One second per second is harder than it sounds

Re:One second per second is harder than it sounds by ChuckMcM on Tuesday July 22, 2014 @01:46AM (#8067537)

For a long time synchronized clocks was the networking code's equivalent joke to Regular Expressions ("Now you've got two problems ...") That said, having implemented a 100ns accurate clock using the PPS on a $30 GPS module (Adafruit) and a Beaglebone Black (multiple) these days there are some interesting alternatives. It would be interesting if the Open Compute folks would specify a 1PPS GPIO input as part of the spec, you could easily deliver rack level 1PPS accurate signals throughout the data center pretty cost effectively (a few wires, a transmitter, a TDR tool. Not that it will ever be a 'solved' problem but it certainly makes some transactional protocols safer.

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Re:One second per second is harder than it sounds by rdtsc on Tuesday July 22, 2014 @01:54AM (#8067550)

I've used PTP with special timestamping hardware (network cards) and could synchronize machines on a LAN pretty well. Now that doesn't guarantee global sync just sync between machines.

GPS NTP server can be obtained for as low as $600 or so. So those are becoming not as exotic now.

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Re:One second per second is harder than it sounds by ChuckMcM on Tuesday July 22, 2014 @01:33PM (#8070331)

So FWIW my beagle bone is a "GPS NTP server" and it was built out of the following:

1) BeagleBone $55 (http://www.adafruit.com/product/1876)

2) Proto cape $10 (http://www.adafruit.com/products/572)

3) Power supply $8 (http://www.adafruit.com/products/276)

4) Enclosure $7 (http://www.makershed.com/products/beaglebone-black-enclosure)

5) GPS board $40 w/ PPS (http://www.adafruit.com/products/746)

6) Ext Antenna ($13) (http://www.adafruit.com/products/960)

7) SMA to Antenna ($4) (http://www.adafruit.com/products/851)

So what $137 + tax and shipping, and the Robert Nelson Ubuntu pack with Openntpd running on it. The trick though in data centers is always getting visibility to GPS satellites. I wonder when the data center guys will figure the can add a 'cross connect' to the GPS on the roof for $75/month or something :-)

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Re:One second per second is harder than it sounds by rdtsc on Tuesday July 22, 2014 @03:02PM (#8070938)

That is pretty neat. I got my Beaglebone but haven't quite figured out what I am going to do with it. A GPS time server is a neat idea.

The price sounds about right. But, you also used your expertise, time and labor. Then add an enclosure box, documentation, support line, CE+FCC certifications and it can quickly get up there in price.

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Re:One second per second is harder than it sounds by ChuckMcM on Tuesday July 22, 2014 @03:37PM (#8071136)

Oh absolutely, I completely get why folks sell them for $600 each. (They were $25,000 each at one time which I found to be rather extortionate). I used to run ntpd on my home server to keep the machines in my house synced until the reflection attack using it came out and folks were dragging down my cheezy home net using it to attack others. Now I only run ntp inside the firewall :-)

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Re:One second per second is harder than it sounds by nick_riviera on Tuesday July 22, 2014 @05:05AM (#8067946)

We had radio time receivers (Rugby MSF, now Anthorn MSF) for a long time in the UK. Just plug them into an RS232 hole and have a site-wide master NTP server on that box, fire up the daemon and you're done.

I think the last one we paid for was in the region of $250 as the hardware is very simple.

http://www.npl.co.uk/science-technology/time-frequency/produ...

GPS is an interesting prospect but they are pretty HF which is a PITA if you have a unit inside a rack inside a machine room inside a building. MSF is 60KHz so is ground wave propagation so it had great coverage.

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Re:One second per second is harder than it sounds by easytiger on Tuesday July 22, 2014 @05:54AM (#8068106)

But worth noting it is only ±1 ms accuracy which is not useful in many applications.

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Re:One second per second is harder than it sounds by nick_riviera on Tuesday July 22, 2014 @06:29AM (#8068203)

Very true, but system-wide consistency across all participants is a different problem to external reference accuracy but it starts as a reasonable basis that you can reason about.

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Re:One second per second is harder than it sounds by sp332 on Tuesday July 22, 2014 @12:03PM (#8069666)

"A man with one watch always knows what time it is. A man with two watches is never quite sure..."

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