Hardware Assisted Precision Time Protocol . Design and case study

Keeping system time closely synchronized among all nodes of a cluster is a hard problem. The Network Time Protocol reliably synchronizes only to an accuracy of a few milliseconds. This is too coarse to compare time stamps of fast events on modern clusters, for example, the send and receive times of a message over a low-latency network. The Precision Time Protocol (PTP), defined in IEEE 1588, specifies a protocol which can substantially enhance the time accuracy across nodes in a local area network. An open source implementation of PTP (PTPd) relies on software time stamping, which is susceptible to jitter introduced by the non-realtime OS. An upcoming Ethernet NIC from Intel solves this problem by providing time stamping in hardware. This paper describes our modifications which allow PTPd to make use of this hardware feature, and evaluates several approaches for synchronizing the system time against the PTP time. Without hardware assistance, PTPd achieved accuracy as good as one microsecond; with hardware assistance, accuracy was reliably improved and dependence on real-time packet time stamping in software was virtually eliminated.