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Monotonic clocks

A monotonic clock is suitable for measuring a duration (time interval), such as a timeout or a service’s response time: clock_gettime(CLOCK_MONOTONIC) on Linux and System.nanoTime() in Java are monotonic clocks, for example. The name comes from the fact that they are guaranteed to always move forward (whereas a time-of- day clock may jump back in time).

You can check the value of the monotonic clock at one point in time, do something, and then check the clock again at a later time. The difference between the two values tells you how much time elapsed between the two checks. However, the absolute value of the clock is meaningless: it might be the number of nanoseconds since the computer was started, or something similarly arbitrary. In particular, it makes no sense to compare monotonic clock values from two different computers, because they don’t mean the same thing.

On a server with multiple CPU sockets, there may be a separate timer per CPU, which is not necessarily synchronized with other CPUs. Operating systems compensate for any discrepancy and try to present a monotonic view of the clock to application threads, even as they are scheduled across different CPUs. However, it is wise to take this guarantee of monotonicity with a pinch of salt [40].

NTP may adjust the frequency at which the monotonic clock moves forward (this is known as slewing the clock) if it detects that the computer’s local quartz is moving faster or slower than the NTP server. By default, NTP allows the clock rate to be speeded up or slowed down by up to 0.05%, but NTP cannot cause the monotonic clock to jump forward or backward. The resolution of monotonic clocks is usually quite good: on most systems they can measure time intervals in microseconds or less.

In a distributed system, using a monotonic clock for measuring elapsed time (e.g., timeouts) is usually fine, because it doesn’t assume any synchronization between different nodes’ clocks and is not sensitive to slight inaccuracies of measurement.

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