Software-implemented fault insertion: An FTMP example

This report presents a model for fault insertion through software; describes its implementation on a fault-tolerant computer, FTMP; presents a summary of fault detection, identification, and reconfiguration data collected with software-implemented fault insertion; and compares the results to hardware fault insertion data. Experimental results show detection time to be a function of time of insertion and system workload. For the fault detection time, there is no correlation between software-inserted faults and hardware-inserted faults; this is because hardware-inserted faults must manifest as errors before detection, whereas software-inserted faults immediately exercise the error detection mechanisms. In summary, the software-implemented fault insertion is able to be used as an evaluation technique for the fault-handling capabilities of a system in fault detection, identification and recovery. Although the software-inserted faults do not map directly to hardware-inserted faults, experiments show software-implemented fault insertion is capable of emulating hardware fault insertion, with greater ease and automation

Fault-free performance validation of fault-tolerant multiprocessors

A validation methodology for testing the performance of fault-tolerant computer systems was developed and applied to the Fault-Tolerant Multiprocessor (FTMP) at NASA-Langley's AIRLAB facility. This methodology was claimed to be general enough to apply to any ultrareliable computer system. The goal of this research was to extend the validation methodology and to demonstrate the robustness of the validation methodology by its more extensive application to NASA's Fault-Tolerant Multiprocessor System (FTMP) and to the Software Implemented Fault-Tolerance (SIFT) Computer System. Furthermore, the performance of these two multiprocessors was compared by conducting similar experiments. An analysis of the results shows high level language instruction execution times for both SIFT and FTMP were consistent and predictable, with SIFT having greater throughput. At the operating system level, FTMP consumes 60% of the throughput for its real-time dispatcher and 5% on fault-handling tasks. In contrast, SIFT consumes 16% of its throughput for the dispatcher, but consumes 66% in fault-handling software overhead

Do bilinguals have different concepts? The case of shape and material in Japanese L2 users of English

An experiment investigated whether Japanese speakers’ categorisation of objects and substances as shape or material is influenced by acquiring English, based on Imai and Gentner (1997). Subjects were presented with an item such as a cork pyramid and asked to choose between two other items that matched it for shape (plastic pyramid) or for material (piece of cork). The hypotheses were that for simple objects the number of shape-based categorisations would increase according to experience of English and that the preference for shape and material-based categorisations of Japanese speakers of English would differ from mono¬lingual speakers of both languages. Subjects were 18 adult Japanese users of English who had lived in English-speaking countries between 6 months and 3 years (short-stay group), and 18 who had lived in English-speaking countries for 3 years or more (long-stay group). Both groups achieved above criterion on an English vocabulary test. Results were: both groups preferred material responses for simple objects and substances but not for complex objects, in line with Japanese mono¬linguals, but the long-stay group showed more shape preference than the short-stay group and also were less different from Americans. These effects of acquiring a second language on categorisation have implications for conceptual representation and methodology

Directional persistence & the optimality of run-and-tumble chemotaxis

E. coli does chemotaxis by performing a biased random walk composed of alternating periods of swimming (runs) and reorientations (tumbles). Tumbles are typically modelled as complete directional randomisations but it is known that in wild type E. coli, successive run directions are actually weakly correlated, with a mean directional difference of not, vert, similar63°. We recently presented a model of the evolution of chemotactic swimming strategies in bacteria which is able to quantitatively reproduce the emergence of this correlation. The agreement between model and experiments suggests that directional persistence may serve some function, a hypothesis supported by the results of an earlier model. Here we investigate the effect of persistence on chemotactic efficiency, using a spatial Monte Carlo model of bacterial swimming in a gradient, combined with simulations of natural selection based on chemotactic efficiency. A direct search of the parameter space reveals two attractant gradient regimes, (a) a low-gradient regime, in which efficiency is unaffected by directional persistence and (b) a high-gradient regime, in which persistence can improve chemotactic efficiency. The value of the persistence parameter that maximises this effect corresponds very closely with the value observed experimentally. This result is matched by independent simulations of the evolution of directional memory in a population of model bacteria, which also predict the emergence of persistence in high-gradient conditions. The relationship between optimality and persistence in different environments may reflect a universal property of random-walk foraging algorithms, which must strike a compromise between two competing aims: exploration and exploitation. We also present a new graphical way to generally illustrate the evolution of a particular trait in a population, in terms of variations in an evolvable parameter

Optimal transport on wireless networks

We present a study of the application of a variant of a recently introduced heuristic algorithm for the optimization of transport routes on complex networks to the problem of finding the optimal routes of communication between nodes on wireless networks. Our algorithm iteratively balances network traffic by minimizing the maximum node betweenness on the network. The variant we consider specifically accounts for the broadcast restrictions imposed by wireless communication by using a different betweenness measure. We compare the performance of our algorithm to two other known algorithms and find that our algorithm achieves the highest transport capacity both for minimum node degree geometric networks, which are directed geometric networks that model wireless communication networks, and for configuration model networks that are uncorrelated scale-free networks.Comment: 5 pages, 4 figure

Breather decay into a vortex/anti-vortex pair in a Josephson Ladder

We present experimental evidence for a new behavior which involves discrete breathers and vortices in a Josephson Ladder. Breathers can be visualized as the creation and subsequent annihilation of vortex/anti-vortex pairs. An externally applied magnetic field breaks the vortex/anti-vortex symmetry and causes the breather to split apart. The motion of the vortex or anti-vortex creates multi-site breathers, which are always to one side or the other of the original breather depending on the sign of the applied field. This asymmetry in applied field is experimentally observed.Comment: 10 pages, 5 figure

Gene expression analysis on small numbers of invasive cells collected by chemotaxis from primary mammary tumors of the mouse

BACKGROUND: cDNA microarrays have the potential to identify the genes involved in invasion and metastasis. However, when used with whole tumor tissue, the results average the expression patterns of different cell types. We have combined chemotaxis-based cell collection of the invasive subpopulation of cells within the primary tumor with array-based gene expression analysis to identify the genes necessary for the process of carcinoma cell invasion. RESULTS: Invasive cells were collected from live primary tumors using microneedles containing chemotactic growth factors to mimic chemotactic signals thought to be present in the primary tumor. When used with mammary tumors of rats and mice, carcinoma cells and macrophages constitute the invasive cell population. Microbeads conjugated with monoclonal anti-CD11b (Mac-1α) antibodies were used to separate macrophages from carcinoma cells. We utilized PCR-based cDNA amplification from small number of cells and compared it to the quality and complexity of conventionally generated cDNA to determine if amplified cDNA could be used with fidelity for array analysis of this cell population. These techniques showed a very high level of correlation indicating that the PCR based amplification technique yields a cDNA population that resembles, with high fidelity, the original template population present in the small number of cells used to prepare the cDNA for use with the chip. CONCLUSIONS: The specific collection of invasive cells from a primary tumor and the analysis of gene expression in these cells are is now possible. By further comparing the gene expression patterns of cells collected by invasion into microneedles with that of carcinoma cells obtained from the whole primary tumor, the blood, and whole metastatic tumors, genes that contribute to the invasive process in carcinoma cells may be identified