32,304 research outputs found
Inner product computation for sparse iterative solvers on\ud distributed supercomputer
Recent years have witnessed that iterative Krylov methods without re-designing are not suitable for distribute supercomputers because of intensive global communications. It is well accepted that re-engineering Krylov methods for prescribed computer architecture is necessary and important to achieve higher performance and scalability. The paper focuses on simple and practical ways to re-organize Krylov methods and improve their performance for current heterogeneous distributed supercomputers. In construct with most of current software development of Krylov methods which usually focuses on efficient matrix vector multiplications, the paper focuses on the way to compute inner products on supercomputers and explains why inner product computation on current heterogeneous distributed supercomputers is crucial for scalable Krylov methods. Communication complexity analysis shows that how the inner product computation can be the bottleneck of performance of (inner) product-type iterative solvers on distributed supercomputers due to global communications. Principles of reducing such global communications are discussed. The importance of minimizing communications is demonstrated by experiments using up to 900 processors. The experiments were carried on a Dawning 5000A, one of the fastest and earliest heterogeneous supercomputers in the world. Both the analysis and experiments indicates that inner product computation is very likely to be the most challenging kernel for inner product-based iterative solvers to achieve exascale
Minimizing synchronizations in sparse iterative solvers for distributed supercomputers
Eliminating synchronizations is one of the important techniques related to minimizing communications for modern high performance computing. This paper discusses principles of reducing communications due to global synchronizations in sparse iterative solvers on distributed supercomputers. We demonstrates how to minimizing global synchronizations by rescheduling a typical Krylov subspace method. The benefit of minimizing synchronizations is shown in theoretical analysis and is verified by numerical experiments using up to 900 processors. The experiments also show the communication complexity for some structured sparse matrix vector multiplications and global communications in the underlying supercomputers are in the order P1/2.5 and P4/5 respectively, where P is the number of processors and the experiments were carried on a Dawning 5000A
Cosmological and Solar-System Tests of f(R) Modified Gravity
We investigate the cosmological and the local tests of the f(R) theory of
modified gravity via the observations of (1) the cosmic expansion and (2) the
cosmic structures and via (3) the solar-system experiments. To fit the possible
cosmic expansion histories under consideration, for each of them we reconstruct
f(R), known as "designer f(R)". We then test the designer f(R) via the
cosmic-structure constraints on the metric perturbation ratio Psi/Phi and the
effective gravitational coupling G_eff and via the solar-system constraints on
the Brans-Dicke theory with the chameleon mechanism. We find that among the
designer f(R) models specified by the CPL effective equation of state w_eff,
only the model closely mimicking general relativity with a cosmological
constant (LambdaCDM) can survive all the tests. Accordingly, these tests rule
out the frequently studied "w_eff = -1" designer f(R) models which are distinct
in cosmic structures from LambdaCDM. When considering only the cosmological
tests, we find that the surviving designer f(R) models, although exist for a
variety of w_eff, entail fine-tuning.Comment: 22 pages, 9 figures, LaTe
Radiative Neutrino Mass, Dark Matter and Leptogenesis
We propose an extension of the standard model, in which neutrinos are Dirac
particles and their tiny masses originate from a one-loop radiative diagram.
The new fields required by the neutrino mass-generation also accommodate the
explanation for the matter-antimatter asymmetry and dark matter in the
universe.Comment: 4 pages, 3 figures. Revised version with improved model. Accepted by
PR
A left-right symmetric model with SU(2)-triplet fermions
We consider an
left-right symmetric model with three Higgs scalars including an
doublet, an doublet and an bidoublet. In
addition to usual SU(2)-doublet fermions, our model contains SU(2)-triplet
fermions with Majorana masses. The neutral components of the left-handed
triplets can contribute a canonical seesaw while the neutral components of the
right-handed triplets associated with the right-handed neutrinos can contribute
a double/inverse-type seesaw. Our model can be embedded into an SO(10) grand
unification theory where the triplets belong to the representations.Comment: 4 pages. To appear in Phys. Rev.
Neutrino masses, leptogenesis and dark matter in hybrid seesaw
We suggest a hybrid seesaw model where relatively ``light''right-handed
neutrinos give no contribution to the neutrino mass matrix due to a special
symmetry. This allows their Yukawa couplings to the standard model particles to
be relatively strong, so that the standard model Higgs boson can decay
dominantly to a left and a right-handed neutrino, leaving another stable
right-handed neutrino as cold dark matter. In our model neutrino masses arise
via the type-II seesaw mechanism, the Higgs triplet scalars being also
responsible for the generation of the matter-antimatter asymmetry via the
leptogenesis mechanism.Comment: 4 page
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