Scalable Distributed Metadata Server Based on Nonblocking Transactions

Metadata performance scalability is critically important in high-performance computing when accessing many small files from millions of clients. This paper proposes a design of a scalable distributed metadata server, PPMDS, for parallel file systems using multiple key-value servers. In PPMDS, hierarchical namespace of a file system is efficiently managed by multiple servers. Multiple entries can be atomically updated using a nonblocking distributed transaction based on an algorithm of dynamic software transactional memory. This paper also proposes optimizations to further improve the metadata performance by introducing a server-side transaction processing, multiple readers, and a shared lock mode, which reduce the number of remote procedure calls and prevent unnecessary blocking. Performance evaluation shows the scalable performance up to 3 servers, and achieves 62,000 operations per second, which is 2.58x performance improvement compared to a single metadata performance.

The Control of the Braking Stability of Active Rear Wheel Steering Vehicle Based on LQR

The aim of this article is to improve the brake stability of active rear wheel steering vehicle. The optimal theory of linear quadratic regulator is used to construct a controller, and the aim of the controller is to maintain the side slip angle is zero, and the control parameter is set according to the change of velocity when braking. An antilock brake model based on the door limit of wheel slip rate is constructed. The analysis is carried on a front wheel steering vehicle, which has two kinds of unti-lock mode. Meanwhile, an active rear wheel steering vehicle with two kinds of unti-lock mode is performed, also. All tests are performed on the bisectional road. The results of analysis show that the active rear wheel steering vehicle using the anti-lock mode of four wheels independent control can give the shortest braking distance, the smaller side slip angle and the smaller deviation from the lane. So it can give more contribution to the braking safety.

MODERATE CONCURRENCY CONTROL IN DISTRIBUTED OBJECT SYSTEMS

Objects are concurrently manipulated only through methods issued by multiple transactions in object-based systems. We first extend traditional read and write lock modes to methods on objects. We newly introduce availability and exclusion types of conflicting relations among methods. Then, we define a partially ordered relation on lock modes showing which modes are stronger than others. We newly propose a moderate concurrency control algorithm. Before manipulating an object through a method, the object is locked in a weaker mode than an intrinsic mode of the method. Then, the lock mode is escalated to the method mode. The weaker the initial mode is, the more concurrency is obtained but the more frequently deadlock occurs.

Analysis of Tiplock Phenomenon in Extension Ladders

Extension ladder accidents injure thousands of people each year in the U.S. One cause of these accidents is false lock of the flylocks. Flylocks are the devices that support the upper or fly section of an extended ladder. False locked flylocks are not fully locked, but nevertheless support a load. False lock is not stable and may release under disturbance of the ladder, e.g. from climbing. False lock modes include tiplock, flipperlock, and camlock. If a false lock releases the ladder will telescope, and may cause serious injury. One false lock mode is tiplock, where the tip of the flylock is resting on the rung of the ladder in a metastable position. The tiplock may be held in place by mechanical locking, or by friction between the tip and the rung. Tiplock may occur when the ladder is being extended and the fly section is not raised enough to fully engage the flylocks. Ladder labels usually instruct “Securely engage ladder locks before climbing,” but typically do not instuct a user how to do this. Ladder users employ various methods to securely engage flylocks. These methods do not always prevent false lock. No gravity or spring actuated flylock design is known which will completely eliminate tiplock, but various designs have different ranges of fly section travel over which tiplock can occur (tiplock “window”). A mathematical expression defines the conditions under which friction tiplock can occur. Two methods of evaluating the tiplock window are discussed: geometric analysis and physical measurement. Tiplock propensity during random ladder extension is calculated. A geometric analysis is made of two flylocks with different tip radii. The flylock with the smaller tip radius is observed to have a smaller tiplock window.