[HN Gopher] Clock Synchronization (2020)
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       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?
        
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