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Analysis of a class of distributed queues with application
Recently we have developed a class of media access control algorithms for different types of Local Area Networks. A common feature of these LAN algorithms is that they represent various strategies by which the processors in the LAN can simulate the availability of a centralized packet transport facility, but whose service incorporates a particular type of change over time known as 'moving sever' overhead. First we describe the operation of moving server systems in general, for both First-Come - First-Served and Head-of-the-Line orders of service, together with an approach for their delay analysis in which we transform the moving server queueing system into a conventional queueing system having proportional waiting times. Then we describe how the various LAN algorithms may be obtained from the ideal moving server system, and how a significant component of their performance characteristics is determined by the performance characteristics of that ideal system. Finally, we evaluate the compatibility of such LAN algorithms with separable queueing network models of distributed systems by computing the interdeparture time distribution for M/M/1 in the presence of moving server overhead. Although it is not exponential, except in the limits of low server utilization or low overhead, the interdeparture time distribution is a weighted sum of exponential terms with a coefficient of variation not much smaller than unity. Thus, we conjecture that a service centre with moving server overhead could be used to represent one of these LAN algorithms in a product form queueing network model of a distributed system without introducing significant approximation errors
GAMER: a GPU-Accelerated Adaptive Mesh Refinement Code for Astrophysics
We present the newly developed code, GAMER (GPU-accelerated Adaptive MEsh
Refinement code), which has adopted a novel approach to improve the performance
of adaptive mesh refinement (AMR) astrophysical simulations by a large factor
with the use of the graphic processing unit (GPU). The AMR implementation is
based on a hierarchy of grid patches with an oct-tree data structure. We adopt
a three-dimensional relaxing TVD scheme for the hydrodynamic solver, and a
multi-level relaxation scheme for the Poisson solver. Both solvers have been
implemented in GPU, by which hundreds of patches can be advanced in parallel.
The computational overhead associated with the data transfer between CPU and
GPU is carefully reduced by utilizing the capability of asynchronous memory
copies in GPU, and the computing time of the ghost-zone values for each patch
is made to diminish by overlapping it with the GPU computations. We demonstrate
the accuracy of the code by performing several standard test problems in
astrophysics. GAMER is a parallel code that can be run in a multi-GPU cluster
system. We measure the performance of the code by performing purely-baryonic
cosmological simulations in different hardware implementations, in which
detailed timing analyses provide comparison between the computations with and
without GPU(s) acceleration. Maximum speed-up factors of 12.19 and 10.47 are
demonstrated using 1 GPU with 4096^3 effective resolution and 16 GPUs with
8192^3 effective resolution, respectively.Comment: 60 pages, 22 figures, 3 tables. More accuracy tests are included.
Accepted for publication in ApJ
Data Dissemination Performance in Large-Scale Sensor Networks
As the use of wireless sensor networks increases, the need for
(energy-)efficient and reliable broadcasting algorithms grows. Ideally, a
broadcasting algorithm should have the ability to quickly disseminate data,
while keeping the number of transmissions low. In this paper we develop a model
describing the message count in large-scale wireless sensor networks. We focus
our attention on the popular Trickle algorithm, which has been proposed as a
suitable communication protocol for code maintenance and propagation in
wireless sensor networks. Besides providing a mathematical analysis of the
algorithm, we propose a generalized version of Trickle, with an additional
parameter defining the length of a listen-only period. This generalization
proves to be useful for optimizing the design and usage of the algorithm. For
single-cell networks we show how the message count increases with the size of
the network and how this depends on the Trickle parameters. Furthermore, we
derive distributions of inter-broadcasting times and investigate their
asymptotic behavior. Our results prove conjectures made in the literature
concerning the effect of a listen-only period. Additionally, we develop an
approximation for the expected number of transmissions in multi-cell networks.
All results are validated by simulations
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