We present a composite design methodology for the simulation and optimization of the solar cell performance. Our method is based on the synergy of different computational techniques and it is especially designed for the thin-film cell technology. In particular, we aim to efficiently simulate light trapping and plasmonic effects to enhance the light harvesting of the cell. The methodology is based on the sequential application of a hierarchy of approaches: (a) full Maxwell simulations are applied to derive the photon’s scattering probability in systems presenting textured interfaces; (b) calibrated Photonic Monte Carlo is used in junction with the scattering matrices method to evaluate coherent and scattered photon absorption in the full cell architectures; (c) the results of these advanced optical simulations are used as the pair generation terms in model implemented in an effective Technology Computer Aided Design tool for the derivation of the cell performance; (d) the models are investigated by qualitative and quantitative sensitivity analysis algorithms , to evaluate the importance of the design parameters considered on the models output and to get a first order descriptions of the objective space; (e) sensitivity analysis results are used to guide and simplify the optimization of the model achieved through both Single Objective Optimization (in order to fully maximize devices efficiency) and Multi Objective Optimization (in order to balance efficiency and cost); (f) Local, Global and “Glocal” robustness of optimal solutions found by the optimization algorithms are statistically evaluated; (g) data-based Identifiability Analysis is used to study the relationship between parameters. The results obtained show a noteworthy improvement with respect to the quantum efficiency of the reference cell demonstrating that the methodology presented is suitable for effective optimization of solar cell devices.

The Markov modulated Poisson process (MMPP) has been shown to well describe the flow of incoming traffic in networked systems, such as the Grid and the WWW. This makes the MMPP/M/1 queue a valuable instrument to evaluate and predict the service level of networked servers. In a recent work we have provided an approximate solution for the response time distribution of the MMPP/M/1 queue, which is based on a weighted superposition of M/M/l queues (i.e. a hyper-exponential process). In this article we address the tradeoff between the accuracy of this approximation and its computational cost. By jointly considering both accuracy and cost, we identify the scenarios where such approximate solution could be effectively used in support of network servers (dynamic) configuration and evaluation strategies, aimed at ensuring the agreed dependability levels in case of, e.g., request redirection due to faults.

The high-level architecture (HLA) standard developed by the Department of Defense in the United States is a key technology to perform distributed simulation. Inside the HLA framework, many different simulators (termed federates) may be interconnected to create a single more complex simulator (federation). Data distribution management (DDM) is an optional subset of services that controls which federates should receive notification of state modifications made by other federates. A simple DDM implementation will usually generate much more traffic than needed, whereas a complex one might introduce too much overhead. In this work, we describe an approach to DDM that delegates a portion of the DDM computation to a processor on the network card in order to provide more CPU time for other federate and Runtime Infrastructure (RTI) computations while still being able to exploit the benefits of a complex DDM implementation to reduce the amount of information exchange.

The high-level-architecture (HLA) is an IEEE standard for the interoperability and integration of (autonomous) simulation packages and applications (termed federates in the HLA context). It is based on a middleware-level component referred to as run-time-infrastructure (RTI) offering a set of interoperability services to the overlying simulation software. Time-management is the suite of services allowing synchronized execution among the federates, which, according to the HLA specification, covers pure conservative and pure optimistic synchronization schemes. In this paper we provide the design and implementation of a software layer, we refer to as risk-free-speculator (RFS), which supports an optimistic oriented intermediate approach to synchronization embedding the aggressiveness property of optimistic systems, but discarding risk. This is done in a totally transparent manner to the overlying applications, and does not even require any modification of the underlying RTI. The effectiveness of RFS has been tested against simulated demonstration exercises using the Joint Semi-Automated Forces (JSAF) simulation program.

Input characterization to describe the flow of incoming traffic in network systems, such as the GRID and the WWW, is often performed by using Markov modulated Poisson processes (MMPP). Therefore, to enact capacity planning and quality-of-service (QoS) oriented design, the model of the hosts that receive the incoming traffic is often described as a MMPP/M/1 queue. The drawback of this model is that no closed form for its solution has been derived. This means that evaluating even the simplest output statistics of the model, such as the average response times of the queue, is a computationally intensive task and its usage in the above contexts is often unadvisable. In this paper we discuss the possibility to approximate the behavior of a MMPP/M/1 queue with a computational effective analytical approximation, thus saving the large amount of calculations required to evaluate the same data by other means. The employed method consists in approximating the MMPP/M/1 queue as a weighted superposition of different M/M/1 queues. The analysis is validated by comparing the results of a discrete event simulator with those obtained from the proposed approximations, in the context of a real case study involving a GRID networked server.

Magic state manager (MASM) is recently developed software architecture for completely transparent checkpointing/recovery in support of optimistic synchronization in the high level architecture. In the original design, MASM relies on: (i) user level machine dependent modules; (ii) patches for specific versions of the LINUX kernel; and (iii) static linking of specific application libraries, all of them required for performing ad-hoc, low level memory management operations associated with optimistic synchronization requirements. In this paper, we propose a complete re-engineering of this software architecture which allows all those memory management tasks to be carried out through user level, machine independent modules, with the additional advantage of avoiding the need for static linking of specific application libraries, thus achieving portability of MASM across different UNIX systems and different computer architectures.

A content delivery network (CDN) is composed by a set of "reverse proxies" placed in proper geographical locations which provide caching and content distribution services to third party Web sites. Client requests are dispatched to one of the cache nodes that constitute the proxy by using content-aware and state-aware switching. Different distributions are generally believed to be more representative of the general traffic behavior; the classical Markovian model well captures the peculiarities of high intensity traffic during the busiest periods. The Markov chain is finite since, it admits a maximum amount of concurrently processed requests and derive the asymptotic state probabilities of the model of the CDN which can be finally used to configure the CDN with proper parameters to sustain the requested service levels, and thus to meet the SLA for each service class.

The notion of spatial uncertainty indicates the lack of exact knowledge about where, within the simulated space, an event actually occurs. In one of our previous works, we have shown how to exploit spatial uncertainty to reduce the synchronization cost in optimistic simulation, in terms of amount of rollback. In this paper we show how to exploit spatial uncertainty also for reducing the expected cost of simulation events during forward computation, thus achieving further reduction of the wall-clock time for the simulation model execution. The application of this proposal to optimistic simulation of a Personal Communication System (PCS) is also presented, together with experimental results supporting the claim of increased execution speed of the simulation system.

The high level architecture (HLA) standard developed by the Department of Defense in the United States is a key technology to perform distributed simulation. Inside the HLA framework, many different simulators (termed federates) may be interconnected to create a single, more complex simulator (federation). Data distribution management (DDM) is an optional subset of services that controls which federates should receive notification of state modifications made by other federates. A simple DDM implementation usually generate much more traffic than needed, while a complex one might introduce too much overhead. In this work, we describe an approach to DDM that delegates a portion of the DDM computation to a processor on the network card, in order to provide more CPU time for other federate and RTI computations, while still being able to exploit the benefits of a complex DDM implementation to reduce the amount of information exchange.

Checkpointing and Communication Library (CCL) is recently developed software in support of optimistic parallel discrete event simulation on myrinet clusters. Beyond low latency message delivery functionalities, CCL also offers non-blocking checkpointing functionalities supported by a programmable PCI DMA engine on board of myrinet cards. CCL employs resynchronization functionality between PCI DMA activities and CPU activities to maintain the consistency of checkpointed information (i.e. to prevent the CPU from updating information that still needs to be copied through DMAing). If re-synchronization is invoked before the checkpoint operation is completed, simulation activities carried out by the CPU may be forced to wait for checkpoint completion. Since data copy through the PCI DMA is slower than what achievable with the CPU, in pathological situations a re-synchronization period may last more than a whole checkpoint operation performed by the CPU, thus nullifying the potential benefit from offloading checkpointing from the CPU. This paper tackles such an issue by presenting the design and implementation of a handoff mechanism of checkpoint operations between PCI (Peripheral Component Interconnect) DMA (direct memory access)and CPU to enhance the effectiveness of checkpointing functionalities offered by CCL. Although a checkpoint operation is initially entrusted to the PCI DMA, whenever re-synchronization forces the simulation application to wait for its completion, the checkpoint operation is dynamically switched to the CPU, namely the fastest available device, since its timely completion has become a performance critical task for the simulation application.

Checkpointing and Communication Library (CCL) is a recently developed software implementing CPU offloaded checkpointing functionalities in support of optimistic parallel simulation on myrinet clusters. Specifically, CCL implements a non-blocking execution mode of memory-to-memory data copy associated with checkpoint operations, based on data transfer capabilities provided by a programmable DMA engine on board of myrinet network cards. Re-synchronization between CPU and DMA activities must sometimes be employed for several reasons, such as maintenance of data consistency, thus adding some overhead to (otherwise CPU cost-free) non-blocking checkpoint operations. In this paper we present a cost model for non-blocking checkpointing and derive a performance effective re-synchronization semantic which we call minimum cost re-synchronization MC. With this semantic, an occurrence of re-synchronization either commits an on-going DMA based checkpoint operation (causing suspension of CPU activities) or aborts the operation (with possible increase in the expected rollback cost due to a reduced amount of committed checkpoints) on the basis of a minimum overhead expectation evaluated through the cost model. We have implemented MC within CCL, and we also report experimental results demonstrating the performance benefits from this optimized re-synchronization semantic, in terms of increase in the execution speed, for a Personal Communication System (PCS) simulation application.

Describes a nonblocking checkpointing mode in support of optimistic parallel discrete event simulation. This mode allows real concurrency in the execution of state saving and other simulation specific operations (e.g, event list update, event execution) with the aim of removing the cost of recording state information from the completion time of the parallel simulation application. We present an implementation of a C library supporting nonblocking checkpointing on a myrinet based cluster, which demonstrates the practical viability of this checkpointing mode on standard off-the-shelf hardware. By the results of an empirical study on classical parameterized synthetic benchmarks, we show that, except for the case of minimal state granularity applications, nonblocking checkpointing allows improvement of the speed of the parallel execution, as compared to commonly adopted, optimized checkpointing methods based on the classical blocking mode. A performance study for the case of a personal communication system (PCS) simulation is additionally reported to point out the benefits from nonblocking checkpointing for a real world application.

In this paper we present a communication layer for Myrinet based clusters, designed to efficiently support preemptive rollback operations in optimistic parallel simulation. Beyond standard low latency message delivery functionalities, this layer also embeds functionalities for allowing the overlying simulation application to efficiently track whether an incoming message will actually produce causality inconsistency of the currently executed simulation event upon its receipt at the application level. Exploiting these functionalities, awareness of the inconsistency precedes the message receipt at the application level, thus allowing timely event execution interruption for activating rollback procedures. Experimental results on a standard simulation benchmark show that the layer we implement allows a strong reduction of the rollback overhead which, in its turn, yields strong performance improvements (up to 33%), especially in case of large parallelism in the simulation model execution.

The benefits of Web caching can be improved by systems of cooperative cache servers that share their cached documents. The increasing number of Web cache servers over the Internet makes the scalability of the cooperation protocol a major issue to be addressed. In this paper, we propose the Two-Tier Cooperation (2TC) protocol, which is specifically designed for systems of dozens or hundreds of cache servers with no centralized control. 2TC embeds two classical cooperation approaches for distributed Web caching systems, namely informed cooperation (IC) and query cooperation (QC), that are applied within different subsets of cache servers in the system. IC is applied within subsets of close servers and lets them cooperate through mutual exchange of state information related to their cache content. QC lets more distant cache servers cooperate through query/reply messages to locate documents within the global cache. Thanks to the use of IC among close cache servers, QC can explore the cache content of several cache servers through a single query message. High scalability arises as few queries explore the cache content of many cache servers and state information is exchanged within small groups of close cache servers. We report experimental results based on real traces that compare a prototype implementation of 2TC with classical protocols of the informed and query classes. The results point out a strong reduction (up to 50%) of the amount of transferred information to manage cooperation. This overhead reduction is achieved with no performance degradation in terms of latency and cache hit rate.