas) said:

Johan> We are considering to replace our current Convex C1 with a mix
Johan> of RS/6000 systems instead of upgrading to a Convex C2.

This is a good idea.

Johan> I would like to hear if e.g. the model 550 is any good to serve
Johan> as a machine for, say, 8 PhD. students (like myself).  We do a
Johan> lot of program development, but also do finite difference
Johan> simulations that run easily 10 hours on our C1.

You should not have any trouble with such a small number of users if
you have adequate memory and swap space.

Johan> I know that the MIPS/MFLOPS performance of the 550 is superior
Johan> to that of the Convex, but there are rumours that the multiuser
Johan> performance of RISC machines is very bad. Is this true?

There is no reason to call the performance "very bad".  The reviews
that I can recall were disappointed with the multi-user performance
because it was no better than other machines with (for example) half
the MIPS rating of the IBM.  Unless your 8 users each insist on
running *lots* of active processes all the time and all at the same
time, you should not have trouble.

Johan> Another issue is the single precision Floating Point
Johan> performance that --according to a lettr to Unix World-- can be
Johan> worse than the double precision performance. We do most of our
Johan> work in single precision.

It is regularly observed that single-precision (32-bit) arithmetic
runs between 10% slower and 10% faster than double precision (64-bit).
The moral is that (unless you are out of memory) you can switch to
64-bit for "free", with resulting increases in accuracy and
reliability.   But even if you run in 32-bit precision, you should not
expect to see a penalty of more than 10-15%, and you might see a
slight speed improvement --- it depends on how well the compiler can
overlap the precision conversions for your application with other work
that has to be done anyway....  Try one of your own applications and
see what happens....
--
John D. McCalpin			mccalpin at perelandra.cms.udel.edu
Assistant Professor			mccalpin at brahms.udel.edu
College of Marine Studies, U. Del.	J.MCCALPIN/OMNET