9,348 research outputs found
PaRiS: Causally Consistent Transactions with Non-blocking Reads and Partial Replication
Geo-replicated data platforms are at the backbone of several large-scale
online services. Transactional Causal Consistency (TCC) is an attractive
consistency level for building such platforms. TCC avoids many anomalies of
eventual consistency, eschews the synchronization costs of strong consistency,
and supports interactive read-write transactions. Partial replication is
another attractive design choice for building geo-replicated platforms, as it
increases the storage capacity and reduces update propagation costs. This paper
presents PaRiS, the first TCC system that supports partial replication and
implements non-blocking parallel read operations, whose latency is paramount
for the performance of read-intensive applications. PaRiS relies on a novel
protocol to track dependencies, called Universal Stable Time (UST). By means of
a lightweight background gossip process, UST identifies a snapshot of the data
that has been installed by every DC in the system. Hence, transactions can
consistently read from such a snapshot on any server in any replication site
without having to block. Moreover, PaRiS requires only one timestamp to track
dependencies and define transactional snapshots, thereby achieving resource
efficiency and scalability. We evaluate PaRiS on a large-scale AWS deployment
composed of up to 10 replication sites. We show that PaRiS scales well with the
number of DCs and partitions, while being able to handle larger data-sets than
existing solutions that assume full replication. We also demonstrate a
performance gain of non-blocking reads vs. a blocking alternative (up to 1.47x
higher throughput with 5.91x lower latency for read-dominated workloads and up
to 1.46x higher throughput with 20.56x lower latency for write-heavy
workloads)
Entropic N-bound and Maximal Mass Conjecture Violations in Four Dimensional Taub-Bolt(NUT)-dS Spacetimes
We show that the class of four-dimensional Taub-Bolt(NUT) spacetimes with
positive cosmological constant for some values of NUT charges are stable and
have entropies that are greater than that of de Sitter spacetime, in violation
of the entropic N-bound conjecture. We also show that the maximal mass
conjecture, which states "any asymptotically dS spacetime with mass greater
than dS has a cosmological singularity", can be violated as well. Our
calculation of conserved mass and entropy is based on an extension of the path
integral formulation to asymptotically de Sitter spacetimes.Comment: 37 pages, 22 figures, 3 tables, few typos corrected, version to
appear in Nucl. Phys.
Okapi: Causally Consistent Geo-Replication Made Faster, Cheaper and More Available
Okapi is a new causally consistent geo-replicated key- value store. Okapi
leverages two key design choices to achieve high performance. First, it relies
on hybrid logical/physical clocks to achieve low latency even in the presence
of clock skew. Second, Okapi achieves higher resource efficiency and better
availability, at the expense of a slight increase in update visibility latency.
To this end, Okapi implements a new stabilization protocol that uses a
combination of vector and scalar clocks and makes a remote update visible when
its delivery has been acknowledged by every data center. We evaluate Okapi with
different workloads on Amazon AWS, using three geographically distributed
regions and 96 nodes. We compare Okapi with two recent approaches to causal
consistency, Cure and GentleRain. We show that Okapi delivers up to two orders
of magnitude better performance than GentleRain and that Okapi achieves up to
3.5x lower latency and a 60% reduction of the meta-data overhead with respect
to Cure
A Review of the N-bound and the Maximal Mass Conjectures Using NUT-Charged dS Spacetimes
The proposed dS/CFT correspondence remains an intriguing paradigm in the
context of string theory. Recently it has motivated two interesting
conjectures: the entropic N-bound and the maximal mass conjecture. The former
states that there is an upper bound to the entropy in asymptotically de Sitter
spacetimes, given by the entropy of pure de Sitter space. The latter states
that any asymptotically de Sitter spacetime cannot have a mass larger than the
pure de Sitter case without inducing a cosmological singularity. Here we review
the status of these conjectures and demonstrate their limitation. We first
describe a generalization of gravitational thermodynamics to asymptotically de
Sitter spacetimes, and show how to compute conserved quantities and
gravitational entropy using this formalism. From this we proceed to a
discussion of the N-bound and maximal mass conjectures. We then illustrate that
these conjectures are not satisfied for certain asymptotically de Sitter
spacetimes with NUT charge. We close with a presentation of explicit examples
in various spacetime dimensionalities.Comment: 49 pages, 17 figures, a few typos corrected, addendum added with
regard to some references that were later brought to our attentio
On the Thermodynamics of NUT charged spaces
We discuss and compare at length the results of two methods used recently to
describe the thermodynamics of Taub-NUT solutions in a deSitter background. In
the first approach (\mathbb{% C}-approach), one deals with an analytically
continued version of the metric while in the second approach
(-approach), the discussion is carried out using the unmodified
metric with Lorentzian signature. No analytic continuation is performed on the
coordinates and/or the parameters that appear in the metric. We find that the
results of both these approaches are completely equivalent modulo analytic
continuation and we provide the exact prescription that relates the results in
both methods. The extension of these results to the AdS/flat cases aims to give
a physical interpretation of the thermodynamics of nut-charged spacetimes in
the Lorentzian sector. We also briefly discuss the higher dimensional spaces
and note that, analogous with the absence of hyperbolic nuts in AdS
backgrounds, there are no spherical Taub-Nut-dS solutions.Comment: 35pages, 4 figures. v.4 references added,few typos corrected, to
appear in Phys. Rev.
Large N Phases, Gravitational Instantons and the Nuts and Bolts of AdS Holography
Recent results in the literature concerning holography indicate that the
thermodynamics of quantum gravity (at least with a negative cosmological
constant) can be modeled by the large N thermodynamics of quantum field theory.
We emphasize that this suggests a completely unitary evolution of processes in
quantum gravity, including black hole formation and decay; and even more
extreme examples involving topology change. As concrete examples which show
that this correspondence holds even when the space-time is only locally
asymptotically AdS, we compute the thermodynamical phase structure of the
AdS-Taub-NUT and AdS-Taub-Bolt spacetimes, and compare them to a 2+1
dimensional conformal field theory (at large N) compactified on a squashed
three sphere, and on the twisted plane.Comment: 20 pages, three figures. (uses harvmac.tex and epsf.tex
Chern-Simons dilaton black holes in 2+1 dimensions
We construct rotating magnetic solutions to the three-dimensional
Einstein-Maxwell-Chern-Simons-dilaton theory with a Liouville potential. These
include a class of black hole solutions which generalize the warped AdS black
holes. The regular black holes belong to two disjoint sectors. The first sector
includes black holes which have a positive mass and are co-rotating, while the
black holes of the second sector have a negative mass and are counter-rotating.
We also show that a particular, non-black hole, subfamily of our
three-dimensional solutions may be uplifted to new regular non-asymptotically
flat solutions of five-dimensional Einstein-Maxwell-Chern-Simons theory.Comment: 26 pages, 3 figures, published versio
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