Practical Parallel Computing

396 Pages·1994·21.039 MB·English

by  H. Stephen Morse (Auth.)

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xxiivv PPrreeffaaccee
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Preface XV
comes perilously close to: If you aren't really sure you need massively parallel
computing, you had probably better think twice before getting involved with
it. On the other hand, there has also been an attempt to debunk some of the
more egregious and technically unsupportable charges sometimes made
against parallel machines (see, for example, the discussion of Amdahl's Law
in Chapter 2). The book is liberally sprinkled with examples and case stud-
ies. The tone is, at times, a trifle irreverent—the result of dealing in a field
that is filled with the most extravagant claims and counterclaims, charges and
countercharges. Still, despite these disclaimers, I hope the reader will also
sense my genuine admiration for the technical pioneers who have made pap
allel computing a reality and who continue to work at the difficult remaining
tasks of making it more broadly available and usable.
HSM
May 1994
Ac knowledgments
The person who got this project going in the fall of 1992 was Tom Wheeler,
at that time a senior technical manager with American Express. MRJ, Inc. had
done some work for Tom on a consulting basis, and he approached the com-
pany with the idea of writing a volume on parallel computing as one in a se-
ries on open systems for which he was the editor. Tom was tragically afflicted
with a brain tumor shortly after work began and died in the spring of 1993.
Without Tom's initial impetus and vision, the book would never have been
written.
Another shock came in the spring of 1993 when the intended publisher
underwent a major reorganization. At that point, Alan Rose of Intertext took
the book under his care and eventually found it a home at AP PROFES-
SIONAL. Being novices in the publishing world, MRJ is very grateful for Alan's
help and patience. Without his initiative, the project might have foundered
for lack of publishing interest.
As author, I am indebted to the management at MRJ for their support dur-
ing the writing. Particular thanks go to my Division Manager, Scott Miller, who
gave me flexibility in my schedule to free time for writing; and to Dr. Ed
McMahon, President of MRJ, who made funding available to support the
project. Ed also read through the entire draft manuscript and made a num-
ber of perceptive and valuable suggestions.
The example programs shown in Chapters 4 through 7 are intended to be
compilable and runnable on the target machines. In many cases, the codes
were actually prepared by technical staff from the vendors involved, and in
all cases the sample code, comments, and annotations were reviewed by the
vendors for accuracy. The companies involved were:
Silicon Graphics, Inc. (Power C on the SGI Challenge)
nCube Corporation (nCube 2)
ParaSoft, Inc. (Express)
Intel Corporation (Paragon)
xvii
xviii Acknowledgments
MasPar, Inc. (MPL on the MP-1)
Thinking Machines Corporation (C* on the CM2 and CM5)
Scientific Computing Associates (Linda)
Many thanks to these companies and to their technical staff for helping to
create and review these programming examples.
In many ways, the real heroes behind this book are the members of the
parallel computing technical staff at MRJ. We are small, but we are very good;
the material in this book owes its immediacy and relevance to lessons learned
from real application development on real parallel equipment. Nothing focuses
the attention like having to meet a deliverable—on time and in budget! Spe-
cial thanks go to Steve Geyer, Alan Broder, and George Wilson who read por-
tions of the text and tempered my more extravagant claims (e.g., on debug-
ging) with a healthy dose of common sense and experience.
Finally, thanks to my wife, Mary, and my family for patiently enduring the
emotional vicissitudes of authorship over the past 18 months.
P AR T I
Preliminaries
The three chapters of Part 1 constitute an introduction to the subject of pap
allel processing. The current status is examined in Chapter 1, including dis-
cussions of the performance potential of these machines and the reasons why
they have, thus far, been so unsuccessful in the marketplace. Chapter 2 con-
siders the merits of a commonly cited charge against parallel processors—that
there are theoretical bounds on how many processors can be used efficiently
on a single problem. The analysis concludes that the size of the problem be-
ing solved must enter into such a calculation in a critical way often overlooked
by advocates of sequential and vector machines. Finally, Chapter 3 provides a
tutorial introduction to the three major classes of parallel architecture and
points out strengths and weaknesses in each case.
1
The Status and Future
of Massively
Parallel Processing
Asked to complete the sentence, "Parallel processing is ...," an honest tech-
nical assessment necessarily includes the characterizations "... painful" and
"... inevitable."
If we view the vendors of high-performance computing as residing in two
camps—traditional vector and sequential processing faced off against upstart
parallel processing—both camps have strong vested interests in exaggerating
or minimizing estimates of the kinds and levels of pain. The strong depend-
ence of this pain both on the application and on the operational environment
will defeat any "one size fits all" resolution of these opposing claims: There is
justice on both sides. However, on the issue of inevitability there is broad agree-
ment. Major vendors of traditional architectures have active R&D programs
focused on the development of hardware and software for massively parallel
processors (MPP). This amounts to a tacit admission, marketing hype aside,
that they, too, have seen the technical handwriting on the wall.
For the most part, we will concentrate on the pain of parallel processing.
What are the sources (expected and unexpected)? How can they be avoided
or at least minimized? What characteristics of an application or an operational
environment might facilitate a transition to parallel processing? At the out-
set, however, it is appropriate to consider carefully the inevitability of this tech-
nology. If MPP is inevitably in an organization's future, an understanding of
3