Rethinking multi-grain timestamps [LWN.net]

Rethinking multi-grain timestamps

Posted Oct 10, 2023 7:24 UTC (Tue) by Wol (subscriber, #4433)
In reply to: Rethinking multi-grain timestamps by NYKevin
Parent article: Rethinking multi-grain timestamps

> > At which point, don't you now get caught by relativity? :-)

> Not yet. The next logical step down from 1 ms resolution is 100 µs resolution, but as long as both machines are within thirty kilometers[1] of each other, the events in question are separated by a timelike interval, and so all observers will agree about the order in which they happen.

Fascinating! Yes really. But I think maybe I should have used the word "causality" rather than "relativity". My bad ...

But what I was trying to get at, is that if that distance is greater than your thirty kilometers, either you don't actually need to know the order, or any attempt to assign an order is essentially throwing dice at random. (I think about that with regard to distributed databases, and I'd certainly try to localise the problem to avoid those network effects ...)

At the end of the day, humans don't like it when the people who know say "it's unknowable". And in the example we appear to be discussing here, "make" running across a distributed file system, I find it hard to grasp how you can make the required sequential determinism work over the randomness of parallel file saves. If the system is running fast enough, or the network is large enough, the results will by the laws of physics be random, and any attempt to solve the problem is doomed to failure.

From what you're saying, we're nowhere near that limit yet, but we might get better results if we planned for hitting it, rather than pretending it's not there.

Cheers,
Wol

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Rethinking multi-grain timestamps

Posted Oct 10, 2023 7:43 UTC (Tue) by NYKevin (subscriber, #129325) [Link] (2 responses)

> But what I was trying to get at, is that if that distance is greater than your thirty kilometers, either you don't actually need to know the order, or any attempt to assign an order is essentially throwing dice at random.

No, that is not what relativity says. Relativity says that the order is *arbitrary*, not that it is random. There is no randomness introduced by events separated by spacelike intervals - you can and should just pick your favorite reference frame, and use Lorentz transformations to correct all observations in other frames to match it. This is an entirely deterministic mathematical process which will produce a total ordering of all events (unless your chosen reference frame says they are exactly simultaneous, which can be disregarded since your measurements are not perfectly precise anyway).

> At the end of the day, humans don't like it when the people who know say "it's unknowable". And in the example we appear to be discussing here, "make" running across a distributed file system, I find it hard to grasp how you can make the required sequential determinism work over the randomness of parallel file saves. If the system is running fast enough, or the network is large enough, the results will by the laws of physics be random, and any attempt to solve the problem is doomed to failure.

Yes, but this is not about relativity. This is about "I don't know how fast my network/SSD/whatever runs," or "I don't know how wrong my clock is." Those are much older problems, which have been well-understood in the world of distributed systems for decades. The most common approach is to use something like Paxos, Raft, or CRDTs, all of which explicitly establish "happens-before" relationships as a natural part of their consensus/convergence algorithms. Or, to put it in even simpler terms: The way you make sure X happens before Y is to have the computers responsible for X and Y talk to each other and arrange for that to be the case.[1]

[1]: It should be acknowledged that this is harder than it sounds. If you only have two computers, it may well be completely intractable, for some definitions of "talk to each other" - see the "two generals problem." But there are versions of this problem which are more tractable, and modern distributed systems are built around solving those versions of the problem.

Rethinking multi-grain timestamps

Posted Oct 10, 2023 12:33 UTC (Tue) by Wol (subscriber, #4433) [Link] (1 responses)

> you can and should just pick your favorite reference frame, and use Lorentz transformations to correct all observations in other frames to match it.

I'm thinking humans here. And stock markets. Where $billions could hang on the precise ordering of events. :-)

And yes, I know that in our macro world all reference frames are - to all intents and purposes - the same. But as soon as you say "pick your favourite frame", you're going to get people fighting for the one that is to their personal advantage.

Which is my point. As clocks get faster (the point of this article) and distances get greater (we're talking about a network), the greater the importance of the chosen reference frame, which is a matter of politics not maths. Which means we cannot appeal to logic for a solution.

Cheers,
Wol

Rethinking multi-grain timestamps

Posted Oct 10, 2023 16:11 UTC (Tue) by wittenberg (guest, #4473) [Link]

At this point, you need some old guy to point out the definitive discussion of this: Time, Clocks, and the Ordering of Events in a Distributed System
https://lamport.azurewebsites.net/pubs/time-clocks.pdf (1978) by Leslie Lamport. As you would expect from him, it's beautifully written. Everybody concerned with this issue should read it.

--David