Leap-second considerations in distributed computer systems
Markus Kühn · 2003
computer clocks accurate user display, distributed activity scheduling, transmission rate control, performance monitoring, interval timing, unique timestamp generation, mergable distributed logs, causality checking, replacement for Lamport/vector clocks Typical application needs → simple and robust link to civilian time zones → monotonicity & robust measurement of time intervals → scalar representation → compatibility & synchrony → no rare events and difficult to test special cases 2 Computer-clock hazards and disruptions → increasingly unpredictable instruction execution times preemptive scheduling, context switches, virtual memory, interrupts, power-saving modes, system bus arbitration, cache latency, pipelining, multiprocessing, hyperthreading → lack of resolution Traditional filesystem timestamp resolution: 1 s → lack of synchronization → crystal frequency error (10 −4) and instability (10 −5) → operator error → UTC leap seconds Standard computer clocks are not well-suited or even designed for precision time-interval measurements and are therefore rarely used for this purpose directly. 3 Scalar time → Computer clock = oscillator + counter → raw counter value C mapped to standardized scalar time scale T, in the simplest case by a (piecewise) linear relation: