Subject: SGI R8000: GROMOS and compiler flags (summary of responses)

From: [email-masked] (Pieter Stouten)
Date: Mon, 10 Jul 1995 00:08:46 -0500

On Fri 30 Jun 1995, I asked the following questions about running GROMOS on
SGI R8000's.

|1) What Fortran compiler flags does one have to use to generate optimal
|   GROMOS single-processor executables? I have heard that the standard
|   flags do not generate code that takes advantage of the R8000's
|   architecture. Is that true?
|
|2) I intend to run GROMOS on an 8 processor (R8000's) Power Challenge. Is
|   there a parallel GROMOS version that also takes advantage of the R8000's
|   architecture? About four years ago Terry Clark had a parallel Stellar
|   version. Does anybody know what happened to that version? Is there another
|   (better?) version that will work on the Power Challenge?

I received many useful responses from the following people: Daan van
Aalten, Jeff Blaney, Terry Clark, Volkhard Helms, Xinjun Hou, Philippe
Hunenberger, Joe Leonard, Andy McCammon, Alain Nanzer, Bill Ross, Vladimir
Shcherbukhin and Shankar Subramaniam. Thank you all.

1) A wide variety of compiler flags seem to affect the performance of the
   code and I got many different recommendations. The concensus was that
   in order to make use of the R8000 architecture, one must use the
   following flags:
      -O3 -mips4 -lfastm -OPT:roundoff=3:IEEE_arithmetic=3
   Other flags that were frequently mentioned as influencing performance
   and accuracy were:
      -old_rl -r8 -TENV:X=3 -SWP:strict_ivdep=OFF (on IRIX6.1)
   I cannot vouch for the correctness of the above recommendations as I
   have not tested different settings myself (yet), nor do we seem to have
   the relevant man pages (?!), but it is evident that not specifying any
   flags results in code that is not optimized for an R8000.

2) If one wants to run GROMOS parallel on relatively few processors,
   there are two options: UHGROMOS and GROMOS_XL. The former is developed
   by Terry Clark <[email-masked]> at the University of Houston.
   For more details contact Terry directly. GROMOS_XL is a GROMOS version
   whose time-critical routines were rewritten by Michael Schlenkrich
   (SGI, Basel) and which is distributed (but not guaranteed) by the van
   Gunsteren group (contact Philippe Hunenberger <[email-masked]>).
   Extensive benchmarking results are available for UHGROMOS on KSR-1, and
   modest ones for GROMOS_XL on an SGI PowerChallenge, but I have not seen
   any data for UHGROMOS on a PowerChallenge. Unfortunately a direct
   comparison of their relative performance on a PowerChallenge has not
   been carried out (yet?).

Hope this helps, Pieter.

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