Bruno Ciciani

Analysis of replication in distributed database systems

The authors develop an approximate analytical model to study the tradeoffs of replicating data in a distributed database environment. Several concurrency control protocols are considered, including pessimistic, optimistic, and semi-optimistic protocols. The approximate analysis captures the effect of the protocol on hardware resource contention and data contention. The accuracy of the approximation is validated through detailed simulations. It is found that the benefit of replicating data and the optimal number of replicates are sensitive to the concurrency control protocol. Under the optimistic and semi-optimistic protocols, replications can significantly improve response time with an additional MIPS (million instructions per second) requirement to maintain consistency among the replicates. The optimal degree of replication is further affected by the transaction mix (e.g. the fraction of read-only transactions), the communications delay and overhead, the number of distributed sites, and the available MIPS. Sensitivity analyses have been carried out to examine how the optimal degree of replication changes with respect to these factors. >

Performance evaluation of deterministic wormhole routing in k -ary n -cubes

Abstract We present a new analytical approach for the performance evaluation of deterministic wormhole routing in k -ary n -cubes. Our methodology achieves closed formulas for average time values through the analysis of network flows. The comparison with simulation models demonstrates that our methodology gives accurate results for both low and high traffic conditions. Another important quality is the flexibility of our approach. We demonstrate that it can be used to model dimension-ordered -routing in several k -ary n -cubes such as hypercubes, 3D symmetric and asymmetric tori, architectures with uni- and bi-directional channels.

Analysis of concurrency-coherency control protocols for distributed transaction processing systems with regional locality

A system structure and protocols for improving the performance of a distributed transaction processing system when there is some regional locality of data reference are presented. A distributed computer system is maintained at each region, and a central computer system with a replication of all databases at the distributed sites is introduced. It provides the advantage of distributed systems principally for local transactions, and has the advantage of centralized systems for transactions accessing nonlocal data. Specialized protocols keep the copies at the distributed and centralized systems consistent without incurring the overhead and delay of generalized protocols for fully replicated databases. The advantages achievable through this system structure and the tradeoffs between protocols for concurrency and coherency control of the duplicate copies of the databases are studied. An approximate analytic model is used to estimate the system performance. It is found that the performance is sensitive to the protocol and that substantial performance improvement can be obtained as compared with distributed systems. >

A VP-accordant checkpointing protocol preventing useless checkpoints

A useless checkpoint corresponds to the occurrence of a checkpoint and communication pattern called Z-cycle. A recent result shows that ensuring a computation without Z-cycles is a particular application of a property, namely Virtual Precedence (VP), defined on an interval-based abstraction of a computation. We first propose a taxonomy of communication-induced checkpointing protocols based on the way they ensure the VP property. Then we derive a sufficient condition ensuring no Z-cycles in a distributed computation. This condition defines a checkpoint and communication pattern, namely suspect Z-cycle, such that if no suspect Z-cycle exists in a distributed computation then no Z-cycle exists. We present finally a communication-induced checkpointing protocol that avoids useless checkpoints by preventing on-the-fly the formation of suspect Z-cycles and discuss its performance with respect to other protocols.

An accurate model for the performance analysis of deterministic wormhole routing

Presents a new analytical approach for the performance evaluation of asynchronous wormhole routing in k-ary n-cubes. Through the analysis of network flows, our methodology furnishes a closed formula for the average message delay in wormhole deterministic routing. In this paper, the focus is on 3D asymmetric torus networks with uni-directional or bi-directional links. However, the model can be easily applied to evaluate the performance of deterministic wormhole policies in any hypercube and torus topology. The comparison with two simulation models demonstrates that our methodology gives accurate results for both low and high traffics.

Machine Learning-Based Self-Adjusting Concurrency in Software Transactional Memory Systems

One of the problems of Software-Transactional-Memory (STM) systems is the performance degradation that can be experienced when applications run with a non-optimal concurrency level, namely number of concurrent threads. When this level is too high a loss of performance may occur due to excessive data contention and consequent transaction aborts. Conversely, if concurrency is too low, the performance may be penalized due to limitation of both parallelism and exploitation of available resources. In this paper we propose a machine-learning based approach which enables STM systems to predict their performance as a function of the number of concurrent threads in order to dynamically select the optimal concurrency level during the whole lifetime of the application. In our approach, the STM is coupled with a neural network and an on-line control algorithm that activates or deactivates application threads in order to maximize performance via the selection of the most adequate concurrency level, as a function of the current data access profile. A real implementation of our proposal within the TinySTM open-source package and an experimental study relying on the STAMP benchmark suite are also presented. The experimental data confirm how our self-adjusting concurrency scheme constantly provides optimal performance, thus avoiding performance loss phases caused by non-suited selection of the amount of concurrent threads and associated with the above depicted phenomena.

Performance analysis of circuit-switching interconnection networks with deterministic and adaptive routing

This paper compares three link conflict resolution strategies applied to multicomputers with symmetric topologies and circuit-switching interconnection networks. Several performance parameters are evaluated through an approximate analytical model based on the flow analysis. The main peculiarity of this method with respect to previous studies is the capacity to take into account actual network delays and all feedback effects among probability of link conflict, routing controller overhead, and message latency. An extensive simulation analysis has been carried out to validate the analytical models. The results show that our approach is quite accurate for a wide range of message traffic loads, independently of the link conflict resolution strategy and message length distribution.

A Performance Model of Multi-Version Concurrency Control

In this article we present a performance model for multi-version concurrency control (MVCC). This type of concurrency control is currently very popular among mainstream commercial and open source database systems thanks to its ability to well cope with read intensive workloads, as in the case of transaction profiles proper of Web applications. To build the model we had to tackle the intrinsic higher complexity of MVCC when compared to traditional concurrency control mechanisms (i.e. 2-phase-locking and optimistic ones), such as the joint use of locks and aborts to resolve direct conflicts among write accesses to the same data item, and the management of multiple data versions. We validate our analytical model via an extensive simulation study, considering both uniform and skewed data accesses, as well as differentiated transaction profiles. To the best of our knowledge, the present study provides the first analytical model of MVCC.

On the No-Z-Cycle Property in Distributed Executions

Given a checkpoint and communication pattern of a distributed execution, the No Z-Cycle property (NZC) states that a dependency between a checkpoint and itself does not exist. In other words, a noncausal sequence of messages that starts after a checkpoint and terminates before that checkpoint does not exist. From an operational point of view, this property corresponds to the fact that each checkpoint belongs to at least one consistent global checkpoint. So it could be used, for example, for restarting a distributed application after the occurrence of a failure. In this paper we derive a characterization of the NZC property (previously an open problem). It identifies a subset of Z-cycles, namely core Z-cycles (CZCs), that has to be empty in order that the checkpoint and communication pattern of the execution satisfies the NZC property. Then, we present a communication-induced checkpointing protocol that prevents CZCs on-the-fly. This protocol actually removes the common causal part to any CZC. Finally we propose a taxonomy of communication-induced checkpointing protocols that ensure the NZC property.

Performability Evaluation of Fault-Tolerant Satellite Systems

A combined performance and reliability (performability) measure for gracefully degradable fault-tolerant satellite systems is introduced, and a closed-form analytic solution is provided in order to compute the performability for a class of unrepairable systems. An efficient algorithm for the solution of Markovian models which take into account the degradation of more than one type of system components is given. By means of an example on a commercial communication-satellite system we intend to illustrate this novel measure and compare it to more conventional ones.