Multiple machines using physical clocks. How can we keep them more or even less synchronized.
Internal versus External synchronization
Perfect synchronization difficult due to communication delays
Even synchronization inside a bound cannot be assured along with guarantee due to unpredictability associated with communication delays.
How clocks work
Computer clocks tend to be crystals which oscillate in a particular rate of frequency
Every H oscillations, the actual timer chip interrupts once. Number associated with interrupts per second is usually 18.2, 50, 60, 100; could be greater, settable in some instances
The interrupt handler increments the counter which retains track associated with number of ticks from a reference previously
Knowing number associated with ticks for each second, we areable to determine year, month, day, time of day and so on.
Clock Drift
Unfortunately, period associated with crystal oscillation differs somewhat
If this oscillates quicker, much more ticks per real second, therefore clock runs faster; comparable with regard to slower clocks
For machine p, when correct reference time is t, let machine clock show time as C = Cp(t)
Ideally, Cp(t) = t for all p, t
In practice, 1 – ρ ≤ dC/dt ≤ 1 + ρ
ρ = max. clock drift rate, usually around 10-5 for cheap oscillators
Drift => Skew between clocks (difference in clock values of two machines)
Resynchronization
Periodic resynchronization required to offset skew
If two clocks tend to be drifting within reverse directions, maximum skew following period capital t is actually 2 ρ t
If application requires which clock skew < δ, after that resynchronization period r < δ /(2 ρ)
