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Living Without Atomic Clocks

cockroachlabs.com

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Re: Living Without Atomic Clocks

#11

Post by danielsju6 »

bigdubs wrote: post #4
tlarkworthy wrote: post #2

I don't understand why 7ms is considered a good bound for atomic clocks? Hafele and Keating Experiment: "During October, 1971, four cesium atomic beam clocks were flown on regularly scheduled commercial jet flights around the world twice, once eastward and once westward, to test Einstein's theory of relativity with macroscopic clocks. From the actual flight paths of each trip, the theory predicted that the flying clo…

Something I never understood. If velocity (?) affects time, wouldn't time operate a different speed at different parts of the universe / solar system?

Yep, that's relativity.

Re: Living Without Atomic Clocks

#12

Post by noselasd »

bigdubs wrote: post #4
tlarkworthy wrote: post #2

I don't understand why 7ms is considered a good bound for atomic clocks? Hafele and Keating Experiment: "During October, 1971, four cesium atomic beam clocks were flown on regularly scheduled commercial jet flights around the world twice, once eastward and once westward, to test Einstein's theory of relativity with macroscopic clocks. From the actual flight paths of each trip, the theory predicted that the flying clo…

Something I never understood. If velocity (?) affects time, wouldn't time operate a different speed at different parts of the universe / solar system?

Yes, time is slower/faster in different parts of the solar system. Atomic clocks on the GPS satellites ticks 38 microseconds faster than on the surface of earth each day (which they correct for).

Re: Living Without Atomic Clocks

#13

Post by hnkimb3558 »

duskwuff wrote: post #3
tlarkworthy wrote: post #2

I don't understand why 7ms is considered a good bound for atomic clocks? Hafele and Keating Experiment: "During October, 1971, four cesium atomic beam clocks were flown on regularly scheduled commercial jet flights around the world twice, once eastward and once westward, to test Einstein's theory of relativity with macroscopic clocks. From the actual flight paths of each trip, the theory predicted that the flying clo…

Maybe they've mixed up ms and µs? Their claim of "between 100ms and 250ms" for NTP also seems way too high.

These are conservative upper bounds and are meant to encompass long tail offsets. You can usually get NTP down to < 10ms offsets, but if you rely on that, you'll likely run into problems which end up violating your database guarantees.

Re: Living Without Atomic Clocks

#14

Post by lifty »

vinkelhake wrote: post #8
tlarkworthy wrote: post #2

I don't understand why 7ms is considered a good bound for atomic clocks? Hafele and Keating Experiment: "During October, 1971, four cesium atomic beam clocks were flown on regularly scheduled commercial jet flights around the world twice, once eastward and once westward, to test Einstein's theory of relativity with macroscopic clocks. From the actual flight paths of each trip, the theory predicted that the flying clo…

The 7ms bound is on the machines that do the actual work, not the ones with access to special clock hardware.

Indeed. The machines that do the actual work will still use a an inaccurate clock and get periodic corrective updates. That being said, you should be able to get a much higher precision using PTP and some decent network cards. There are plenty of factors that come into play (network latency, congestion, etc) but 7 ms sets the bar low.

Re: Living Without Atomic Clocks

#15

Post by obastani »

bigdubs wrote: post #4
tlarkworthy wrote: post #2

I don't understand why 7ms is considered a good bound for atomic clocks? Hafele and Keating Experiment: "During October, 1971, four cesium atomic beam clocks were flown on regularly scheduled commercial jet flights around the world twice, once eastward and once westward, to test Einstein's theory of relativity with macroscopic clocks. From the actual flight paths of each trip, the theory predicted that the flying clo…

Something I never understood. If velocity (?) affects time, wouldn't time operate a different speed at different parts of the universe / solar system?

This is based on my limited physics education, but I believe there are two things at play:

1) Velocity doesn't affect time, acceleration does. So if your twin flies to Mars and back, you age more quickly. But if your twin flies to Mars, and you join him after a year (following the same trajectory), you age the same. This is special relativity.

2) Acceleration due to gravity doesn't count. In fact, the opposite is true -- by standing on the ground, you are accelerating up at 32 feet per second squared. This is general relativity.

So, to answer your question. On the one hand, yes, if I am accelerating (not counting gravity), then I observe time pass more slowly. On the other hand, most of the universe is dominated by gravitational forces, so most systems wouldn't notice these effects.

The fascinating thing is that both (1) and (2) happen because of a very basic physics principle: if I can't use an experiment to tell two frames of reference apart, then the physics in the two frames of reference are identical.

For (1), I observe the same speed of light as someone moving at velocity v relative to me. Einstein used this simple axiom to derive special relativity. For (2), I can't tell the difference between free fall and being at rest (alternatively, I can't tell the difference between being on Earth and being in an elevator in space). This is because the inertial mass equals gravitational mass, a "coincidence" that dates back to Newton's law of gravitation. From this (and a lot of math) Einstein derived general relativity. Beautiful!

Re: Living Without Atomic Clocks

#16

Post by jasonwatkinspdx »

First, CockroachDB's time api is based on the Hybrid Logical Clocks paper: http://www.cse.buffalo.edu/tech-reports/2014-04.pdf

This paper is one of the most interesting published in the last couple of years IMO. I remain surprised at how many people have overlooked it. I'm not sure why this blog post makes no mention of the work, in the past the Cockroach folks have been quite explicit about crediting the research documents they're drawing from.

Second, cloudera has a patent on this work: http://www.freepatentsonline.com/y2015/0156262.html

Re: Living Without Atomic Clocks

#17

Post by nickpsecurity »

Nice writeup. I disagree that we need chip-scale, atomic clocks. My idea was a dedicated, battery-backed piece of hardware that reliably stored time plus could sync other machines. Plugs into an interconnect with ultra-low latency. One for each datacenter.

You can plug them into each machine in the cluster periodically to sync them. Or you can plug it into a master node that connects with low-latency management interface separate from main data line. Occasionally, time server gets exclusive access to that line, assesses latency, and then syncs its time. Time server might be custom built to avoid its own skew or keep one of the timekeeping devices attached. Those are periodically shipped to a central location to resync themselves against an atomic clock or each other.

What yall think?

Re: Living Without Atomic Clocks

#18

Post by [deleted] »

obastani wrote: post #15
bigdubs wrote: post #4

Earlier quoted context omitted.

Something I never understood. If velocity (?) affects time, wouldn't time operate a different speed at different parts of the universe / solar system?

This is based on my limited physics education, but I believe there are two things at play: 1) Velocity doesn't affect time, acceleration does. So if your twin flies to Mars and back, you age more quickly. But if your twin flies to Mars, and you join him after a year (following the same trajectory), you age the same. This is special relativity. 2) Acceleration due to gravity doesn't count. In fact, the opposite is tru…

[deleted]

Re: Living Without Atomic Clocks

#19

Post by Retric »

obastani wrote: post #15
bigdubs wrote: post #4

Earlier quoted context omitted.

Something I never understood. If velocity (?) affects time, wouldn't time operate a different speed at different parts of the universe / solar system?

This is based on my limited physics education, but I believe there are two things at play: 1) Velocity doesn't affect time, acceleration does. So if your twin flies to Mars and back, you age more quickly. But if your twin flies to Mars, and you join him after a year (following the same trajectory), you age the same. This is special relativity. 2) Acceleration due to gravity doesn't count. In fact, the opposite is tru…

That's not really correct, if you sit at the center of a large object you don't accelerate, but you experience time dilation.

Further, as you dig deeper your acceleration decreases but time dilation continues to increase.

Re: Living Without Atomic Clocks

#20

Post by NeutronBoy »

nickpsecurity wrote: post #17

Nice writeup. I disagree that we need chip-scale, atomic clocks. My idea was a dedicated, battery-backed piece of hardware that reliably stored time plus could sync other machines. Plugs into an interconnect with ultra-low latency. One for each datacenter. You can plug them into each machine in the cluster periodically to sync them. Or you can plug it into a master node that connects with low-latency management inter…

> My idea was a dedicated, battery-backed piece of hardware that reliably stored time plus could sync other machines. Plugs into an interconnect with ultra-low latency. One for each datacenter.

Google have a variant on this, where they use a GPS receiver in each data centre to provide an accurate time source for local machines.

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