On the classification and evaluation of prefetching schemes

Abstract available: p. [2

Seamless mobility with personal servers

We describe the concept and the taxonomy of personal servers, and their implications in seamless mobility. Personal servers could offer electronic services independently of network availability or quality, provide a greater flexibility in the choice of user access device, and support the key concept of continuous user experience. We describe the organization of mobile and remote personal servers, define three relevant communication modes, and discuss means for users to exploit seamless services on the personal server

Management of Long-Running High-Performance Persistent Object Stores

The popularity of object-oriented programming languages, such as Java and C++, for large application development has stirred an interest in improved technologies for high-performance, reliable, and scalable object storage. Such storage systems are typically referred to as Persistent Object Stores. This thesis describes the design and implementation of Sphere, a new persistent object store developed at the University of Glasgow, Scotland. The requirements for Sphere included high performance, support for transactional multi-threaded loads, scalability, extensibility, portability, reliability, referential integrity via the use of disk garbage collection, provision for flexible schema evolution, and minimised interaction with the mutator. The Sphere architecture is split into two parts: the core and the application-specific customisations. The core was designed to be modular, in order to encourage research and experimentation, and to be as light-weight as possible, in an attempt to achieve high performance through simplicity. The customisation part includes the code that deals with and is optimised for the specific load of the application that Sphere has to support: object formats, free-space management, etc. Even though specialising this part of the store is not trivial, it has the benefit that the interaction between the mutator and Sphere is direct and more efficient, as translation layers are not necessary. Major design decisions for Sphere included (i) splitting the store into partitions, to facilitate incremental disk garbage collection and schema evolution, (ii) using a flexible two-level free-space management, (Hi) introducing a three-dimensional method-dispatch matrix to invoke store operations, which contributes to Sphere's ease-of-extensibility, (iv) adopting a logical addressing scheme, to allow straightforward object and partition relocation, (v) requiring that Sphere can identify reference fields inside objects, so that it does not have to interact with the mutator in order to do so, and (vi) adopting the well-known ARIES recovery algorithm to ensure fault-tolerance. The thesis contains a detailed overview of Sphere and the context in which it was developed. Then, it concentrates on two areas that were explored using Sphere as the implementation platform. First, bulk object-loading issues are discussed and the Ghosted Allocation promotion algorithm is described. This algorithm was designed to allocate large numbers of objects to a store efficiently and with minimal log traffic and was evaluated using large-scale experiments. Second, the disk garbage collection framework of Sphere is overviewed and the implemented compacting, relocating garbage collector is described, along with the model of synchronisation with the mutator

Models of higher-order, type-safe, distributed computation over autonomous persistent object stores

A remote procedure call (RPC) mechanism permits the calling of procedures in another address space. RPC is a simple but highly effective mechanism for interprocess communication and enjoys nowadays a great popularity as a tool for building distributed applications. This popularity is partly a result of their overall simplicity but also partly a consequence of more than 20 years of research in transpaxent distribution that have failed to deliver systems that meet the expectations of real-world application programmers. During the same 20 years, persistent systems have proved their suitability for building complex database applications by seamlessly integrating features traditionally found in database management systems into the programming language itself. Some research. effort has been invested on distributed persistent systems, but the outcomes commonly suffer from the same problems found with transparent distribution. In this thesis I claim that a higher-order persistent RPC is useful for building distributed persistent applications. The proposed mechanism is: realistic in the sense that it uses current technology and tolerates partial failures; understandable by application programmers; and general to support the development of many classes of distributed persistent applications. In order to demonstrate the validity of these claims, I propose and have implemented three models for distributed higher-order computation over autonomous persistent stores. Each model has successively exposed new problems which have then been overcome by the next model. Together, the three models provide a general yet simple higher-order persistent RPC that is able to operate in realistic environments with partial failures. The real strength of this thesis is the demonstration of realism and simplicity. A higherorder persistent RPC was not only implemented but also used by programmers without experience of programming distributed applications. Furthermore, a distributed persistent application has been built using these models which would not have been feasible with a traditional (non-persistent) programming language

Safe Class and Data Evolution in Large and Long-Lived Java Applications

There is a growing class of applications implemented in object-oriented languages that are large and complex, that exploit object persistence, and need to run uninterrupted for long periods of time. Development and maintenance of such applications can present challenges in the following interrelated areas: consistent and scalable evolution of persistent data and code, optimal build management, and runtime changes to applications. The research presented in this thesis addresses the above issues. Since Java is becoming increasingly popular platform for implementing large and long-lived applications, it was chosen for experiments. The first part of the research was undertaken in the context of the PJama system, an orthogonally persistent platform for Java. A technology that supports persistent class and object evolution for this platform was designed, built and evaluated. This technology integrates build management, persistent class evolution, and support for several forms of eager conversion of persistent objects. Research in build management for Java has resulted in the creation of a generally applicable, compiler-independent smart recompilation technology, which can be re-used in a Java IDE, or as a standalone Java-specific utility similar to make. The technology for eager object conversion that we developed allows the developers to perform arbitrarily complex changes to persistent objects and their collections. A high level of developer's control over the conversion process was achieved in part due to introduction of a mechanism for dynamic renaming of old class versions. This mechanism was implemented using minor non-standard extensions to the Java language. However, we also demonstrate how to achieve nearly the same results without modifying the language specification. In this form, we believe, our technology can be largely re-used with practically any persistent object solution for Java. The second part of this research was undertaken using as an implementation platform the HotSpot Java Virtual Machine (JVM), which is currently Sun's main production JVM. A technology was developed that allows the engineers to redefine classes on-the-fly in the running VM. Our main focus was on the runtime evolution of server-type applications, though we also address modification of applications running in the debugger. Unlike the only other similar system for Java known to us, our technology supports redefinition of classes that have methods currently active. Several policies for handling such methods have been proposed, one of them is currently operational, another one is in the experimental stage. We also propose to re-use the runtime evolution technology for dynamic fine-grain profiling of applications

Limited Copies and Leased References for Distributed Persistent Objects

As businesses become global organisations and as e-commerce opens up markets to customers across the Internet, demand grows for increasingly ambitious distributed software applications and platforms. Where these applications run over potentially huge collections of data, sophisticated management of data storage and communication is required. There is a need for well-integrated persistence and distribution support that considers the implications for long-term maintenance of valuable persistent data. Orthogonal persistence is intended to ease the programmer's job by providing support for data management that is integrated with a programming language. The simplicity of the orthogonal persistence model argues for its use in distributed systems, in order to make life simpler for the application programmer. PJRMI is an implementation of Java RMI for the orthogonally-persistent PJama platform. This dissertation addresses two problem areas raised by combining orthogonal persistence with support for distributed applications. These problem areas are illustrated by PJRMI. The first problem is raised as a consequence of attempting to provide the illusion of a persistent connection between stores. Distribution-related errors easily break this illusion. In an open system, it can be difficult to determine when an object should become persistent by remote reachability. In the long term, persistent references to remote objects threaten the maintainability of the persistent stores involved. A solution has been implemented to address the problems raised by maintaining persistent references between distributed stores. Greater autonomy of individual stores is achieved by limiting remote access to objects to a duration of time associated with a specific distributed application's lifetime. Within the application's lifetime, the benefits are retained of persistence of inter-store references for resilience. The second problem is encountered when copying object graphs between stores. Large object graphs tend to build up in persistent stores over time. Copying such large object graphs can be prohibitively expensive in terms of resources and performance. A programmer may assume that the size of graph they are copying is acceptable, based on their knowledge of a system in its infancy. However, the problem is that, in a long-lived system, their assumptions may be challenged, since the size of an object graph and the context in which it is used are more likely to change during a persistent object graph's lifetime. The combination of a typically statically-defined policy for passing objects to remote sites and programmer assumptions that fail to take into account the lifetime of an object can also result in other problems. These problems include failure to support different requirements on remote use of the same object graph by different applications during that object graph's lifetime. A solution has been implemented to address the problems raised by remote copying of large object graphs. Flexibility of control over such copying is achieved. Separation of policy from object definition ensures flexibility. Choice of object-copying policy for a specific distributed application's lifetime provides control, while ensuring it is adaptable to changes in size of persistent object graphs over their lifetime and to changes in the context in which these graphs are used

Neutron and Photon Imaging Capabilities of Bismuth-loaded Plastic

Plastic scintillators utilizing iridium complex fluorophores offer substantial improvements in light yield, and their light yield is not significantly quenched in compositions with bismuth metalorganic loading at 21% weight. These advances may resolve significant capability gaps for low-cost, portable, and durable dual-particle imaging (DPI) systems for nuclear safety, security, and safeguard purposes. However, all candidate materials should first undergo investigation utilizing industry standards to quantify and evaluate their capabilities. As such, a 21% bismuth-loaded polyvinyl toluene (BiPVT) scintillator fabricated by Lawrence Livermore National Laboratory (LLNL) is computationally and experimentally evaluated as a small, pixelated radiographic array, with individual pixel dimensions of 2×2×19 mm. To facilitate direct comparisons, the same evaluations are conducted for two same-sized arrays made from EJ-200 and EJ-256 scintillator, respectively. ASTM standard test methods and practices are utilized to calculate the modulation transfer function and basic spatial resolution for each array, both from measured and simulated data. Measurements are recorded by pressure coupling all three arrays to a commercial a-Si digital radiographic panel, and the computational model replicates the experimental design. Computational and experimental results are compared for all three arrays in the x-ray and fast neutron environments. The x-ray results demonstrate equivalent performance between the evaluated BiPVT array and the more ideally manufactured EJ-200 array, while the BiPVT array outperforms a similar array made from EJ-256. The agreement between simulated and experimental x-ray results validates the applied computational methodology and suggests more ideally manufactured BiPVT arrays may significantly outperform similar arrays made from EJ-200. Experimental results in a fast neutron environment demonstrate superior performance of the BiPVT array compared to the EJ-256 array, while the EJ-200 array is found to outperform both. Additionally, the performance of a second array made from a separate 21% bismuth-loaded plastic (Ir-Bi-Plastic) is evaluated experimentally in both x-ray and neutron environments using the same radiographic panel and methodology. The Ir-Bi-Plastic array consists of 64 pixels with individual dimensions of 5×5×20 mm, and the results suggest it will outperform similar arrays made from EJ-200 in both x-ray and neutron environments. These findings suggest plastic scintillators with iridium complex fluorophores and 21% weight bismuth-loading hold promise over more traditional material alternatives for DPI applications supporting nuclear safety, security, and safeguard missions

An Architecture for the Compilation of Persistent Polymorphic Reflective Higher-Order Languages

Persistent Application Systems are potentially very large and long-lived application systems which use information technology: computers, communications, networks, software and databases. They are vital to the organisations that depend on them and have to be adaptable to organisational and technological changes and evolvable without serious interruption of service. Persistent Programming Languages are a promising technology that facilitate the task of incrementally building and maintaining persistent application systems. This thesis identifies a number of technical challenges in making persistent programming languages scalable, with adequate performance and sufficient longevity and in amortising costs by providing general services. A new architecture to support the compilation of long-lived, large-scale applications is proposed. This architecture comprises an intermediate language to be used by front-ends, high-level and machine independent optimisers, low-level optimisers and code generators of target machine code. The intermediate target language, TPL, has been designed to allow compiler writers to utilise common technology for several different orthogonally persistent higher-order reflective languages. The goal is to reuse optimisation and code-generation or interpretation technology with a variety of front-ends. A subsidiary goal is to provide an experimental framework for those investigating optimisation and code generation. TPL has a simple, clean type system and will support orthogonally persistent, reflective, higher-order, polymorphic languages. TPL allows code generation and the abstraction over details of the underlying software and hardware layers. An experiment to build a prototype of the proposed architecture was designed, developed and evaluated. The experimental work includes a language processor and examples of its use are presented in this dissertation. The design space was covered by describing the implications of the goals of supporting the class of languages anticipated while ensuring long-term persistence of data and programs, and sufficient efficiency. For each of the goals, the design decisions were evaluated in face of the results

Object-Oriented Recovery for Non-volatile Memory

New non-volatile memory (NVM) technologies enable direct, durable storage of data in an application's heap. Durable, randomly accessible memory facilitates the construction of applications that do not lose data at system shutdown or power failure. Existing NVM programming frameworks provide mechanisms to consistently capture a running application's state. They do not, however, fully support object-oriented languages or ensure that the persistent heap is consistent with the environment when the application is restarted. In this paper, we propose a new NVM language extension and runtime system that supports object-oriented NVM programming and avoids the pitfalls of prior approaches. At the heart of our technique is \emph{object reconstruction}, which transparently restores and reconstructs a persistent object's state during program restart. It is implemented in NVMReconstruction, a Clang/LLVM extension and runtime library that provides: (i) transient fields in persistent objects, (ii) support for virtual functions and function pointers, (iii) direct representation of persistent pointers as virtual addresses, and (iv) type-specific reconstruction of a persistent object during program restart. In addition, NVMReconstruction supports updating an application's code, even if this causes objects to expand, by providing object migration. NVMReconstruction also can compact the persistent heap to reduce fragmentation. In experiments, we demonstrate the versatility and usability of object reconstruction and its low runtime performance cost