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323e727a4c
Most systems (Linux, OpenBSD) adjust 0.5 ms per second, or 0.5 us per 1 ms tick. That is, the clock is sped up or slowed down by at most 0.05%. This means adjusting the clock by 1 s takes 2000 s, and the clock an be adjusted by at most 1.8 s per hour. FreeBSD adjusts 5 ms per second if the remaining time adjustment is >= 1 s (0.5%) , else it adjusts by 0.5 ms as well. This allows adjusting by (almost) 18 s per hour. Since Serenity OS can lose more than 22 s per hour (#3429), this picks an adjustment rate up to 1% for now. This allows us to adjust up to 36s per hour, which should be sufficient to adjust the clock fast enough to keep up with how much time the clock currently loses. Once we have a fancier NTP implementation that can adjust tick rate in addition to offset, we can think about reducing this. adjtime is a bit old-school and most current POSIX-y OSs instead implement adjtimex/ntp_adjtime, but a) we have to start somewhere b) ntp_adjtime() is a fairly gnarly API. OpenBSD's adjfreq looks like it might provide similar functionality with a nicer API. But before worrying about all this, it's probably a good idea to get to a place where the kernel APIs are (barely) good enough so that we can write an ntp service, and once we have that we should write a way to automatically evaluate how well it keeps the time adjusted, and only then should we add improvements ot the adjustment mechanism. |
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