woah hold on a sec. that's not how these clocks are actually used though.

It's a huge huge huge misconception that you can just plunk down an "atomic clock", discipline an NTP server with it and get perfect wallclock time out of it forever. That is just not how it works. Two hydrogen masers sitting next to each other will drift. Two globally distributed networks of hydrogen masers will drift. They cannot NOT drift. The universe just be that way.

UTC is by definition a consensus; there is no clock in the entire world that one could say is exactly tracking it.

Google probably has the gear and the global distribution that they could probably keep pretty close over 30-60 days, but they are assuredly not trying to keep their own independent time standard. Their goal is to keep events correlated on their own network, and for that they just need good internal distribution and consensus, and they are at the point where doing that internally makes sense. But this is the same problem on any size network.

Honestly for just NTP, I've never really seen evidence that anything better than a good GPS disciplined TCXO even matters. The reason they offer these oscillators in such devices is because they usually do additional duties like running PtP or distributing a local 10mhz reference where their specific performance characteristics are more useful. Rubidium, for instance, is very stable at short timescales but has awful long term stability.

It is also important to realize that an atomic clock will only give you a steady pulse. It will count seconds for you, and do so very accurately, but that is not the same as knowing what time it is.

If you get a rubidium clock for your garage, you can sync it up with GPS to get an accurate-enough clock for your hobby NTP project, but large research institutions and their expensive contraptions are more elaborate to set up.

Sure, but F2 is a bit more accurate: "As of February 2016 the IT-CsF2 cesium fountain clock started reporting a uB of 1.7 × 10−16 in the BIPM reports of evaluation of primary frequency standards." ( from https://web.archive.org/web/20220121090046/ftp://ftp2.bipm.o... )

Actually interesting to think about what UTC actually means and there is seems to be no absolute source of truth [0]. I guess the worry is not that much about the NTP servers (for which people anyways should configure fail overs) but the clocks themselves.

[0] https://www.septentrio.com/en/learn-more/insights/how-gps-br...

And more importantly, the tighter the time bound, the higher the performance, so more accurate clocks easily pay for themselves in other saved infrastructure costs to service the same number of users.

I don't know about all hyperscalers, but I have knowledge of one of them that has a large enough fleet of atomic frequency standards to warrant dedicated engineering. Several dozen frequency standards at least, possibly low hundreds. Definitely not one per machine, but also not just one per datacenter.

As you say, the goal is to keep the system clocks on the server fleet tightly aligned, to enable things like TrueTime. But also to have sufficient redundancy and long enough holdover in the absence of GNSS (usually due to hardware or firmware failure on the GNSS receivers) that the likelihood of violating the SLA on global time uncertainty is vanishingly small.

The "global" part is what pushes towards having higher end frequency standards, they want to be able to freewheel for O(days) while maintaining low global uncertainty. Drifting a little from external timescales in that scenario is fine, as long as all their machines drift together as an ensemble.

The deployment I know of was originally rubidium frequency standards disciplined by GNSS, but later that got upgraded to cesium standards to increase accuracy and holdover performance. Likely using an "industrial grade" cesium standard that's fairly readily available, very good but not in the same league as the stuff NIST operates.

GPS satellites have their own atomic clocks. They're synchronized to clocks at the GPS control center at Schriever Space Force Base, Colorado, formerly Falcon AFB. They in turn synchronize to NIST in Boulder, Colorado. GPS has a lot of ground infrastructure checking on the satellites, and backup control centers. GPS should continue to work fine, even if there's some absolute error vs. NIST. Unless there have been layoffs.

> There's a lot of focus in this thread on the atomic clocks but in most datacenters, they're not actually that important and I'm dubious that the hyperscalers actually maintain a "fleet" of them, in the sense that there are hundreds or thousands of these clocks in their datacenters.

I mean, fleets come in all sizes; but if you put one atomic reference in each AZ of each datacenter, there's a fleet. Maybe the references aren't great at distributing time, so you add a few NTP distributors per datacenter too and your fleet is a little bigger. Google's got 42 regions in GCP, so they've got a case for hundreds of machines for time (plus they've invested in spanner which has some pretty strict needs); other clouds are likely similar.

If NIST NTP goes down, the internet doesn’t go down. But atomic clocks drifting does upset many scientific experiments, which would effectively go down for the duration of the outage.

Mostly scientific stuff like astronomical observations — e.g. did this event observed at one telescope coincide with neutrinos detected at this other observatory.

Note I didn’t say they are more important than the Internet. That’s a value judgement in any case. I said that NIST level 0 NTp servers are more important to these use cases than they are to the Internet.

The ability for humankind to communicate across the entire globe at nearly 1/4 of the speed of light has drastically accelerated our technological advancement. There is no doubt that the internet is a HUGE addition to society.

It's not super important when compared to basic needs like plumbing, food, electricity, medical assistance and other silly things we take for granted but are heavily dependent on. We all saw what happened to hospitals during the early stages of the COVID pandemic; we had plenty of internet and electricity but were struggling on the medical part. That was quite bad... I'm not sure if it's any worse if an entire country/continent lost access to the Internet. Quite a lot of our core infrastructure components in society rely on this. And a fair bit of it relies on a common understanding of what time "now" is.

I think it wont be affected by this but on the top of my head:

- GPS

- industrial complex that synchronize operations (we could include trains)

- telecoms in general (so a level higher than the internet)