Most computers have several high-resolution timing sources, from the programmable interrupt timer to the cycle counter. Yet, even at a precision of one cycle in ten millions, clocks may drift significantly in a single second at a clock frequency of several GHz. When tracing the low-level system events in computer clusters, such as packet sending or reception, each computer system records its own events using an internal clock. In order to properly understand the global system behavior and performance, as reported by the events recorded on each computer, it is important to estimate precisely the clock differences and drift between the different computers in the system. This article studies the clock precision and stability of several computer systems, with different architectures. It also studies the typical network delay characteristics, since time synchronization algorithms rely on the exchange of network packets and are dependent on the symmetry of the delays. A very precise clock, based on the atomic time provided by the GPS satellite network, was used as a reference to measure clock drifts and network delays. The results obtained are of immediate use to all applications which depend on computer clocks or network time synchronization accuracy.

Gaining a deep understanding of the detailed execution behavior and performance of complex software systems requires accurate tracing and analysis tools. Tracing tools such as the Linux Trace Toolkit (LTT) provide a detailed log of both system and user level events which may later be accessed and visualized. The information logged for each event includes the event type, a timestamp, and event type specific information. Most computer central processing units (CPU) now come with a cycle counter, thus enabling fast access, very high resolution (cycle level), timestamp values for events.Problems arise when tracing events from computer clusters containing numerous CPUs with independent clocks. Even CPUs in the same multiprocessor computer may not be fully synchronized, and in some cases run at different clock frequencies. Visualization tools, when presenting a timeline of events logged on different CPUs must therefore translate the individual timestamps to a common time reference, thus taking into account the clock delay for each CPU. Several experiments were conducted in order to characterize the CPU cycle counter accuracy and measure the synchronization accuracy achievable using conventional computer networks. The resulting data is of immediate use for all applications concerned with computer clocks or network synchronization accuracy. It will be used in the forthcoming version of the Linux Trace Toolkit to select the most appropriate mechanisms to compute a common time reference for events coming from different CPUs.