213 research outputs found
Exploiting Semantic Proximity in Peer-to-Peer Content Searching
A lot of recent work has dealt with improving performance of content searching in peer-to-peer file sharing systems. In this paper we attack this problem by modifying the overlay topology describing the peer relations in the system. More precisely, we create a semantic overlay, linking nodes that are "semantically close", by which we mean that they are interested in similar documents. This semantic overlay provides the primary search mechanism, while the initial peer-to-peer system provides the fail-over search mechanism. We focus on implicit approaches for discovering semantic proximity. We evaluate and compare three candidate methods, and review open questions
Distributed Slicing in Dynamic Systems
Peer to peer (P2P) systems are moving from application specific architectures
to a generic service oriented design philosophy. This raises interesting
problems in connection with providing useful P2P middleware services capable of
dealing with resource assignment and management in a large-scale, heterogeneous
and unreliable environment. The slicing service, has been proposed to allow for
an automatic partitioning of P2P networks into groups (slices) that represent a
controllable amount of some resource and that are also relatively homogeneous
with respect to that resource. In this paper we propose two gossip-based
algorithms to solve the distributed slicing problem. The first algorithm speeds
up an existing algorithm sorting a set of uniform random numbers. The second
algorithm statistically approximates the rank of nodes in the ordering. The
scalability, efficiency and resilience to dynamics of both algorithms rely on
their gossip-based models. These algorithms are proved viable theoretically and
experimentally
Yes, Topology Matters in Decentralized Optimization: Refined Convergence and Topology Learning under Heterogeneous Data
One of the key challenges in federated and decentralized learning is to
design algorithms that efficiently deal with highly heterogeneous data
distributions across agents. In this paper, we revisit the analysis of
Decentralized Stochastic Gradient Descent algorithm (D-SGD), a popular
decentralized learning algorithm, under data heterogeneity. We exhibit the key
role played by a new quantity, that we call neighborhood heterogeneity, on the
convergence rate of D-SGD. Unlike prior work, neighborhood heterogeneity is
measured at the level of the neighborhood of an agent in the graph topology. By
coupling the topology and the heterogeneity of the agents' distributions, our
analysis sheds light on the poorly understood interplay between these two
concepts in decentralized learning. We then argue that neighborhood
heterogeneity provides a natural criterion to learn sparse data-dependent
topologies that reduce (and can even eliminate) the otherwise detrimental
effect of data heterogeneity on the convergence time of D-SGD. For the
important case of classification with label skew, we formulate the problem of
learning such a good topology as a tractable optimization problem that we solve
with a Frank-Wolfe algorithm. Our approach provides a principled way to design
a sparse topology that balances the number of iterations and the per-iteration
communication costs of D-SGD under data heterogeneity
Adaptive gossip-based broadcast
This paper presents a novel adaptation mechanism that allows every node of a gossip-based broadcast algorithm to adjust the rate of message emission 1) to the amount of resources available to the nodes within the same broadcast group and 2) to the global level of congestion in the system. The adaptation mechanism can be applied to all gossip-based broadcast algorithms we know of and makes their use more realistic in practical situations where nodes have limited resources whose quantity changes dynamically with time without decreasing the reliability.(undefined
Dynamics of Rumor Spreading in Complex Networks
We derive the mean-field equations characterizing the dynamics of a rumor
process that takes place on top of complex heterogeneous networks. These
equations are solved numerically by means of a stochastic approach. First, we
present analytical and Monte Carlo calculations for homogeneous networks and
compare the results with those obtained by the numerical method. Then, we study
the spreading process in detail for random scale-free networks. The time
profiles for several quantities are numerically computed, which allow us to
distinguish among different variants of rumor spreading algorithms. Our
conclusions are directed to possible applications in replicated database
maintenance, peer to peer communication networks and social spreading
phenomena.Comment: Final version to appear in PR
NEEM: network-friendly epidemic multicast
Epidemic, or probabilistic, multicast protocols have emerged as a viable mechanism to circumvent the scalabil- ity problems of reliable multicast protocols. However, most existing epidemic approaches use connectionless transport protocols to exchange messages and rely on the intrinsic robustness of the epidemic dissemination to mask network omissions. Unfortunately, such an approach is not network- friendly, since the epidemic protocol makes no effort to re- duce the load imposed on the network when the system is congested. In this paper, we propose a novel epidemic protocol whose main characteristic is to be network-friendly. This property is achieved by relying on connection-oriented transport connections, such as TCP/IP, to support the com- munication among peers. Since during congestion mes- sages accumulate in the border of the network, the pro- tocol uses an innovative buffer management scheme, that combines different selection techniques to discard messages upon overflow. This technique improves the quality of the information delivered to the application during periods of network congestion. The protocol has been implemented and the benefits of the approach are illustrated using a com- bination of experimental and simulation results
Total Angular Momentum Conservation During Tunnelling through Semiconductor Barriers
We have investigated the electrical transport through strained
p-Si/Si_{1-x}Ge_x double-barrier resonant tunnelling diodes. The confinement
shift for diodes with different well width, the shift due to a central
potential spike in a well, and magnetotunnelling spectroscopy demonstrate that
the first two resonances are due to tunnelling through heavy hole levels,
whereas there is no sign of tunnelling through the first light hole state. This
demonstrates for the first time the conservation of the total angular momentum
in valence band resonant tunnelling. It is also shown that conduction through
light hole states is possible in many structures due to tunnelling of carriers
from bulk emitter states.Comment: 4 pages, 4 figure
Gapped and gapless short range ordered magnetic states with wavevectors in the pyrochlore magnet TbTiO
Recent low temperature heat capacity (C) measurements on polycrystalline
samples of the pyrochlore antiferromagnet TbTiO
have shown a strong sensitivity to the precise Tb concentration , with a
large anomaly exhibited for at K and no such
anomaly and corresponding phase transition for . We have grown single
crystal samples of TbTiO, with approximate
composition , and , where the single
crystal exhibits a large C anomaly at =0.45 K, but neither the
nor the single crystals display any such anomaly. We
present new time-of-flight neutron scattering measurements on the
and the samples which show strong
quasi-Bragg peaks at low
temperatures characteristic of short range antiferromagnetic spin ice (AFSI)
order at zero magnetic field but only under field-cooled conditions, as was
previously observed in our single crystal. These results show that
the strong quasi-Bragg peaks
and gapped AFSI state at low temperatures under field cooled conditions are
robust features of TbTiO, and are not correlated with the presence
or absence of the C anomaly and phase transition at low temperatures.
Further, these results show that the ordered state giving rise to the C
anomaly is confined to for
TbTiO, and is not obviously connected with
conventional order of magnetic dipole degrees of freedom.Comment: 7 pages, 3 figure
Quenched crystal field disorder and magnetic liquid ground states in Tb2Sn2-xTixO7
Solid-solutions of the "soft" quantum spin ice pyrochlore magnets Tb2B2O7
with B=Ti and Sn display a novel magnetic ground state in the presence of
strong B-site disorder, characterized by a low susceptibility and strong spin
fluctuations to temperatures below 0.1 K. These materials have been studied
using ac-susceptibility and muSR techniques to very low temperatures, and
time-of-flight inelastic neutron scattering techniques to 1.5 K. Remarkably,
neutron spectroscopy of the Tb3+ crystal field levels appropriate to at high
B-site mixing (0.5 < x < 1.5 in Tb2Sn2-xTixO7) reveal that the doublet ground
and first excited states present as continua in energy, while transitions to
singlet excited states at higher energies simply interpolate between those of
the end members of the solid solution. The resulting ground state suggests an
extreme version of a random-anisotropy magnet, with many local moments and
anisotropies, depending on the precise local configuration of the six B sites
neighboring each magnetic Tb3+ ion.Comment: 6 pages, 6 figure
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