Clock Sync User Manual

CLKSYNC provides high-resolution network time synchronization for constructing global timelines of distributed experiments. This is accomplished by allowing remote machines to synchronize their clocks to a timeserver running a modified Network Time Protocol (NTP) daemon. Clock resolution is significantly increased over that of the standard NTP. The exact clock synchronization level depends on the network over which CLKSYNC messages are exchanged between the NTP server and client machines, with the message delay variance having the biggest influence. The key point is that this subsystem includes Linux kernel and Ethernet driver modifications that, in conjunction with the associated user-level tools, maintain clock synchronization among a set of machines on a LAN at a resolution significantly below that of the minimum message delay among machines, which the standard NTP does not. This improved clock synchronization resolution, in conjunction with Datastreams data gathering and post-processing, permit the construction of a common global time line for a wide range of events within the user and system software on a distributed system. This is useful for any experiments where activities of interest span machine boundaries, but it is obviously vital for our current main interest which is the evaluation and control of MPI based distributed scientific computations.


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