26 research outputs found
Expander Graph and Communication-Efficient Decentralized Optimization
In this paper, we discuss how to design the graph topology to reduce the
communication complexity of certain algorithms for decentralized optimization.
Our goal is to minimize the total communication needed to achieve a prescribed
accuracy. We discover that the so-called expander graphs are near-optimal
choices. We propose three approaches to construct expander graphs for different
numbers of nodes and node degrees. Our numerical results show that the
performance of decentralized optimization is significantly better on expander
graphs than other regular graphs.Comment: 2016 IEEE Asilomar Conference on Signals, Systems, and Computer
Decentralized projected Riemannian gradient method for smooth optimization on compact submanifolds
We consider the problem of decentralized nonconvex optimization over a
compact submanifold, where each local agent's objective function defined by the
local dataset is smooth. Leveraging the powerful tool of proximal smoothness,
we establish local linear convergence of the projected gradient descent method
with unit step size for solving the consensus problem over the compact
manifold. This serves as the basis for analyzing decentralized algorithms on
manifolds. Then, we propose two decentralized methods, namely the decentralized
projected Riemannian gradient descent (DPRGD) and the decentralized projected
Riemannian gradient tracking (DPRGT) methods. We establish their convergence
rates of and , respectively, to
reach a stationary point. To the best of our knowledge, DPRGT is the first
decentralized algorithm to achieve exact convergence for solving decentralized
optimization over a compact manifold. The key ingredients in the proof are the
Lipschitz-type inequalities of the projection operator on the compact manifold
and smooth functions on the manifold, which could be of independent interest.
Finally, we demonstrate the effectiveness of our proposed methods compared to
state-of-the-art ones through numerical experiments on eigenvalue problems and
low-rank matrix completion.Comment: 32 page
Asynchronous Distributed ADMM for Large-Scale Optimization- Part I: Algorithm and Convergence Analysis
Aiming at solving large-scale learning problems, this paper studies
distributed optimization methods based on the alternating direction method of
multipliers (ADMM). By formulating the learning problem as a consensus problem,
the ADMM can be used to solve the consensus problem in a fully parallel fashion
over a computer network with a star topology. However, traditional synchronized
computation does not scale well with the problem size, as the speed of the
algorithm is limited by the slowest workers. This is particularly true in a
heterogeneous network where the computing nodes experience different
computation and communication delays. In this paper, we propose an asynchronous
distributed ADMM (AD-AMM) which can effectively improve the time efficiency of
distributed optimization. Our main interest lies in analyzing the convergence
conditions of the AD-ADMM, under the popular partially asynchronous model,
which is defined based on a maximum tolerable delay of the network.
Specifically, by considering general and possibly non-convex cost functions, we
show that the AD-ADMM is guaranteed to converge to the set of
Karush-Kuhn-Tucker (KKT) points as long as the algorithm parameters are chosen
appropriately according to the network delay. We further illustrate that the
asynchrony of the ADMM has to be handled with care, as slightly modifying the
implementation of the AD-ADMM can jeopardize the algorithm convergence, even
under a standard convex setting.Comment: 37 page
Fully Distributed Robust Singular Value Decomposition
Articulated Funiculator is a new and innovative concept developed by TyrĂ©ns forachieving a more efficient vertical transportation with a higher space utilization.Having a variety of merits, i.e.: simple construction, direct electromagneticthrust propulsion, and high safety and reliability in contrast to rotary inductionmotor, linear induction motor (LIM) is considered to be one of the cases as thepropulsion system for Articulated Funiculator. The thesis is then carried outwith the purpose of determining the feasibility of this study case by designing theLIMs meeting some specific requirements. The detailed requirements include: aset of identical LIMs are required to jointly produce the thrust that is sufficientto vertically raise the moving system up to 2 m/s2; the size of the LIMs cannotexceed the specification of the funiculator; the maximum flux density in the airgap for each LIM is kept slightly below 0.6 T; no iron saturation of any part ofthe LIMs is allowed.In this thesis report, an introduction of LIM is firstly presented. Followingthe introduction, relevant literature has been reviewed for a strengthenedtheoretical fundamentals and a better understanding of LIMâs history and applications. A general classification of LIMs is subsequently introduced. In addtion,an analytical model of the single-sided linear induction motor (SLIM) has beenbuilt based on an approximate equivalent circuit, and the preliminary geometryof the SLIM is thereby obtained. In order to acquire a more comprehensiveunderstanding of the machine characteristics and a more precise SLIM design, atwo-dimensional finite element method (2D-FEM) analysis is performed initiallyaccording to the preliminary geometry. The results, unfortunately, turn out tobe iron severely saturated in the teeth and yoke, and a excessive maximumvalue of air-gap flux density. Specific to the problems, different parameters ofthe SLIM are marginally adjusted and a series of design scenarios are run inFlux2D for 8-pole and 6-pole SLIM. The comparisons between the results areconducted and the final solution is lastly chosen among them.Articulated Funiculator Ă€r ett nytt och innovativt koncept som utvecklats av TyrĂ©ns för att möjilggöra en mer effektiv vertikal transport och bĂ€ttre utnyttjautrymme. Tack vare fördelar sĂ„som en enkel konstruktion, direkt elektromagnetiskdragkraftsframdrivning, samt hög sĂ€kerhet och tillförlitlighet i motsatstill roterande induktionsmotor, Ă€r en linjĂ€r induktionsmotor (LIM) aktuell somframdrivningssystem. Detta examensarbete Ă€r utfört med syfte att utforma enLIM för att uppfylla vissa specifika krav. De detaljerade kraven inkluderar: enuppsĂ€ttning identiska LIM krĂ€vs för att gemensamt producera tillrĂ€cklig dragkraftför att vertikalt höja det rörliga systemet upp till 2 m/s2; storleken pĂ„LIM fĂ„r inte överstiga specifikation; den maximala flödestĂ€theten i luftgapet förvarje LIM hĂ„lls Ă€r begrĂ€nsad till knappt 0.6 T; ingen jĂ€rnmĂ€ttnad av nĂ„gon delav LIM Ă€r tillĂ„tet. I denna rapport ges först en introduktion av LIM-konceptet. Efter introduktionenhar relevant litteratur granskats för att stĂ€rka teoretiska grundkunskapersamt ge en bĂ€ttre belysning av historiken kring LIMs samt dess applikationer. Utöver detta har en analytisk modell av den ensidiga linjĂ€ra induktionsmotorn(SLIM) byggts, baserat pĂ„ en ungefĂ€rlig ekvivalent krets med vilket den preliminĂ€rageometrin för SLIM. För att erhĂ„lla en mer grundlĂ€ggande förstĂ„else avmaskinens egenskaper Ă€r en tvĂ„dimensionell analys med finita elementmetoden(2D-FEM) utförd, initialt med anvĂ€ndande av en preliminĂ€r geometri erhĂ„llenmed hjĂ€lp av analytisk dimensionering. Resultaten frĂ„n dessa simuleringar visadedock att jĂ€rnet mĂ€ttats kraftigt i bĂ„de tĂ€nderna och oket och ett överdrivetstort maximivĂ€rde av luftgapets flödestĂ€thet erhĂ„lls. Specifikt för applikationenjusteras olika parametrar och en rad driftscenarier körs i Flux2D för en 8-poligoch en 6-polig SLIM. En slutgiltig jĂ€mförelse mellan de olika maskindesignernapresenteras och den rekommenderade lösningen vĂ€ljs slutligen