117 research outputs found
Writing an Efficient Device Driver for a Multimedia Teleconferencing System
Modern high speed networks, such as ATM, can provide the
bandwidth and the QoS guarantees to demanding real-time multimedia
applications. However, overall performance of a networked multimedia
application will greatly depend on the in-host data movement. Analyzing
the characteristics and requirements of those applications, we came to
several conclusions about the operation of the multimedia devices'
drivers. We applied these conclusions in the design and implementation of
a device driver for a multimedia teleconferencing system, based on IBM
RS/6000 servers, running the AIX 3.2 operating system. Tracing the
complete in-host data path, we found that though our device driver
minimized the movement of data between the teleconferencing card and user
main memory, the UDP/IP stack proved to be a cause of delay in the
movement of data between user main memory and the network interface.
(Also cross-referenced as UMIACS-TR-96-18
Parallel and Distributed Simulation of Discrete Event Systems
The achievements attained in accelerating the simulation of the dynamics of
complex discrete event systems using parallel or distributed multiprocessing
environments are comprehensively presented. While parallel discrete event
simulation (DES) governs the evolution of the system over simulated time in
an iterative SIMD way, distributed DES tries to spatially decompose the event
structure underlying the system, and executes event occurrences in spatial
subregions by logical processes (LPs) usually assigned to different (physical)
processing elements. Synchronization protocols are necessary in this approach
to avoid timing inconsistencies and to guarantee the preservation of event
causalities across LPs.
Included in the survey are discussions on the sources and levels of parallelism,
synchronous vs. asynchronous simulation and principles of LP simulation.
In the context of conservative LP simulation (Chandy/Misra/Bryant) deadlock
avoidance and deadlock detection/recovery strategies, Conservative Time
Windows and the Carrier Nullmessage protocol are presented. Related to
optimistic LP simulation (Time Warp), Optimistic Time Windows, memory
management, GVT computation, probabilistic optimism control and adaptive
schemes are investigated.
(Also cross-referenced as UMIACS-TR-94-100
Measuring NFS Performance in Wireless Networks
Technological trends suggest that soon communication networks will
consist of a high speed wired backbone with numerous wireless Local Area
Networks. Mobile computing and wireless subnetworks are increasingly in
demand. Mobile routing solutions provide wireless LANs with seamless
connectivity to backbone wired systems. However, these solutions do not
provide acceptable performance. Wireless networks have distinct transmission
characteristics which present challenges to achieving efficient performance.
Performance over wireless links is limited by high error rates, mobility,
and low bandwidth. We have studied the performance of TCP and NFS over
a wireless network. The prevalence of these protocols means that mobile
hosts will frequently use them when communicating with stationary hosts.
Measurements have been collected to determine the response of these protocols
in the presence of various error patterns. These measurements show that
NFS and TCP performance suffer extreme degradation due to these wireless
link characteristics. Unexpectedly, NFS performance is not better than an
TCP FTP file transfer. NFS performance over wireless links is limited by
large packet sizes, long retransmission timeouts, and slow response to losses.
Our goal is to understand the effects of wireless communication on these
protocols and improve performance without requiring changes to the current
network Infrastructure.
(Also cross-referenced as UMIACS-TR-95-125
Multiprocessor Priority Ceiling Based Protocols
We study resource synchronization in multiprocessor hard
real-time systems. Specifically, we propose a multiprocessor resource
control protocol which allows a job to simultaneously lock multiple
global resources, removing a restriction from previous protocols.
Allowing nested critical sections may permit a finer granularity of
synchronization, increasing parallelism and throughput. All the protocols
discussed belong to the class of priority inheritance protocols and rely
in some fashion on priority ceilings for global semaphores. We consider
both static and dynamic priorities, building upon the multiprocessor
priority ceiling protocol (MPCP) proposed by Rajkumar et al. and the
dynamic priority ceiling protocol (DPCP) proposed by Chen and Lin.
The extended protocols prevent deadlock and transitive blocking.
We derive bounds for worse case blocking time, and describe sufficient
conditions to guarantee that m sets of periodic tasks can be scheduled on
an rn multiprocessor system. Performance comparisons of these protocols
with MPCP shows that the proposed protocols increase schedulability.
(Also cross-referenced as UMIACS-TR-94-42
Network Layer Mobility: an Architecture and Survey
In this paper we explore various network layer concepts that pertain to
the design of mobile networking systems. We show that mobility is
essentially an {\em address translation} problem and is best resolved at
the network layer. We have identified the fundamental services that must
be supported at the network layer to carry out the task of address
translation. Using these service primitives as building blocks, we propose
a network layer architecture which enables smooth integration of mobile
end systems within the existing Internet. The architecture is modularized
into well-defined logical components. In this paper our objective is not
to propose {\em a specific scheme} for supporting mobility, rather it is
to highlight and analyze the essential aspects of supporting mobile
end-systems, as well as to better understand the trade-off between various
design alternatives.
(Also cross-referenced as UMIACS-TR-95-117
Carry-Over Round Robin: A Simple Cell Scheduling Mechaniasm for ATM Networks
We propose a simple cell scheduling mechanism for
ATM networks. The proposed mechanism, named Carry-Over
Round Robin (CORR), is an extension of weighted round
robin scheduling. We show that albeit its simplicity, CORR
achieves tight bounds on end-to-end delay and near perfect
fairness. Using a variety of video traffic traces we
show that CORR often outperforms some of the more complex
scheduling disciplines such as Packet-by-Packet
Generalized Processor Sharing (PGPS).
(Also cross-referenced as UMIACS-TR-96-45
Improving NFS Performance over Wireless Links
NFS is a widely used remote file access protocol that has been tuned to
perform well on traditional LANs which exhibit low error rates. Users migrating
to mobile-hosts would like to continue to use NFS for remote file accesses.
However, low bandwidth and high error-rates degrade performance
on mobile-hosts using wireless links thus hindering the use of NFS. In this
paper, we present two mechanisms to improve NFS performance over wireless
links : an aggressive NFS client and link-level retransmissions. Our
experiments show that these mechanisms improve throughput by up to 200%,
which brings the performance to within 5% of that obtained in zero error
conditions.
(Also cross-referenced as UMIACS-TR-95-126
Theoretical study of Oldroyd-b visco-elastic fluid flow through curved pipes with slip effects in polymer flow processing
The characteristics of the flow field of both viscous and viscoelastic fluids passing through a curved pipe with a Navier slip boundary condition have been investigated analytically in the present study. The Oldroyd-B constitutive equation is employed to simulate realistic transport of dilute polymeric solutions in curved channels. In order to linearize the momentum and constitutive equations, a perturbation method is used in which the ratio of radius of cross section to the radius of channel curvature is employed as the perturbation parameter. The intensity of secondary and main flows is mainly affected by the hoop stress and it is demonstrated in the present study that both the Weissenberg number (the ratio of elastic force to viscous force) and slip coefficient play major roles in determining the strengths of both flows. It is also shown that as a result of an increment in slip coefficient, the position of maximum velocity markedly migrates away from the pipe center towards the outer side of curvature. Furthermore, results corresponding to Navier slip scenarios exhibit non-uniform distributions in both the main and lateral components of velocity near the wall which can notably vary from the inner side of curvature to the outer side. The present solution is also important in polymeric flow processing systems because of experimental evidence indicating that the no-slip condition can fail for these flows, which is of relevance to chemical engineers
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