Several Software Transactional Memory (STM) algorithms have been developed in the last few years. These algorithms differ in many different properties, such as progress and safety guarantees, contention management, etc. Some STM frameworks (DSTM2, DeuceSTM) were developed to allow the implementation and comparison of many different algorithms using the same transactional interface. The information required by an STM algorithm is either associated with the execution of a transaction (frequently referred as transaction descriptor), or associated with each memory location (or object reference) accessed within the transaction. The transaction descriptor is typically stored in a thread-local memory space and maintains the information necessary to validate and commit a memory transaction. The information associated with each memory location depends on the nature of the STM algorithm, which we will henceforth refer to as metadata, and may be composed by e.g. locks, timestamps or version lists. Since metadata is associated with each memory location, there are two main strategies regarding the location for storing such metadata: The metadata is stored either near each memory location (in-place strategy); or in a mapping table that associates the metadata with the corresponding memory location (out- place strategy). DeuceSTM is one of the most efficient STM frameworks available for the Java programming language and provides a well defined interface that allows to implement several STM algorithms using an out-place strategy. This work describes the adaptation and extension of DeuceSTM to support the in-place metadata strategy. Our approach complies to the following properties: Efficiency –- The STM implementation does not rely on an auxiliary mapping table, thus providing faster direct access to the transactional metadata; Non-transactional code is oblivious to the presence of metadata in objects, hence there is no performance overhead whatsoever for non-transactional code; Transactional code avoids the extra memory dereference imposed by the decorator pattern; Primitive types are fully supported, even in transactional code; And we provide a solution for fully supporting transactional N-dimensional arrays, which again impose no overhead on non-transactional code. Transparency –- The STM implementation automatically identifies, creates and initializes the necessary additional metadata fields in the objects; Non-transactional code is oblivious to the presence of metadata in objects, hence no code changes are required; The new transactional array types (supporting metadata for individual cells) are compatible with the standard arrays, hence not requiring any pre-/post- processing of the arrays when invoking standard or third-party non-transactional libraries. Performance benchmarking has confirmed that our approach for supporting in-place metadata in DeuceSTM, alongside with the existing out-place strategy, is a valid functional complement, widening the range of algorithms that can be efficiently implemented in DeuceSTM, enabling their fair comparison in a common infrastructure.

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The traditional lock-based concurrency control is complex and error-prone due to its low-level nature and composability challenges. Software transactional memory (STM), inherited from the database world, has risen as an exciting alternative, sparing the programmer from dealing explicitly with such low-level mechanisms. In real world scenarios, software is often faced with requirements such as high availability and scalability, and the solution usually consists on building a distributed system. Given the benefits of STM over traditional concurrency controls, Distributed Software Transactional Memory (DSTM) is now being investigated as an attractive alternative for distributed concurrency control. Our long-term objective is to transparently enable multithreaded applications to execute over a DSTM setting. In this work we intend to pave the way by defining a modular DSTM framework for the Java programming language. We extend an existing, efficient, STM framework with a new software layer to create a DSTM framework. This new layer interacts with the local STM using well-defined interfaces, and allows the implementation of different distributed memory models while providing a non-intrusive, familiar, programming model to applications, unlike any other DSTM framework. Using the proposed DSTM framework we have successfully, and easily, implemented a replicated STM which uses a Certification protocol to commit transactions. An evaluation using common STM benchmarks showcases the efficiency of the replicated STM, and its modularity enables us to provide insight on the relevance of different implementations of the Group Communication System required by the Certification scheme, with respect to performance under different workloads.