Internal Clock Drift Estimation in Computer Clusters

Eric S. Raymond esr at
Thu Aug 25 09:05:57 UTC 2016

ABSTRACT: 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.
		<a href="">Eric S. Raymond</a>

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