77 research outputs found
Space-Time Block Preconditioning for Incompressible Resistive Magnetohydrodynamics
This work develops a novel all-at-once space-time preconditioning approach
for resistive magnetohydrodynamics (MHD), with a focus on model problems
targeting fusion reactor design. We consider parallel-in-time due to the long
time domains required to capture the physics of interest, as well as the
complexity of the underlying system and thereby computational cost of long-time
integration. To ameliorate this cost by using many processors, we thus develop
a novel approach to solving the whole space-time system that is parallelizable
in both space and time. We develop a space-time block preconditioning for
resistive MHD, following the space-time block preconditioning concept first
introduced by Danieli et al. in 2022 for incompressible flow, where an
effective preconditioner for classic sequential time-stepping is extended to
the space-time setting. The starting point for our derivation is the continuous
Schur complement preconditioner by Cyr et al. in 2021, which we proceed to
generalise in order to produce, to our knowledge, the first space-time block
preconditioning approach for the challenging equations governing incompressible
resistive MHD. The numerical results are promising for the model problems of
island coalescence and tearing mode, with the overhead computational cost
associated with space-time preconditioning versus sequential time-stepping
being modest and primarily in the range of 2x-5x, which is low for
parallel-in-time schemes in general. Additionally, the scaling results for
inner (linear) and outer (nonlinear) iterations are flat in the case of fixed
time-step size and only grow very slowly in the case of time-step refinement.Comment: 25 pages, 4 figures, 3 table
Space-time block preconditioning for incompressible flow
Parallel-in-time methods have become increasingly popular in the simulation
of time-dependent numerical PDEs, allowing for the efficient use of additional
MPI processes when spatial parallelism saturates. Most methods treat the
solution and parallelism in space and time separately. In contrast, all-at-once
methods solve the full space-time system directly, largely treating time as
simply another spatial dimension. All-at-once methods offer a number of
benefits over separate treatment of space and time, most notably significantly
increased parallelism and faster time-to-solution (when applicable). However,
the development of fast, scalable all-at-once methods has largely been limited
to time-dependent (advection-)diffusion problems. This paper introduces the
concept of space-time block preconditioning for the all-at-once solution of
incompressible flow. By extending well-known concepts of spatial block
preconditioning to the space-time setting, we develop a block preconditioner
whose application requires the solution of a space-time (advection-)diffusion
equation in the velocity block, coupled with a pressure Schur complement
approximation consisting of independent spatial solves at each time-step, and a
space-time matrix-vector multiplication. The new method is tested on four
classical models in incompressible flow. Results indicate perfect scalability
in refinement of spatial and temporal mesh spacing, perfect scalability in
nonlinear Picard iterations count when applied to a nonlinear Navier-Stokes
problem, and minimal overhead in terms of number of preconditioner applications
compared with sequential time-stepping.Comment: 28 pages, 7 figures, 4 table
Influence of dietary vitamin E supplementation on cholesterol oxidation and fresh colour in beef aged for 3 and 14 days
The effects of dietary vitamin E supplementation on the susceptibility to lipid oxidation and colour of the Longissimus thoracis (LT) muscle aged in vacuum packaged conditions for 3 or 14 days were studied. For this purpose, Charolais cattle were fed on a diet providing daily 60 mg (control) or 5500 mg of vitamin E per animal (supplemented) for 30 and 60 days before slaughter. Dietary vitamin E supplementation increased the liver vitamin E content, but not in the LT muscle of treated animals. The vitamin supplementation for 30 and 60 days has shown non-consistent effects in reducing cholesterol oxidation products of vacuum-packed aged meat. However, the vitamin E supplementation for 60 days was effective on Lightness stability in LT muscle during vacuum-packed ageing. Overall, from the practical standpoint, this study suggests that supranutritional supplementation up to 60 days may not increase the vitamin E content of Charolais LT muscle giving little, if any, benefits on meat colour and cholesterol oxidation. However, the present study suggests that it would be interesting to determine in which extent specific oxysterols are related to the meat colour and whether colour parameters can be useful for predicting the formation of cholesterol oxidation products along the industrial meat production chain.The effects of dietary vitamin E supplementation on the susceptibility to lipid oxidation and colour
of the Longissimus thoracis (LT) muscle aged in vacuum packaged conditions for 3 or 14 days
were studied. For this purpose, Charolais cattle were fed on a diet providing daily 60mg (control)
or 5500mg of vitamin E per animal (supplemented) for 30 and 60 days before slaughter. Dietary
vitamin E supplementation increased the liver vitamin E content, but not in the LT muscle of
treated animals. The vitamin supplementation for 30 and 60 days has shown non-consistent
effects in reducing cholesterol oxidation products of vacuum-packed aged meat. However, the
vitamin E supplementation for 60 days was effective on Lightness stability in LT muscle during
vacuum-packed ageing. Overall, from the practical standpoint, this study suggests that supranutritional
supplementation up to 60 days may not increase the vitamin E content of Charolais
LT muscle giving little, if any, benefits on meat colour and cholesterol oxidation. However, the
present study suggests that it would be interesting to determine in which extent specific oxysterols
are related to the meat colour and whether colour parameters can be useful for predicting
the formation of cholesterol oxidation products along the industrial meat production chain
DeepPCR: Parallelizing Sequential Operations in Neural Networks
Parallelization techniques have become ubiquitous for accelerating inference
and training of deep neural networks. Despite this, several operations are
still performed in a sequential manner. For instance, the forward and backward
passes are executed layer-by-layer, and the output of diffusion models is
produced by applying a sequence of denoising steps. This sequential approach
results in a computational cost proportional to the number of steps involved,
presenting a potential bottleneck as the number of steps increases. In this
work, we introduce DeepPCR, a novel algorithm which parallelizes typically
sequential operations in order to speed up inference and training of neural
networks. DeepPCR is based on interpreting a sequence of steps as the
solution of a specific system of equations, which we recover using the Parallel
Cyclic Reduction algorithm. This reduces the complexity of computing the
sequential operations from to , thus
yielding a speedup for large . To verify the theoretical lower complexity of
the algorithm, and to identify regimes for speedup, we test the effectiveness
of DeepPCR in parallelizing the forward and backward pass in multi-layer
perceptrons, and reach speedups of up to for the forward and
for the backward pass. We additionally showcase the flexibility of
DeepPCR by parallelizing training of ResNets with as many as 1024 layers, and
generation in diffusion models, enabling up to faster training and
faster generation, respectively, when compared to the sequential
approach
REALM: Robust Entropy Adaptive Loss Minimization for Improved Single-Sample Test-Time Adaptation
Fully-test-time adaptation (F-TTA) can mitigate performance loss due to
distribution shifts between train and test data (1) without access to the
training data, and (2) without knowledge of the model training procedure. In
online F-TTA, a pre-trained model is adapted using a stream of test samples by
minimizing a self-supervised objective, such as entropy minimization. However,
models adapted with online using entropy minimization, are unstable especially
in single sample settings, leading to degenerate solutions, and limiting the
adoption of TTA inference strategies. Prior works identify noisy, or
unreliable, samples as a cause of failure in online F-TTA. One solution is to
ignore these samples, which can lead to bias in the update procedure, slow
adaptation, and poor generalization. In this work, we present a general
framework for improving robustness of F-TTA to these noisy samples, inspired by
self-paced learning and robust loss functions. Our proposed approach, Robust
Entropy Adaptive Loss Minimization (REALM), achieves better adaptation accuracy
than previous approaches throughout the adaptation process on corruptions of
CIFAR-10 and ImageNet-1K, demonstrating its effectiveness.Comment: Accepted at WACV 2024, 17 pages, 7 figures, 11 table
DUET: 2D Structured and Approximately Equivariant Representations
Multiview Self-Supervised Learning (MSSL) is based on learning invariances
with respect to a set of input transformations. However, invariance partially
or totally removes transformation-related information from the representations,
which might harm performance for specific downstream tasks that require such
information. We propose 2D strUctured and EquivarianT representations (coined
DUET), which are 2d representations organized in a matrix structure, and
equivariant with respect to transformations acting on the input data. DUET
representations maintain information about an input transformation, while
remaining semantically expressive. Compared to SimCLR (Chen et al., 2020)
(unstructured and invariant) and ESSL (Dangovski et al., 2022) (unstructured
and equivariant), the structured and equivariant nature of DUET representations
enables controlled generation with lower reconstruction error, while
controllability is not possible with SimCLR or ESSL. DUET also achieves higher
accuracy for several discriminative tasks, and improves transfer learning.Comment: Accepted at ICML 202
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