This paper proposes and validates a methodology to measure explicitly the increase in the risk of a processor error with increasing workload. By relating the occurrence of a CPU related error to the system activity just prior to the occurrence of an error, the approach measures the dynamic CPU workload/failure relationship. The measurements show that the probability of a CPU related error (the load hazard) increases nonlinearly with increasing workload; i.e., the CPU rapidly deteriorates as end points are reached. The load hazard is observed to be most sensitive to system CPU utilization, the I/O rate, and the interrupt rates. The results are significant because they indicate that it may not be useful to push a system close to its performance limits (the previously accepted operating goal) since what we gain in slightly improved performance is more than offset by the degradation in reliability. Importantly, they also indicate that conventional reliability models need to be reevaluated so as to take system work-load explicity into account.

This paper demonstrates a practical approach to the study of the failure behavior of computer systems. Particular attention is devoted to the analysis of permanent failures. A number of important techniques, which may have general applicability in both failure and workload analysis, are brought together in this presentation. These include: smeared averaging of the workload data, clustering of like failures, and joint analysis of workload and failures. Approximately 17 percent of all failures affecting the CPU were estimated to be permanent. The manifestation of a permanent failure was found to be strongly correlated with the level and type of workload prior to the failure. Although, in strict terms, the results only relate to the manifestation of permanent failures and not to their occurrence, there are strong indications that permanent failures are both caused and discovered by increased activity. More measurements and experiments are necessary to determine their respective contributions to the measured workload/failure relationship.

This paper presents an analysis of operating system failures on an IBM 3081 running VM/SP. We find three broad categories of software failures: error handling (ERH), program control or logic (CTL), and hardware related (HS); it is found that more than 25 percent of software failures occur in the hardware/software interface. Measurements show that results on software reliability cannot be considered representative unless the system workload is taken into account. For example, it is shown that the risk of a software failure increases in a nonlinear fashion with the amount of interactive processing, as measured by parameters such as the paging rate and the amount of overhead (operating system CPU time). The overall CPU execution rate, although measured to be close to 100 percent most of the time, is not found to correlate strongly with the occurrence of failures. The paper discusses possible reasons for the observed workload failure dependency based on detailed investigations of the failure data.