Efficient Parallelism vs Reliable Distribution: A Trade-Off for Concurrent Computations

Concurrent computations should combine efficiency with reliability, where efficiency is usually associated with parallel and reliability with distributed computing. Such a desirable combination is not always possible, because of an intuitive trade-off: efficiency requires removing redundancy from computations whereas reliability requires some redundancy. We survey a spectrum of algorithmic models (from fail-stop, synchronous to asynchronous and from approximate to exact computations) in which reliability is guaranteed with small trade-offs in efficiency. We illustrate a number of cases where optimal trade-offs are achievable. A basic property of all these models, which is of some interest in the study of concurrency, is that "true" read/write concurrency is necessary for fault tolerance. In particular, we outline (from [14]) how algorithms can be designed so that, in each execution, the total "true" concurrency used can be closely related to the faults that can be tolerated

Common Subexpression Isolation in Multiple Query Optimization

Abstract: The simultaneous optimization of multiple queries submitted to a database system may lead to substantial savings over the current approach of optimizing each query separately. Isolating common subexpressions in multiple queries and treating their execution as a sharable resource are important prerequisites. This chapter presents techniques for recognizing, supporting, and exploiting common subexpressions in record-oriented, relational algebra, domain relational calculus, and tuple relational calculus query representations, It also investigates preconditions that transaction management mechanisms must satisfy to make multiple query optimization effective. The joint execution of batches of queries and update operations has been a standard technique in the conventional, record-at-a-time file systems of the sixties and early seventies. However, with the introduction of interactive database systems based on direct access to specific subsets of data, th

Interruption of antiretroviral therapy is associated with increased plasma cystatin C.

Collaboratore della suddetta ricerca in quanto membro del INSIGHT SMART Study Grou