The Berkeley algorithm is a method of clock synchronisation in distributed computing which assumes no machine has an accurate time source. It was developed by Gusella and Zatti at the University of California, Berkeley in 1989. Like Cristian's algorithm, it is intended for use within intranets.
The algorithm
Unlike Cristian's algorithm, the server process in the Berkeley algorithm, called the master, periodically polls other slave processes. Generally speaking, the algorithm is:
- A master is chosen via an election process such as Chang and Roberts algorithm.
- The master polls the slaves who reply with their time in a similar way to Cristian's algorithm.
- The master observes the round-trip time (RTT) of the messages and estimates the time of each slave and its own.
- The master then averages the clock times, ignoring any values it receives far outside the values of the others.
- Instead of sending the updated current time back to the other process, the master then sends out the amount (positive or negative) that each slave must adjust its clock. This avoids further uncertainty due to RTT at the slave processes.
With this method the average cancels out individual clock's tendencies to drift. Gusella and Zatti released results involving 15 computers whose clocks were synchronised to within about 20-25 milliseconds using their protocol.
Computer systems normally avoid rewinding their clock when they receive a negative clock alteration from the master. Doing so would break the property of monotonic time, which is a fundamental assumption in certain algorithms in the system itself or in programs such as make. A simple solution to this problem is to halt the clock for the duration specified by the master, but this simplistic solution can also cause problems, although they are less severe. For minor corrections, most systems slow the clock (known as "clock slew"), applying the correction over a longer period of time.