86,807 research outputs found
Concurrent counting is harder than queuing
We compare the complexities of two fundamental distributed coordination problems, distributed counting and distributed queuing, in a concurrent setting. In both distributed counting and queuing, processors in a distributed system issue operations which are organized into a total order. In counting, each participating processor receives the rank of its operation in the total order, where as in queuing, a processor receives the identity of its predecessor in the total order. Many coordination applications can be solved using either distributed counting or queuing, and it is useful to know which of counting or queuing is the easier problem. Our results show that concurrent counting is harder than concurrent queuing on a variety of processor interconnection topologies, including high and low diameter graphs. For all these topologies, we show that the concurrent delay complexity of a particular solution to queuing, the arrow protocol, is asymptotically smaller than a lower bound on the complexity of any solution to counting
Two snap-stabilizing point-to-point communication protocols in message-switched networks
A snap-stabilizing protocol, starting from any configuration, always behaves
according to its specification. In this paper, we present a snap-stabilizing
protocol to solve the message forwarding problem in a message-switched network.
In this problem, we must manage resources of the system to deliver messages to
any processor of the network. In this purpose, we use information given by a
routing algorithm. By the context of stabilization (in particular, the system
starts in an arbitrary configuration), this information can be corrupted. So,
the existence of a snap-stabilizing protocol for the message forwarding problem
implies that we can ask the system to begin forwarding messages even if routing
information are initially corrupted. In this paper, we propose two
snap-stabilizing algorithms (in the state model) for the following
specification of the problem: - Any message can be generated in a finite time.
- Any emitted message is delivered to its destination once and only once in a
finite time. This implies that our protocol can deliver any emitted message
regardless of the state of routing tables in the initial configuration. These
two algorithms are based on the previous work of [MS78]. Each algorithm needs a
particular method to be transform into a snap-stabilizing one but both of them
do not introduce a significant overcost in memory or in time with respect to
algorithms of [MS78]
Query processing of geometric objects with free form boundarie sin spatial databases
The increasing demand for the use of database systems as an integrating
factor in CAD/CAM applications has necessitated the development of database
systems with appropriate modelling and retrieval capabilities. One essential
problem is the treatment of geometric data which has led to the development of
spatial databases. Unfortunately, most proposals only deal with simple geometric
objects like multidimensional points and rectangles. On the other hand, there has
been a rapid development in the field of representing geometric objects with free
form curves or surfaces, initiated by engineering applications such as mechanical
engineering, aviation or astronautics. Therefore, we propose a concept for the realization
of spatial retrieval operations on geometric objects with free form
boundaries, such as B-spline or Bezier curves, which can easily be integrated in
a database management system. The key concept is the encapsulation of geometric
operations in a so-called query processor. First, this enables the definition of
an interface allowing the integration into the data model and the definition of the
query language of a database system for complex objects. Second, the approach
allows the use of an arbitrary representation of the geometric objects. After a
short description of the query processor, we propose some representations for free
form objects determined by B-spline or Bezier curves. The goal of efficient query
processing in a database environment is achieved using a combination of decomposition
techniques and spatial access methods. Finally, we present some experimental
results indicating that the performance of decomposition techniques is
clearly superior to traditional query processing strategies for geometric objects
with free form boundaries
Credit Card Fraud: A New Perspective On Tackling An Intransigent Problem
This article offers a new perspective on battling credit card fraud. It departs from a focus on post factum liability, which characterizes most legal scholarship and federal legislation on credit card fraud and applies corrective mechanisms only after the damage is done. Instead, this article focuses on preempting credit card fraud by tackling the root causes of the problem: the built-in incentives that keep the credit card industry from fighting fraud on a system-wide basis. This article examines how credit card companies and banks have created a self-interested infrastructure that insulates them from the liabilities and costs of credit card fraud. Contrary to widespread belief, retailers, not card companies or banks, absorb much of the loss caused by thieves who shop with stolen credit cards. Also, credit card companies and banks earn fees from every credit card transaction, including those that are fraudulent. In addressing these problems, this article advocates broad reforms, including legislation that would mandate data security standards for the industry, empower multiple stakeholders to create the new standards, and offer companies incentives to comply by capping bank fees for those that are compliant, while deregulating fees for those that are not compliant
Universality and optimality of programmable quantum processors
We analyze and compare the optimality of approximate and probabilistic
universal programmable quantum processors. We define several characteristics
how to quantify the optimality and we study in detail performance of three
types of programmable quantum processors based on (1) the C-NOT gate, (2) the
SWAP operation, and (3) the model of the quantum information distributor - the
QID processor. We show under which conditions the measurement assisted QID
processor is optimal. We also investigate optimality of the so-called
U-processors and we also compare the optimal approximative implementation of
U(1) qubit rotations with the known probabilistic implementation as introduced
by Vidal, Masanes and Cirac [ {\em Phys. Rev. Lett.} {\bf 88}, 047905 (2002)].Comment: 9 page
GRAPE-6: The massively-parallel special-purpose computer for astrophysical particle simulation
In this paper, we describe the architecture and performance of the GRAPE-6
system, a massively-parallel special-purpose computer for astrophysical
-body simulations. GRAPE-6 is the successor of GRAPE-4, which was completed
in 1995 and achieved the theoretical peak speed of 1.08 Tflops. As was the case
with GRAPE-4, the primary application of GRAPE-6 is simulation of collisional
systems, though it can be used for collisionless systems. The main differences
between GRAPE-4 and GRAPE-6 are (a) The processor chip of GRAPE-6 integrates 6
force-calculation pipelines, compared to one pipeline of GRAPE-4 (which needed
3 clock cycles to calculate one interaction), (b) the clock speed is increased
from 32 to 90 MHz, and (c) the total number of processor chips is increased
from 1728 to 2048. These improvements resulted in the peak speed of 64 Tflops.
We also discuss the design of the successor of GRAPE-6.Comment: Accepted for publication in PASJ, scheduled to appear in Vol. 55, No.
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