[Beowulf] breaking Amdahl's law

Eugen Leitl eugen at leitl.org
Wed Jun 19 02:25:48 PDT 2013


Breaking The Law

Posted: 05-22-2013 17:00

As a theoretical physicist, I am fascinated by “fundamental” laws of physics
describing basic phenomena of nature. Many of these laws are tested through
numerous experiments and are “valid” within the constraints defined by these
experiments. In this sense we “believe”, for example, in Newton's law of

However, any further experiment might challenge our fundamental concepts.
Sometimes, they even can be broken. For example. Michelson's interferometer
experiment questioned some of Newton's basic assumptions on the structure of
space and time and paved the way for new ideas.  Eventually this led to
Einstein's theories of special and general relativity, thus changing our
understanding of the structure of space-time once and forever.

It is the experiments that differentiate natural science and engineering from
mathematics.  Mathematicians can "choose" their axioms – often inspired by
real world phenomena – while successful physicists, chemists and engineers
infer relevant axioms from controlled, robust and reproducible experiments
and might formulate far reaching theories. However, while theories often are
believed to be true laws of nature, experiments might teach us better.
Conclusion: challenging fundamental laws by experiments is crucial for
progress in science and engineering.
In parallel computing, there is a fundamental law stating that the fastest
speedup achievable through parallelization is restricted by the part of the
program that cannot be parallelized. This law, named after Gene Amdahl,
appears to be fundamental for strong scaling. Its generalization governs
efficiencies in the presence of different concurrency level. From Gustafson
we learned how weak scaling can explain why many problems scale very far,
sometimes to hundreds of thousands of cores. He showed us a specific loophole
in Amdahl’s law. Still the exponentially growing numbers of processors of
future supercomputers will entail increasing restrictions on the efficiency
of  massively parallel computing and will make life very hard in the Exascale
I believe, time is ripe to challenge Amdahl's generalized law by exposing it
to a new class of  experiments in parallel computing. With the DEEP project
we are about to demonstrate that the pitfalls of Amdahl’s law can be avoided
in specific situations.

DEEP keeps the code parts of a simulation that can only be parallelized up to
a concurrency of p = L on a Cluster Computer equipped with fast general
purpose processors. The highly parallelizable parts of the simulation are run
on a massively parallel Booster-system with a concurrency of p = H,  H >> L.
The booster is equipped with many-core Xeon Phi processors and connected by a
3D-torus network of sub-microsecond latency based on EXTOLL technology.
The DEEP system software allows to dynamically distribute the tasks to the
most appropriate parts of the hardware in order to achieve highest
computational efficiency. The MPI programming paradigm in combination with an
improved version of BSC's OmpSs task-based programming environment allows
application programmers to abstract from the system software by simply
requesting the necessary resources. The rest is done dynamically by the
system. Hence the name DEEP, the “Dynamical Exascale Entry Platform”.
The applications adapted to DEEP are selected in order to investigate and
demonstrate the usefulness of the combination of hardware, system software
and the programming model to leave ground and leap beyond the limits of
Amdahl’s law of parallel computing. We are eager to show our first results at
the ISC’13 in Leipzig.
The DEEP project (www.deep-project.eu), comprising 16 partners from 8
different countries and funded by the European commission, started in
December 2011. In the first BoF session of ISC’13 we will present results
achieved since then and demonstrate the hardware that already is up and
running at the Jülich Supercomputing Centre.
So, learn more about our experiment aimed at breaking the fundamental law of
parallel computing! Join the BoF 1 “Exascale Research ­ The European
Approach” of the three EU funded projects in exascale computing DEEP, CRESTA
and Mont-Blanc on Tuesday, June 18, 2013, 9 am – 10 am at Hall 4.

Prof. Dr. Dr. Thomas Lippert received his diploma in Theoretical Physics in
1987 from the University of Würzburg. He completed Ph.D. theses in
theoretical physics at Wuppertal University on simulations of lattice quantum
chromodynamics and at Groningen University in the field of parallel computing
with systolic algorithms. He is director of the Jülich Supercomputing Centre
at Forschungzentrum Jülich, member of the board of directors of the John von
Neumann Institute for Computing (NIC), and he holds the chair for
Computational Theoretical Physics at the University of Wuppertal. His
research interests include lattice gauge theories, quantum computing,
numerical and parallel algorithms, and cluster computing.

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