[dmazin]

I highly recommend anyone to look up how PTP works and how it compares to NTP. Clock sync is very interesting. When I joined an HFT company, first thing I did was understand this stuff. We care about it a lot[1].

If you want a specific question to answer, answer this: why does PTP need hardware timestamping to achieve high precision (where the network card itself assigns timestamps to packets, rather than having the kernel do it as part of TCP/IP processing)? If we use software timestamps, why can we do microsecond precision at best? If you understand this, it goes a very long way to understanding the core ideas behind precise clock sync.

Once you have a solid understanding of PTP, look into White Rabbit. They’re able to sync two clocks with sub-ns precision. In case that isn’t obvious, that is absolutely insane.

[1] So do a lot of people. For example audio engineers. Once, an audio engineer absolutely talked my ear off about ptp. I had no idea that audio people understood clock sync so well but they do!

[RossBencina]

> So do a lot of people. For example audio engineers.

Indeed. PTP (various, not-necessarily compatible, versions) is at the core of modern ethernet-based audio networking: Dante (proprietary, PTP: IEEE 1588 v1), AVB (IEEE standard, PTP: 802.1AS), AES67 (AES standard, PTP: IEEE 1588 v2). And now the scope of the AVB protocol stack has been expanded to TSN for industrial and automotive time sensitive network applications.

[dmazin]

Yeah, the audio engineer then talked my ear off about networking!

[baby_souffle]

Not just audio, anybody in the live events / production space needs all equipment marching in lock step.

[8n4vidtmkvmk]

If it's for an event, can they not bring all the devices together in close proximity and sync them somehow? That at least removes network delays

[baby_souffle]

> That at least removes network delays

But that's often _the source_ you need to work around; there is no way in hell that they're going to get all the light/sound/video/graphics ... etc people in the same 2 square meter area to put on a show like the super bowl :).

For smaller events like a touring act or even a venue with a few hundred people capacity you still need a single master clock but this time it's not "wall time" and is "absolute" time. E.g.: a musician at the front of the house chooses when to start and the video/lighting guy in the back needs to be on the same page so the visuals line up [0].

[0]: https://en.wikipedia.org/wiki/SMPTE_timecode

[varjag]

You can't sync individual oscillators precisely for very long.

[hdjrudni]

Not even if you test hundreds of pairs to find a match?

[baby_souffle]

> Not even if you test hundreds of pairs to find a match?

Assuming you do find _a_ match, you still need everything else to be in sync across the different temperature(s) that each component will be operating at

[tombert]

I find time accuracy to be ridiculously interesting, and I have had to talk myself out of buying those a used atomic clock to play with [1]. I think precision time is very cool, and a small part of me wants to create the most overly engineered wall-clock using a Raspberry Pi or something to have sub-microsecond level accuracy.

Sadly, they're generally just a bit too expensive for me to justify it as a toy.

I don't work in trading (though not for lack of trying on my end), so most of the stuff I work on has been a lot more about "logical clocks", which are cool in their own right, but I have always wondered how much more efficient we could be if we had nanosecond-level precision to guarantee that locks are almost always uncontested.

[1] I'm not talking about those clocks that radio to Colorado or Greenwich, I mean the relatively small ones that you can buy that run locally.

[nickpsecurity]

The Wikipedia article has a good summary:

https://en.wikipedia.org/wiki/White_Rabbit_Project