Simulation model of dependability of redundant computer systems with recurrent information recovery

Today’s digital nanotechnology-based information management systems are especially sensitive to highly-energized particles during operation in irradiated areas. This sensitivity is most often manifested in the form of intermittent soft errors, i.e. distortion of information bits in the system’s memory elements with no hardware failure. The cause is in the afterpulses at the output of the logical elements that occur as the result of ionization of the gate area of the transistor’s semiconductor after it is exposed to a highly-energized particle. In order to counter soft errors the system is equipped with self-repair mechanisms that ensure regular replacement of distorted data with correct data. If this approach to design is employed, the significance of dependability analysis of the system under development increases significantly. Since regular occurrence of soft errors is essentially normal operating mode of a system in conditions of increased radiation, dependability analysis must be repeatedly conducted at the design stage, as that is the only way to duly evaluate the quality of the taken design decisions. The distinctive feature of fault-tolerant hardware and software systems that consists in the presence of nonprobabilistic recovery process limits the applicability of the known methods of dependability analysis. It is difficult to formalize the behaviour of such systems in the form of a dependability model in the context of the classic dependability theory that is geared towards the evaluation of hardware structure. As it has been found out, the application of conventional methods of dependability analysis (such as the Markovian model or probabilistic logic) requires making a number of assumptions that result in unacceptable errors in the evaluation results or its inapplicability.

Aim. Development of the model and methods of dependability analysis that would allow evaluating the dependability of hardware and software systems with periodic recovery.

Results. A simulation model was developed that is intended for dependability evaluation of complex recoverable information management systems. The model is a network of oriented state graphs that allows describing the behaviour of a recoverable system subject to the presence of computation processes and recovery processes that operate according to non-stochastic algorithms. Based on the simulation model, a software tool for dependability analysis was developed that enables probabilistic estimation of dependability characteristics of individual system units and its overall structure by means of computer simulation of failures and recoveries. This tool can be used for comprehensive dependability evaluation of hardware and software systems that involves the analysis of recoverable units with complex behaviour using the developed simulation model, and their operation along with simple hardware components, such as power supplies and fuses, using conventional analytical methods of dependability analysis. Such approach to dependability evaluation is implemented in the Digitek Reliability Analyzer dependability analysis software environment.

Practical significance.The application of the developed simulation model and dependability analysis tool at the design stage enables due evaluation of the quality of the produced fault tolerant recoverable system in terms of dependability and choose the best architectural solution, which has a high practical significance.

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