[HN Gopher] Clock Synchronization (2020) ___________________________________________________________________ Clock Synchronization (2020) Author : tosh Score : 42 points Date : 2021-08-08 17:49 UTC (5 hours ago) (HTM) web link (signalsandthreads.com) (TXT) w3m dump (signalsandthreads.com) | xrayarx wrote: | Link is to the transcript of a podcast, intro: Clock | synchronization, keeping all of the clocks on your network set to | the "correct" time, sounds straightforward: our smartphones sure | don't seem to have trouble with it. Next, keep them all accurate | to within 100 microseconds, and prove that you did -- now things | start to get tricky. In this episode, Ron talks with Chris Perl, | a systems engineer at Jane Street about the fundamental | difficulty of solving this problem at scale and how we solved it. | ithkuil wrote: | It's not hard to make something that works ok most of the time. | It's much harder to make it work all the time or at least to | know for sure when it didn't work. It all depends on what is at | stakes: if your clocks are wrong, are you just mildly | inconvenienced? Do you get a cached result that should have | expired? Is it a security problem (are you accepting a | certificate that has expired)? Is it a correctness problem (is | the causal order of a database update been inverted by | mistake)? There is a spectrum of severity of problems caused by | clock sync, and whether the solutions are trivially solved or | provably impossible (or anything in between) ultimately depends | on what you're trying to do. | gerdesj wrote: | Not sure why you were DVd. You have mentioned many valid | points related to timesync. | gerdesj wrote: | "the fundamental difficulty of solving this problem at scale | and how we solved it" | | Time is hard and the funny thing is - it's relative! Anyone who | says they have "solved" time is delusional. The best you can do | is reduce errors and problems related to timekeeping to an | acceptable level. | | I've "solved" time at work with three Raspberry Pis with GPS | boards/hats and aerials. My requirements are that logs be | correlatable and Kerberos etc work - so milli-second is enough. | My Pis have a spread of around 0.002ms ie 2us according to ntpq | -p. It doesn't take much to make it far worse. When I update | them, I do one at a time per day. Accuracy drops a bit. | | I might put in three more at home. ntp needs a lot of sources | to converge properly. Ideally five or more. I have quite a | decent connection to work. | | ... reads transcript. | | Oh well done - you discovered PTP. | | Anyone who actually knows about this stuff like to comment on | ntp vs ptp? ___________________________________________________________________ (page generated 2021-08-08 23:00 UTC)