How Precision Time Protocol is being deployed at Meta

How Precision Time Protocol is being deployed at Meta
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You need both.

PTP synchronizes clocks to a ridiculous precision, like down to nanoseconds. To do that, it uses support in the ethernet hardware. Hardware adds precision stamps to the PTP-related ethernet frames, so that the time calculations are free of jitters induced by the higher network stacks.

A cool thing is that PTP-aware routers/switches can actually rewrite the timestamps in PTP packets to subtract their own propagation delay accurately.

Something in the network has to use NTP, if it is important for the devices to have the correct calendar date and time; you don't use PTP over the Internet to get time from a remote server.

The real-time clock being synchronized isn't even the system clock in the first place, but a high resolution RTC that is in the network adapters and switches.

As an additional utility on top of the PTP stack, there is the possibility of synchronizing the host system's clock with the help of PTP.

PTP is used for things like generating precise signals in certain applications. E.g. Some devices that have PTP support can be programmed to generate periodic pulses which are clocked by the sychronized real-time clock. With PTP, two such signals from separate devices on the network can align very precisely. Such an application doesn't care about the time of day, just that the signals are synced.

> One could argue that we don’t really need PTP for that. NTP will do just fine. Well, we thought that too. But experiments we ran comparing our state-of-the-art NTP implementation and an early version of PTP showed a roughly 100x performance difference:

While I'm not necessarily against more accuracy/precision, what problems specifically are experiencing? They do mention some use cases of course:

> There are several additional use cases, including event tracing, cache invalidation, privacy violation detection improvements, latency compensation in the metaverse, and simultaneous execution in AI, many of which will greatly reduce hardware capacity requirements. This will keep us busy for years ahead.

But given that NTP (either ntpd or chrony) tends to give me an estimated error of around (tens of) 1e-6 seconds, and PTP can get down to 1e-9 seconds, I'm not sure how many data centre applications need that level of accuracy.

> We believe PTP will become the standard for keeping time in computer networks in the coming decades.

Given the special hardware needed for the grand master clock to get down to nanosecond time scales, I'm doubtful this will be used in most data centres of most corporate networks. Adm. Grace Hopper elegantly illustrates 'how long' a nanosecond is:

* https://www.youtube.com/watch?v=9eyFDBPk4Yw

How many things need to worry the latency of signal travelling ~300mm?

Is it really their "special sauce" though? Do these types of releases actually give away how these companies make money?

In this particular case, telling the world how to get to nanosecond levels of timekeeping doesn't really help any competitors take away Metabook's revenues or profits.