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Most systems fail when a certain amount of reliability quantities have exceeded their threshold levels. The typical example is cumulative damage model in which a system is subjected to shocks and suffers some damage due to shocks, and fails when the total damage has exceeded a failure level K. This paper proposes the following reliability model: Faults occur at a nonhomogeneous Poisson process and the system fails when N faults have occurred, which could be applied to optimization problems in computer systems with fault tolerance, and we suppose that the system is replaced before failure at a planned time T. Two cases where the threshold fault number N is constantly given and is a random variable are considered, we obtain the expected cost rates and discuss their optimal policies.

In the computer science community, the technique of garbage collection [5] is an automatic process of memory recycling, which refers to those objects in the memory no longer referenced by programs are called garbage and should be thrown away. A garbage collector determines which objects are garbage and makes the heap space occupied by such garbage available again for the subsequent new objects. Garbage collection plays an important role in Java’s security strategy, however, it adds a large overhead that can deteriorate the program performances. From related studies which are summarized in [5], a garbage collector spends between 25 and 40 percent of execution time of programs for its work in general, and delays caused by such garbage collection are obtrusive.

Hydraulic transformer can improve the transmission efficiency of the hydraulic system. As the manual hydraulic transformer cannot achieve automatic control and remote control in the hydraulic system, to meet this challenge, the electro-hydraulic servo plate-inclined plunger hydraulic transformer (EHHT) was proposed in this paper. First, through analyzing the principle of the EHHT, the structure of EHHT is designed based on the theoretical analysis; Secondly, the mathematical model and simulation model of output pressure were created. Finally, the fuzzy control strategy was designed and tested on the experiment bench. Both the simulated and experimental results show that the control strategy can effectively improve the pressure control performance of hydraulic transformer and the theoretical analysis is correct.

When a computer system executes successive jobs and processes with random time intervals, it would be impossible or impractical to make some inspections to check faults that occur intermittently in a strict periodic fashion. From such a viewpoint, by considering both factors such that periodic and random inspections, this paper proposes two new inspection policies in which the computer system is checked at a planned time T or at a random time Y, whichever occurs first and last. These policies are called inspection first and inspection last, respectively. Expected cost rates of each model until failure detection are obtained, and optimal inspection policies which minimize them are derived analytically. The comparison results of such two policies indicate that either inspection first or last is better than the other according to the ratio of checking cost to downtime cost from failure to its detection.

When failure occurs in a database, we execute the backup operations to the latest checking times. We formulate periodic and random inspection policies for a database. When the system is checked at the N th interval of working times, two total expected costs are obtained and optimal numbers N* which minimize them are derived analytically, when failure and random times are exponential. One modified backup model in which failures are detected only at checking times is proposed and two expected costs are obtained.

This paper proposes asymptotic maintenance models by using the hazard rate function, i.e., the expected number of failures in the time interval, when the failure time has a Weibull distribution. First, when failures occur at a nonhomogeneous Poisson process, the mean times between failures are given by a simple equation. The expected cost rate and optimal replacement time for the age replacement policy are also given by simple equations. Both periodic replacements at time and number of failures are shown to be a simple identical equation. Finally, an optimal imperfect preventive policy is derived easily and simply. It is shown numerically that such asymptotic policies for four models have good approximations to the standard ones.

For some computer processes, the processing time intervals may be ephemeral enough to consider the total damage of such system approximately increases continuously with time t. From such viewpoint, this paper proposes a basic replacement policy for continuous damage model and its application to garbage collection in computer science. First, we take up the reliability properties of the unit suffered for continuous damage when the parameters of the total damage are normal and exponential distributions. Second, a basic continuous damage model, where the unit is replaced at a planned time or when the total damage has exceeded a failure level, is given. Third, as the application of such model to garbage collection policies, optimal tenuring collection times are derived analytically and numerically.

This paper considers the following backup scheme for a database system: a database is updated at a nonhomogeneous Poisson process and an amount of updated files accumulates additively. To ensure the safety of data, full backup are performed at time NT=L or when the database fails, whichever occurs first, and between them, incremental backups are made at periodic times iT (i=1, 2, … , N-1) so as to make the backup efficiently. Using the theory of cumulative processes, the expected cost is obtained, and optimal number s N* of incremental backup which minimizes it are analytically discussed when incremental backup interval T or full backup interval L is given. Finally, it is shown as example s that optimal numbers are numerically computed.

It is an important problem to determine the tenuring threshold to meet the pause time goal for a generational garbage collector. From such viewpoint, this paper proposes two stochastic models based on the working schemes of a generational garbage collector: One is random collection which occurs at a nonhomogeneous Poisson process and the other is periodic collection which occurs at periodic times. Since the cost suffered for minor collection increases, as the amount of surviving objects accumulates, tenuring minor collection should be made at some tenuring threshold. Using the techniques of cumulative processes and reliability theory, expected cost rates with tenuring threshold are obtained, and optimal policies which minimize them are discussed analytically and computed numerically.

In order to study partial discharge pulses under impulse voltage effectively, a high-frequency current transducer for measurement of PD is developed. According to the equivalent circuit model, its principle and transmission characteristics are analyzed, and the effects of coil turns integral resistance and stray capacitance on frequency properties are studied. Moreover, calibration and analysis of the actual dynamic response are performed by adopting function generator and square-wave source. Performance tests show that it has good linearity and small phase shift with a wide bandwidth of 800 kHz∼105MHz and high sensitivity of 2.4V A−1. Based on the transducer, measurements of PD of needle-plate model under impulse voltage is identical to the results of theory analysis. It could be effective approach not only for deep study on PD character, but also to explore the mechanism under impulse voltage.

To counteract software aging more effectively, a new preventive software rejuvenation policy is proposed in this paper. This technique is implemented when additive consumption of resources has reached level Z(0 < Z les K), it is different from the conventional measurement-based rejuvenation policy, because software system is in low-efficient state and will face high-risk crash probability causing high-cost rejuvenation when additive consumption of resources has reached the threshold level K. As an important resource, physical memory is considered, and its consumption caused by aging-related bugs could be known by tests made at periodic times nT(n = 1, 2, hellip). Using the theory of cumulative processes, expected cost is obtained, and an optimal level Z* which minimizes it is analytically discussed.

Compared with the parallel system with independent failure, the factor, cascading failure, could reduce the reliability of the parallel system. This paper analyzes reliability of two-identical-units parallel system with cascading failure, using the theory of the renewal process and Markov process. To counteract the impact of cascading failure, preventive maintenance policy is put forward into this paper. Expected cost model is obtained. Optimal maintenance policy which minimizes the expected cost is analytically derived. At last, a numerical example for the model is presented.

This paper proposes two modified replacement policies which combine random replacement with standard age and periodic policies, where units undergo replacement at the first completion of some random working time over a planned time T, which are called replacement overtime. The expected cost rates of age and periodic replacement overtime polices are obtained, and optimal policies which minimize them are discussed analytically. Comparisons between such two overtime policies and standard age and periodic replacements are made in detail. When two costs for replacement overtime and standard age or periodic policies are the same, it is shown as estimated previously that the standard replacements are better than their overtime policies. Furthermore, two modified replacement overtime costs are discussed and computed numerically when such new policies would be better than the standard age and periodic replacements.