Probing the latencies of software timestamping

Dealing effectively with latency is the key to accurate and reliable timekeeping over networks. Software components of timekeeping, including synchronisation algorithms such as ntpd, RADclock, and ptpd, must deal with the significant and highly variable latencies inherent to common operating systems. Using the DTrace system profiling tool, we provide an accurate breakdown of the latencies between common timestamping locations in the FreeBSD Operating System. We report on how these latency components react to stress patterns of different kinds, and determine which timestamping strategies result in the lowest latency, and the smallest in-host asymmetry. Our results can be used to improve timestamping and timekeeping for software clocks.