DRUM Community: UMIACS Technical Reports
http://hdl.handle.net/1903/2277
Mon, 21 Apr 2014 10:18:19 GMT2014-04-21T10:18:19ZRecursive computation of spherical harmonic rotation coefficients of large degree
http://hdl.handle.net/1903/15013
Title: Recursive computation of spherical harmonic rotation coefficients of large degree
Authors: Gumerov, Nail A.; Duraiswami, Ramani
Abstract: Computation of the spherical harmonic rotation coefficients or elements
of Wigner's d-matrix is important in a number of quantum mechanics and
mathematical physics applications. Particularly, this is important for
the Fast Multipole Methods in three dimensions for the Helmholtz,
Laplace and related equations, if rotation-based decomposition of
translation operators are used. In these and related problems related to
representation of functions on a sphere via spherical harmonic
expansions computation of the rotation coefficients of large degree n
(of the order of thousands and more) may be necessary. Existing
algorithms for their computation, based on recursions, are usually
unstable, and do not extend to n. We develop a new recursion and study
its behavior for large degrees, via computational and asymptotic
analyses. Stability of this recursion was studied based on a novel
application of the Courant-Friedrichs-Lewy condition and the von Neumann
method for stability of finite-difference schemes for solution of PDEs.
A recursive algorithm of minimal complexity O(n^2) for degree n and
FFT-based algorithms of complexity O(n^2 log n) suitable for computation
of rotation coefficients of large degrees are proposed, studied
numerically, and cross-validated. It is shown that the latter algorithm
can be used for n <~ 10^3 in double precision, while the former
algorithm was tested for large n (up to 10^4 in our experiments) and
demonstrated better performance and accuracy compared to the FFT-based
algorithm.Fri, 28 Mar 2014 00:00:00 GMThttp://hdl.handle.net/1903/150132014-03-28T00:00:00ZStudying Directory Access Patterns via Reuse Distance Analysis and Evaluating Their Impact on Multi-Level Directory Caches
http://hdl.handle.net/1903/14972
Title: Studying Directory Access Patterns via Reuse Distance Analysis and Evaluating Their Impact on Multi-Level Directory Caches
Authors: Zhao, Minshu; Yeung, Donald
Abstract: The trend for multicore CPUs is towards increasing core count. One of
the key limiters to scaling will be the on-chip directory cache. Our
work investigates moving portions of the directory away from the cores,
perhaps to off-chip DRAM, where ample capacity exists. While such
multi-level directory caches exhibit increased latency, several aspects
of directory accesses will shield CPU performance from the slower
directory, including low access frequency and latency hiding underneath
data accesses to main memory.
While multi-level directory caches have been studied previously, no work
has of yet comprehensively quantified the directory access patterns
themselves, making it difficult to understand multi-level behavior in
depth. This paper presents a framework based on multicore reuse
distance for studying directory cache access patterns. Using our
analysis framework, we show between 69-93% of directory entries are
looked up only once or twice during their liftimes in the directory
cache, and between 51-71% of dynamic directory accesses are latency
tolerant. Using cache simulations, we show a very small L1 directory
cache can service 80% of latency critical directory lookups. Although a
significant number of directory lookups and eviction notifications must
access the slower L2 directory cache, virtually all of these are latency
tolerant.Mon, 13 Jan 2014 00:00:00 GMThttp://hdl.handle.net/1903/149722014-01-13T00:00:00ZA Block Preconditioner for an Exact Penalty Formulation for Stationary MHD
http://hdl.handle.net/1903/14970
Title: A Block Preconditioner for an Exact Penalty Formulation for Stationary MHD
Authors: Phillips, Edward G.; Elman, Howard C.; Cyr, Eric C.; Shadid, John N.; Pawlowski, Roger P.
Abstract: The magnetohydrodynamics (MHD) equations are used to model the flow of electrically conducting fluids in such applications as liquid metals and plasmas. This system of non-self adjoint, nonlinear PDEs couples the Navier-Stokes equations for fluids and Maxwell's equations for electromagnetics. There has been recent interest in fully coupled solvers for the MHD system because they allow for fast steady-state solutions that do not require pseudo-time stepping. When the fully coupled system is discretized, the strong coupling can make the resulting algebraic systems difficult to solve, requiring effective preconditioning of iterative methods for efficiency. In this work, we consider a finite element discretization of an exact penalty formulation for the stationary MHD equations. This formulation has the benefit of implicitly enforcing the divergence free condition on the magnetic field without requiring a Lagrange multiplier. We consider extending block preconditioning techniques developed for the Navier-Stokes equations to the full MHD system. We analyze operators arising in block decompositions from a continuous perspective and apply arguments based on the existence of approximate commutators to develop new preconditioners that account for the physical coupling. This results in a family of parameterized block preconditioners for both Picard and Newton linearizations.
We develop an automated method for choosing the relevant parameters and demonstrate the robustness of these preconditioners for a range of the physical non-dimensional parameters and with respect to mesh refinement.Tue, 04 Feb 2014 00:00:00 GMThttp://hdl.handle.net/1903/149702014-02-04T00:00:00ZProceedings of the 2013 Annual Conference on Advances in Cognitive Systems: Workshop on Metacognition about Artificial Situated Agents
http://hdl.handle.net/1903/14744
Title: Proceedings of the 2013 Annual Conference on Advances in Cognitive Systems: Workshop on Metacognition about Artificial Situated Agents
Authors: Josyula, Darsana; Robertson, Paul; Cox, Michael T.
Abstract: Metacognition is the process of thinking about thinking. It provides cognitive systems the ability to note and deal with anomalies, changes, opportunities, surprises and uncertainty. It includes both monitoring of cognitive activities and control of such activities. Monitoring helps to evaluate and explain the cognitive activities, while control helps to adapt or modify the cognitive activities. Situated agents are agents embedded in a dynamic environment that they can sense or perceive and manipulate or change through their actions. Similarly, they can act in order to manipulate other agents among which they are situated. Examples might include robots, natural language dialog interfaces, web-based agents or virtual-reality bots. An agent can leverage metacognition of its own thinking about other agents in its situated environment. It can equally benefit from metacognition of the thinking of other agents towards itself. Metacognitive monitoring can help situated agents in negotiations, conflict resolution and norm-awareness. Metacognitive control can help coordination and coalition formations of situated social agents. In this workshop, we investigate the monitoring and control aspects of metacognition about self and other agents, and their application to situated artificial agents. The papers in this report cover some of the current work related to metacognition in the areas of meta-knowledge representation, meta-reasoning and meta-cognitive architecture. Perlis et al. outlines a high-level view of architectures for real-time situated agents and the reliance of such agents on metacognition. Mbale, K. and Josyula, D. present a generic metacognitive component based on preserving the homeostasis of a host agent. Pickett, M. presents a framework for representing, learning, and processing meta-knowledge. Riddle, P. et al. discuss meta-level search through a problem representation space for problemâ€“reformulation. Caro, M et al. use metamemory to adapt to changes in memory retrieval constraints. Langley, P. et al. abstract general problem specific abilities into strategic problem solving knowledge in an architecture for flexible problem solving across various domains. Samsonovich, A. examines metacognition as a means to improve fluid intelligence in a cognitive architecture. Perlis, D. and Cox, M. discuss the application of metacognitive monitoring to anomaly detection and goal generation.Sat, 14 Dec 2013 00:00:00 GMThttp://hdl.handle.net/1903/147442013-12-14T00:00:00Z