Open-sourcing a more precise time appliance • Hacker News Board

Re: Open-sourcing a more precise time appliance

#151

Post by eutectic » Wed, Aug 11, 2021, 9:10 PM UTC

joshvm wrote: post #144

eutectic wrote: post #95

Earlier quoted context omitted.

But does that really require an absolute time reference?

In these cases mostly not, they require low jitter references and a good counter. An example that does - I work for IceCube, a big neutrino detector at the South Pole. The detector is essentially a time of flight system that detects emitted light as it propagates through the ice when a neutrino interaction happens. We need good time distribution in the system so we can marry up hits from the 5000 odd sensors in the i…

Neat!

Re: Open-sourcing a more precise time appliance

#152

Post by jlgaddis » Wed, Aug 11, 2021, 9:13 PM UTC

adolph wrote: post #142

I don't see ESP8266 mentioned here: Overview: https://hackaday.com/2021/07/25/portable-gps-time-server-pow... Developer story: https://www.linkedin.com/pulse/iot-maker-tale-stratum-1-time... Repo: https://github.com/Montecri/GPSTimeServer A simpler version from the comments: https://enginemonitor.blogspot.com/2020/09/gps-module.html?m...

Probably because an ESP8266 isn't anywhere close enough to "real-time" to be of any benefit.

Why bother getting a nanosecond-level accurate time signal if you're going to feed it to an ESP8266 to distribute over Wi-Fi (or, in other words, reducing your accuracy to ~10 milliseconds or so)?

Re: Open-sourcing a more precise time appliance

#153

Post by jlgaddis » Wed, Aug 11, 2021, 9:18 PM UTC

riobard wrote: post #137

kube-system wrote: post #131

Earlier quoted context omitted.

I believe they usually get it from the network.

That was my impression as well but now I’m puzzled about why. Since most smartphones have GPS builtin why not get time from GPS?

Mostly because 1) your phone is getting time from the tower anyways (they have to be closely sync'd) and 2) GPS doesn't always work very well -- especially indoors.

Re: Open-sourcing a more precise time appliance

#154

Post by jlgaddis » Wed, Aug 11, 2021, 9:26 PM UTC

Well, dammit.

Of course they come out with this now, after I've got a few antennas up on the roof, half a dozen GPS-disciplined OCXOs providing 10 MHz and PPS signals throughout the house, which are being fed into SolarFlare PTP NICs, and sync'd throughout the network using PTP.

Apparently my workstations' offsets (RMS) are currently 17 and 21 nanoseconds, though, so I suppose there's still room for improvement.

Re: Open-sourcing a more precise time appliance

#155

Post by anon_tor_12345 » Wed, Aug 11, 2021, 9:39 PM UTC

LeifCarrotson wrote: post #146

daddylonglegs wrote: post #99

I cannot find any source files for the FPGA code in the repository (no .v .vhd .vhdl files and I can't see anything by looking around manually). The FPGA directory [1] contains some binaries and some documents relating to commercial code from a Swiss firm [2]. I've seen plenty of projects described as open source when the firmware is open source and the hardware is closed but this is the first time I've seen one wher…

It's a particular module for the Xilinx Artix 7 XC7A100T, made by a Shanghai-based integrator. As an individual user, you could buy these on Aliexpress, from eBay resellers, or from Amazon resellers. Facebook presumably has an arrangement with the contract manufacturer that's not going to be possible for an individual developer ordering ones and twos to replicate. However, to modify the HDL, you'd need a $3,000 licen…

you don't need vivado to modify the hdl, you need vivado (or some other proprietary xilinx toolchain) to produce the bitstream which actually deploys to the fpga.

Re: Open-sourcing a more precise time appliance

#156

Post by daddylonglegs » Wed, Aug 11, 2021, 9:54 PM UTC

LeifCarrotson wrote: post #146

daddylonglegs wrote: post #99

I cannot find any source files for the FPGA code in the repository (no .v .vhd .vhdl files and I can't see anything by looking around manually). The FPGA directory [1] contains some binaries and some documents relating to commercial code from a Swiss firm [2]. I've seen plenty of projects described as open source when the firmware is open source and the hardware is closed but this is the first time I've seen one wher…

It's a particular module for the Xilinx Artix 7 XC7A100T, made by a Shanghai-based integrator. As an individual user, you could buy these on Aliexpress, from eBay resellers, or from Amazon resellers. Facebook presumably has an arrangement with the contract manufacturer that's not going to be possible for an individual developer ordering ones and twos to replicate. However, to modify the HDL, you'd need a $3,000 licen…

Thank you for id'ing the module. I can now see how using this would work for Facebook's needs.

I think the XC7A100T is supported by the free (and limited) edition of Vivado [1]. Of course, the confusion about what you can and can't do with the HDL is in itself a barrier to open source and hobby work. This is true even when the work is theoretically possible and everyone has the best of intentions.

[1] https://www.xilinx.com/products/design-tools/vivado/vivado-m...

Re: Open-sourcing a more precise time appliance

#157

Post by howenterprisey » Wed, Aug 11, 2021, 10:20 PM UTC

myself248 wrote: post #94

amelius wrote: post #53

Earlier quoted context omitted.

Curious, why did telephony require such accurate clocks?

It's not about accuracy (nobody really cares if the whole network is running fast or slow compared to some other clock) so much as synchrony (the whole network needs to be running from the same clock). Asynchronous networks can't guarantee latency or jitter performance, and they require buffers. Synchronous networks (remember, this is voice-grade stuff) can deliver bits out at the same rate as bits in, anywhere in th…

Cool, thanks for the writeup. Would you be able to suggest any books or other sources for further reading about this?

Re: Open-sourcing a more precise time appliance

#158

Post by m-ee » Wed, Aug 11, 2021, 10:41 PM UTC

PragmaticPulp wrote: post #143

m-ee wrote: post #119

Earlier quoted context omitted.

Valve did something similar for the Vive tracker dev kit. There was a time before the standalone tracker was available https://www.vive.com/us/accessory/vive-tracker/ where if you wanted a tracker that wasn't a full controller you had to build it yourself. This involved a paid workshop in Seattle to get access to the dev kit and learn their tooling. Hardware was open source, but firmware was closed. If you wanted fir…

The Valve situation sounds like they attempted to open source after the fact, which doesn’t work with large hardware projects. The only way to really open source a hardware project of this magnitude is to set open source as a constraint from the start. If you don’t, you inevitably end up with someone licensing some code somewhere from a vendor that can’t be open sourced. It gets integrated deeply enough that the proj…

I shouldn't have called it open source, it was source available to customers who paid for the training course and signed a license agreement. Don't think they ever had intention of making it open. I found the model especially strange because of the requirement that any custom changes you asked for would be available to everyone else (your competitors). I guess they didn't actually want the workload of custom changes so this was a way of further disincentivizing it?

https://www.synapse.com/steamvr

Re: Open-sourcing a more precise time appliance

#159

Post by alpb » Wed, Aug 11, 2021, 10:56 PM UTC

abyagowi wrote: post #48

mananaysiempre wrote: post #9

Earlier quoted context omitted.

To improve the performance of their distributed systems by imposing stronger assumptions than full asynchronicity on their environment, I expect. (See also Google’s Spanner.)

Of course we go inspired by the Google Spanner project and what they have published over a decade ago. Hats off to the Google Spanner team

It seems Google has a better time determination method than the initial TrueTime used in Spanner now. https://www.usenix.org/system/files/osdi20-li_yuliang.pdf

Re: Open-sourcing a more precise time appliance

#160

Post by CrazyCatDog » Wed, Aug 11, 2021, 11:00 PM UTC

Are there further details available about the MAC?

Growing up in Boulder, THE atomic clock (NIST) was touted as a modern marvel (circa early 90’s childhood), it’s incredible to think that similar performance is now available on a pci-e card! Any relation to DARPA’s “making progress” announcement in August 2018 [1]?

[1] https://www.darpa.mil/news-events/2019-08-20