Subject: RE: CCL:G:cache size affecting G94 calcs

From: [email-masked]
Date: Thu, 28 Aug 1997 12:21:57 CET

Dear Dr. Bausch,

I think you are right with your guess. In workstation-type computers
including well-known SMP machines, the bandwidth to main memory is a serious
bottleneck. It slows down any calculation tremendously for which the instructions of 
a loop plus the data it works on do not fit into the
processor cache.

An example:

A 75 MHz PowerChallenge with R8000 processors has a bandwidth of about
135 MBytes/sec to main memory (provided the other processors do not use the
common bus). The scalar product with two floating-point operations between 
R*8 vector or matrix elements requires the exchange of 32 bytes. Therefore 
the Mflops drop to 2*135/32 = 8.4 Mflops as soon as the data have to be 
fetched from and stored back to the main memory -- far below the nominal peak
performance of 300 Mflops. By bus interference it can become even worse.

That effect has been investigated in detail by John D. McCalpin, and he has
introduced the STREAM benchmark, which measures the bandwidth and the 
resulting Mflops for c(i)=a(i) (COPY), c(i)=d*a(i) (SCALE), c(i)=a(i)+b(i) (ADD)
and c(i)=c(i)+a(i)*b(i) (TRIAD). The programs and results are available 
via anonymous ftp from ftp.cs.virginia.edu in the directory /pub/stream/ (tables 
Balance.tbl, Bandwidth.tbl, and MFLOPS.tbl in /pub/stream/Tables/standard/).

The SPEC_95 benchmarks catch a bit of the memeory-bandwidth effect by using medium-
size test programs, whereas earlier SPECs reflect more or less only the cache 
performance. It would be extremely helpful for the Comp Chem community to agree
on a test suite of benchmarks with varying size (say, S, M, L, XL), preferably
GAMESS for ab initio because of its free availability and its being part of SPEC_95
and some MD program. Such a test suite would answer the numerous questions we are
seeing in the CCL again and again, and the Comp Chem community is commercially
important enough to convince all major computer manufacturers to determine and
publish the benchmark data already when announcing a product.

To all CCL subscribers: It is only a question of our agreement!
I would like to see comments (and encouragements).

Sincerely yours,

Wolf Weyrich
(Professor of Physical Chemistry)

-----------------------------------------------------------------------------------