Tuning the Level of Concurrency in Software Transactional Memory: An Overview of Recent Analytical, Machine Learning and Mixed Approaches

Synchronization transparency offered by Software Transactional Memory (STM) must not come at the expense of run-time efficiency, thus demanding from the STM-designer the inclusion of mechanisms properly oriented to performance and other quality indexes. Particularly, one core issue to cope with in STM is related to exploiting parallelism while also avoiding thrashing phenomena due to excessive transaction rollbacks, caused by excessively high levels of contention on logical resources, namely concurrently accessed data portions. A means to address run-time efficiency consists in dynamically determining the best-suited level of concurrency (number of threads) to be employed for running the application (or specific application phases) on top of the STM layer. For too low levels of concurrency, parallelism can be hampered. Conversely, over-dimensioning the concurrency level may give rise to the aforementioned thrashing phenomena caused by excessive data contention—an aspect which has reflections also on the side of reduced energy-efficiency. In this chapter we overview a set of recent techniques aimed at building “application-specific” performance models that can be exploited to dynamically tune the level of concurrency to the best-suited value. Although they share some base concepts while modeling the system performance vs the degree of concurrency, these techniques rely on disparate methods, such as machine learning or analytic methods (or combinations of the two), and achieve different tradeoffs in terms of the relation between the precision of the performance model and the latency for model instantiation. Implications of the different tradeoffs in real-life scenarios are also discussed

The views of five participating undergraduate students of the Student Associates Scheme in England

This paper reports findings from a study which explored undergraduate perceptions of the Student Associates Scheme in England (SAS). The scheme was established by the Training and Development Agency for Schools in an attempt to increase the number of graduates entering the teaching profession, particularly in shortage subjects such as the physical sciences and mathematics. The scheme places undergraduate students on short-term placements in secondary schools throughout England to provide them with experiences that may encourage them to consider teaching as a career option. Findings show that the SAS school placements were a positive experience for the students participating in this study. However, a question emerged as to whether or not the scheme is targeting students who have yet to decide upon teaching as a career or just reinforcing the existing aspirations of students who have already decided to teach. As the scheme is attempting to increase the number of teachers entering the profession this question has important implications for this study and further work which will focus on undergraduates who think that their career ambitions would not be fulfilled by teaching

Partial Dynamical Symmetry in the Symplectic Shell Model

We present an example of a partial dynamical symmetry (PDS) in an interacting fermion system and demonstrate the close relationship of the associated Hamiltonians with a realistic quadrupole-quadrupole interaction, thus shedding new light on this important interaction. Specifically, in the framework of the symplectic shell model of nuclei, we prove the existence of a family of fermionic Hamiltonians with partial SU(3) symmetry. We outline the construction process for the PDS eigenstates with good symmetry and give analytic expressions for the energies of these states and E2 transition strengths between them. Characteristics of both pure and mixed-symmetry PDS eigenstates are discussed and the resulting spectra and transition strengths are compared to those of real nuclei. The PDS concept is shown to be relevant to the description of prolate, oblate, as well as triaxially deformed nuclei. Similarities and differences between the fermion case and the previously established partial SU(3) symmetry in the Interacting Boson Model are considered.Comment: 9 figure

Transactional Memory: Glimmer of a Theory

Transactional memory (TM) is a promising paradigm for concurrent programming. This paper is an overview of our recent theoretical work on defining a theory of TM. We first recall some TM correctness properties and then overview results on the inherent power and limitations of TMs

The Cost of Privatization

Software transactional memory (STM) guarantees that a transaction, consisting of a sequence of operations on the memory, appears to be executed atomically. In practice, it is important to be able to run transactions together with nontransactional legacy code accessing the same memory locations, by supporting privatization. Privatization should be provided without sacrificing the parallelism offered by today’s multicore systems and multiprocessors. This paper proves an inherent cost for supporting privatization, which is linear in the number of privatized items. Specifically, we show that a transaction privatizing k items must have a data set of size at least k, in an STM with invisible reads, which is oblivious to different non-conflicting executions and guarantees progress in such executions. When reads are visible, it is shown that Ω(k) memory locations must be accessed by a privatizing transaction, where k is the minimum between the number of privatized items and the number of concurrent transactions guaranteed to make progresss, thus capturing the tradeoff between the cost of privatization and the parallelism offered by the STM