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221 points mfiguiere | 1 comments | | HN request time: 0.235s | source
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throw0101a ◴[] No.33696297[source]
So they state:

> 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?

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pclmulqdq ◴[] No.33698277[source]
I have never seen NTP in a datacenter of reasonable size get below an error of 1e-4. PTP without custom hardware can easily get 3 orders of magnitude better.
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1. bradknowles ◴[] No.33716051[source]
PTP is harder to do within a datacenter. You either need hardware support in every switch and network interface, or you're doing software timestamping at which point you might as well be using NTP. And PTP doesn't support DNS, only IPv4 or IPv6 or Layer 2 MAC addresses.

PTP also requires some intelligence with regards to configuring your Ordinary Clocks, your Transparent Clocks, and your Border Clocks. And you have to have these configured on every device in the network path.

PTP does have a unicast mode as well as multicast, which can help eliminate unknowable one-way latencies.

It's a pain.

Check the documentation at https://linuxptp.nwtime.org/documentation/ and especially https://linuxptp.nwtime.org/documentation/ptp4l/