Clock Synchronization
Last updated
Last updated
Clock synchronization is naturally related to time, although it may not be necessary to have an accurate account of the real time.
It may be sufficient that every node in a distributed systems agrees on a current time.
For running make it is adequate that two nodes agree that input.o is outdated by a new version of input.c, for example.
In this case, keep tracking of each other's events is what matters.
Nearly all computers have a circuit for keeping track of time.
All processes on the same computer use the same clock.
Across multiple computers, clock skew happens.
Let clients contact a time server.
The problem is that when contacting the server, message delays will have outdated the report time.
The trick is to find a good estimation for these delays:
Step 1: A sends a request to B, timestamped with T1
Step 2: B records the time of receipt T2
Step 3: B returns a response timestamped with T3, piggybacking T2
Step 4: A records the time of the response's arrival, T4
If , A's clock is faster than B (the time server)
A can slow down its clock by advancing less time every clock tick
A can estimate its offset relative to B as:
The time server actively coordinates the clocks across the whole system.
This is useful when there is no accurate clock in the system.
For many purposes, it is sufficient for all computers to agree on the same time, not necessarily the real time.
The time daemon computes the average and tells each machine how to adjust its clock.
Suppose the client clock reads 5:30 and the server clock reads 5:20, then both the client and the server will be set to 5:25