Clock instability and its effect on time intervals in performance studies
Workstations in a local area network environment must provide a time service that is synchronized with the other clocks on the LAN. To stay synchronized, computers on a network exchange information about the time via a time service protocol and adjust their own clock accordingly. These adjustments make the system clocks unstable (or tick unevenly). Unstable clocks introduce errors in the measurement of intervals as is done in performance studies, workload characterization, and serialization of distributed events. In this paper, we first present techniques for detecting clock instabilities in a LAN environment. We describe sources of instability of Sun and DEC workstations clocks on three levels: hardware, kernel, and network time service. Next, we discuss approaches to stabilizing clocks, and to collecting data correctly in spite of them. We describe how to build a performance clock in UNIX that is independent of the native system clock, and how to apply the Central Limit Theorem to compute confidence intervals for repeated measurements.
