A comparative study of the performance of concurrency control algorithms in a centralised database

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Space station data system analysis/architecture study. Task 2: Options development, DR-5. Volume 2: Design options

The primary objective of Task 2 is the development of an information base that will support the conduct of trade studies and provide sufficient data to make key design/programmatic decisions. This includes: (1) the establishment of option categories that are most likely to influence Space Station Data System (SSDS) definition; (2) the identification of preferred options in each category; and (3) the characterization of these options with respect to performance attributes, constraints, cost and risk. This volume contains the options development for the design category. This category comprises alternative structures, configurations and techniques that can be used to develop designs that are responsive to the SSDS requirements. The specific areas discussed are software, including data base management and distributed operating systems; system architecture, including fault tolerance and system growth/automation/autonomy and system interfaces; time management; and system security/privacy. Also discussed are space communications and local area networking

Improving Parallel I/O Performance Using Interval I/O

Today\u27s most advanced scientific applications run on large clusters consisting of hundreds of thousands of processing cores, access state of the art parallel file systems that allow files to be distributed across hundreds of storage targets, and utilize advanced interconnections systems that allow for theoretical I/O bandwidth of hundreds of gigabytes per second. Despite these advanced technologies, these applications often fail to obtain a reasonable proportion of available I/O bandwidth. The reasons for the poor performance of application I/O include the noncontiguous I/O access patterns used for scientific computing, contention due to false sharing, and the somewhat finicky nature of parallel file system performance. We argue that a more fundamental cause of this problem is the legacy view of a file as a linear sequence of bytes. To address these issues, we introduce a novel approach for parallel I/O called Interval I/O. Interval I/O is an innovative approach that uses application access patterns to partition a file into a series of intervals, which are used as the fundamental unit for subsequent I/O operations. Use of this approach provides superior performance for the noncontiguous access patterns which are frequently used by scientific applications. In addition, the approach reduces false contention and the unnecessary serialization it causes. Interval I/O also significantly increases the performance of atomic mode operations. Finally, the Interval I/O approach includes a technique for supporting parallel I/O for cooperating applications. We provide a prototype implementation of our Interval I/O system and use it to demonstrate performance improvements of as much as 1000% compared to ROMIO when using Interval I/O with several common benchmarks

Defending secrets, sharing data: new locks and keys for electronic information

This report examines Federal policies directed at protecting information, particularly in electronic communications systems

DESIGN AND ASSESSMENT OF A LEO GNSS MINICONSTELLATION FOR POSITIONING, NAVIGATION, AND TIMING (PNT)

Recently, there has been a resurgent demand in the United Arab Emirates for more accurate positioning, navigation, and timing signals, especially for some targeted applications such as autonomous vehicles and flying taxis. The existing Global Navigation Satellite Systems (GNSS) provide real-time positioning accuracy for up to several meters, while the targeted applications require fast convergence of centimeterlevel positioning accuracy. Recent studies have shown that transmitting GNSS signals from a Low Earth Orbit (LEO) instead of a Medium Earth Orbit (MEO) would enhance positioning accuracy. The main objective of this thesis is to design and simulate an optimum scenario of a mini-LEO constellation transmitting GNSS signals in LEO and assess its performance using a GNSS simulator tool. The second objective is to evaluate the performance of a ground-based GNSS receiver receiving GNSS signals from LEO regarding the receiver’s time to lock, locking period, continuity, Position Dilution of Precision (PDOP) and 3D positioning accuracy. The final objective is to compare the performance of the simulated mini-LEO GNSS constellation with the existing MEO GPS and Galileo. Skydel GNSS simulator tool, single frequency L1/E1 ublox receiver, Systems Tool Kit (STK), and u-center software were used to conduct this research. The best simulated LEO scenario had a design consisting of 35 satellites at 800 km altitude, distributed into 5 planes, with 7 satellites in each plane, the planes were 45° apart and the satellites were 30° in each plane. The results showed a range of PDOP values from 2.1 to 3.3, 3D positioning accuracy of 5.86 m, and the time the receiver took to lock was about 1 minute with a maximum locking period of 3 minutes and with no continuity. The results obtained from the simulated LEO constellation assessed using the ublox receiver were no better than those of the simulated MEO GPS and Galileo. The main reason behind the obtained results is that the current GNSS receivers are not designed to cope with the higher dynamics of the satellites in LEO