23,754 research outputs found
Improving Performance of Iterative Methods by Lossy Checkponting
Iterative methods are commonly used approaches to solve large, sparse linear
systems, which are fundamental operations for many modern scientific
simulations. When the large-scale iterative methods are running with a large
number of ranks in parallel, they have to checkpoint the dynamic variables
periodically in case of unavoidable fail-stop errors, requiring fast I/O
systems and large storage space. To this end, significantly reducing the
checkpointing overhead is critical to improving the overall performance of
iterative methods. Our contribution is fourfold. (1) We propose a novel lossy
checkpointing scheme that can significantly improve the checkpointing
performance of iterative methods by leveraging lossy compressors. (2) We
formulate a lossy checkpointing performance model and derive theoretically an
upper bound for the extra number of iterations caused by the distortion of data
in lossy checkpoints, in order to guarantee the performance improvement under
the lossy checkpointing scheme. (3) We analyze the impact of lossy
checkpointing (i.e., extra number of iterations caused by lossy checkpointing
files) for multiple types of iterative methods. (4)We evaluate the lossy
checkpointing scheme with optimal checkpointing intervals on a high-performance
computing environment with 2,048 cores, using a well-known scientific
computation package PETSc and a state-of-the-art checkpoint/restart toolkit.
Experiments show that our optimized lossy checkpointing scheme can
significantly reduce the fault tolerance overhead for iterative methods by
23%~70% compared with traditional checkpointing and 20%~58% compared with
lossless-compressed checkpointing, in the presence of system failures.Comment: 14 pages, 10 figures, HPDC'1
A magnetic damper for first mode vibration reduction in multimass flexible rotors
Many rotating machines such as compressors, turbines and pumps have long thin shafts with resulting vibration problems, and would benefit from additional damping near the center of the shaft. Magnetic dampers have the potential to be employed in these machines because they can operate in the working fluid environment unlike conventional bearings. An experimental test rig is described which was set up with a long thin shaft and several masses to represent a flexible shaft machine. An active magnetic damper was placed in three locations: near the midspan, near one end disk, and close to the bearing. With typical control parameter settings, the midspan location reduced the first mode vibration 82 percent, the disk location reduced it 75 percent and the bearing location attained a 74 percent reduction. Magnetic damper stiffness and damping values used to obtain these reductions were only a few percent of the bearing stiffness and damping values. A theoretical model of both the rotor and the damper was developed and compared to the measured results. The agreement was good
Marching to different drum beats: a temporal perspective on coordinating occupational work
In this paper, we contribute a temporal perspective on work coordination across collaborating occupations. Drawing on an ethnographic study of medical specialists – surgeons, pathologists, oncologists and radiologists – we examine how their temporal orientations are shaped through the temporal structuring of occupational work. Our findings show that temporal structuring of occupational practices develop in relation to the contingencies and materialities of their work, and that this shapes, and is shaped by, specialists’ temporal orientations. Further, we show that differences in occupations’ temporal orientation have important implications for coordinating work. More specifically, our study reveals how the domination of one temporal orientation can lead to recurrent strain, promoting a competitive trade-off between the different temporal orientations in guiding interaction. This temporal orientation domination is accompanied by a persistent emotional strain and potential conflict. Finally, we suggest that, alternatively, different temporal orientations can be resourced in solving coordination challenges through three inter-related mechanisms, namely juxtaposing, temporal working, and mutual adjusting. In so doing, we show how temporal resourcing can be productive in coordinating work.Eivor Oborn is supported by the National Institute for Health Research (NIHR) Collaborations for Leadership in Applied Health Research and Care West Midlands
How to measure the spreading width for decay of superdeformed nuclei
A new expression for the branching ratio for the decay via the E1 process in
the normal-deformed band of superdeformed nuclei is given within a simple
two-level model. Using this expression, the spreading or tunneling width
Gamma^downarrow for superdeformed decay can be expressed entirely in terms of
experimentally known quantities. We show how to determine the tunneling matrix
element V from the measured value of Gamma^downarrow and a statistical model of
the energy levels. The accuracy of the two-level approximation is verified by
considering the effects of the other normal-deformed states.Comment: 4 pages, 4 figure
The Vampire and the FOOL
This paper presents new features recently implemented in the theorem prover
Vampire, namely support for first-order logic with a first class boolean sort
(FOOL) and polymorphic arrays. In addition to having a first class boolean
sort, FOOL also contains if-then-else and let-in expressions. We argue that
presented extensions facilitate reasoning-based program analysis, both by
increasing the expressivity of first-order reasoners and by gains in
efficiency
An analysis of the impact of bioenergy and geosequestration in the UK future energy system
Three different energy scenarios have been considered to analyse the impact of bioenergy and geosequestration to GHG emissions in the UK for 2050. The analysis was accomplished with the use of the DECC 2050 Pathways Calculator. The outcomes focused on energy demand and supply and GHG emissions. The results showed that bioenergy and geosequestration are key factors for a low carbon energy system as they are capable of reducing significanlty carbon emissions, in parallel with the deployment of other clean energy techlogies
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