How Precision Time Protocol is being deployed at Meta

I’m wondering whether the extremely careful GNSS part is really needed. A microsecond of offset between two servers in the same datacenter could easily matter, but I suspect that, if an entire datacenter were off by a microsecond, everything would be fine — communicating from that datacenter to anywhere else will take well over a microsecond, so an offset of this type would be a bit like the datacenter wiggling around in space a bit.

On a different note, there’s an Intel feature called the Always Running Timer. In theory one ought to be able to directly determine the TSC <-> NIC clock offset using the ART. I’m not sure anyone has gotten this to work, though.

Having rooftop mounted GNSS receive antennas for GPS+GLONASS is extremely common in telecom and ISP infrastructure applications. It's sort of a belt and suspenders approach to obtaining time from low stratum NTP sources and also having a local GNSS timing source to reference from.

Or for use in a case where the network has a total absence of connectivity to any internet-based NTP sources (maybe because your management network doesn't talk to the internet at all, for many good reasons), and in the event of loss of transport connectivity to your own somewhere-in-the-region NTP servers in your management system, you want to be absolutely sure the system clocks on your local radio transport equipment, DWDM equipment, metro ethernet stuff are all precise.

Using receive-only GPS data as a master time reference source is effectively saying "we think that getting the time as it's set by the USAF/Space Force people at Schriever AFB, who run the GPS system, should be treated as our master point of reference and all other clocks should derive from that". It's not a bad policy, as such things go, because GPS maintaining extremely accurate time is of great interest to the US federal government as a whole.

Even a fairly small LTE cellular rooftop site, monopole or tower often has a similar receiver. It doesn't add a lot of cost.

Meta seems to have gone with the expensive option. I think it’s this:

https://www.mouser.com/ProductDetail/HUBER%2bSUHNER/Direct-G...

Admittedly, at their scale, this is peanuts. But I wouldn’t buy one of these for a scrappy startup :). SparkFun will sell a perfectly serviceable kit for a few hundred dollars.

(If you are worried about lightning, then GPSoF looks like cheap insurance.)