[Starlink] Microstate Accounting and the Nyquist problem

Fri Jun 11 19:09:00 EDT 2021

So the GPS is an STA8090 automotive-grade, makes sense as Dishy gets proper hot! Also supports sensor fusion for eg pointing in the right azimuth. https://www.st.com/en/automotive-infotainment-and-telematics/gnss-ics.html

However, the beauty of GPS sync is we can choose our own time base external to Dishy :-)

Best,

Mike
On Jun 12, 2021, 01:00 +0200, Dave Taht <davet at teklibre.net>, wrote:
>
>
> > On Jun 11, 2021, at 3:39 PM, Dave Taht <davet at teklibre.net> wrote:
> >
> >
> >
> > > On Jun 11, 2021, at 3:34 PM, Mike Puchol <mike at starlink.sx> wrote:
> > >
> > > We know that Starlink recalculates topology every 15 seconds (this guy, who obviously has way too much spare time, came up with an indirect observation of this interval: https://blog.beerriot.com/2021/02/14/starlink-raster-scan/ )
> > >
> > > If we could align with this, we could at least know when potential changes in path delays happen, and try to observe other changes that happen at a similar cadence.
> > >
> > > Other thoughts, try to plug more details out of the gRPC data, setup GPS-synced probes with a device at the exit PoP, measure differences between time-sync probes to an array of endpoints.
> > >
> >
> > It’s ironic that the device has to have gps in it, and thus should be  able to provide perfect time to clients directly behind it, isn’t.
> >
> > I haven’t captured a dhcp or dhcpv6 transaction yet myself,
> > do they have a ntp option?
> >
> > What gps software or driver might they have used? (esr’s gpsd is quite popular, but there are others)
> >
> > What’s the gps chip?
> >
>
> It would be good to have solid time everywhere, as I am seeing clocks not synced even close to 40ms accuracy of late.
>
> BTW, Eric Raymond (esr) is also one of the driving forces behind ntpsec, along with gary and a few other people now on our list.
>
> For more details, see:
> https://www.ntpsec.org/
>
> Once upon a time, I sat in esr's basement hearing him rip much crud out of the old ntpd codebase over the course of a very few days. The shouts “What? WHAAAT?” and most of the other pithy comments he made never made the git log.
>
> Over the years following the codebase got better and better, but adoption has been slow.
>
> An intro to that woefully underfunded project:
>
> NTPsec project - a secure, hardened, and improved implementation of Network Time Protocol derived from NTP Classic, Dave Mills’s original.
> NTPsec, as its name implies, is a more secure NTP. Our goal is to deliver code that can be used with confidence in deployments with the most stringent security, availability, and assurance requirements.
> Towards that end we apply best practices and state-of-the art technology in code auditing, verification, and testing. We begin with the most important best practice: true open-source code review. The NTPsec code is available in a public git repository. One of our goals is to support broader community participation.
>
> >
> > > Has nobody attacked the JTAG connector on a Dishy yet?
>
> I reached out to one of the teardown folk (mike (mikeonsoftware?)) months ago to get the debris but the rightest answer was to drill down into it on a still-alive ones.
>
> > >
> > > Best,
> > >
> > > Mike
> > > On Jun 12, 2021, 00:14 +0200, David Collier-Brown <davecb.42 at gmail.com>, wrote:
> > > > OK, Oh Smarter Colleagues, the challenge to you is to say if there is a "natural" place to capture state changes to get the data we want, and if so, is it common or similar enough between drivers to be worthy of attention?
> > > > --dave
> > > > On 2021-06-09 9:15 a.m., Dave Taht wrote:
> > > > >
> > > > >
> > > > > > Begin forwarded message:
> > > > > >
> > > > > > From: David Collier-Brown <davecb.42 at gmail.com>
> > > > > > Subject: Microstate Accounting and the Nyquist problem
> > > > > > Date: June 9, 2021 at 4:44:14 AM PDT
> > > > > > To: Dave Taht <davet at teklibre.net>
> > > > > > Cc: Dave Collier-Brown <dave.collier-brown at indexexchange.com>
> > > > > > Reply-To: davecb at spamcop.net
> > > > > >
> > > > > > A million years ago (roughly around Solaris 9), Sun was suffering from the same problems in measuring their dispatcher as you are with "sloshing".
> > > > > > A CPU would be 100% busy in one microsecond, 10% busy in the next gazillion, and the average CPU utilization for our sample period would be maybe 10.1, if the sampler happened to sample right when the spike was happening.
> > > > > > This was utterly useless for things like the fair-share scheduler, so it got fixed in Solaris 10, by having the dispatcher record the time a process (well, kernel thread) had spent in a state when the state changed.
> > > > > > Initially "microstate accounting" could be toggled on and off, but the branch-around cost more time than always doing the calculation (as discovered by my mad friend Fred) and the kernel folks left it on. It's on to this day.
> > > > > > In Simon Sundberg's talk, the opportunity to measure occurs every 1,000 packets, when a suitable timestamp is provided. While the eBPF program can look at every packet and do after-the-fact book-keeping in a map, that's only good if the phenomenon you're measuring is persistent enough that it's around for ~2,000 packets.
> > > > > > I'm going to suggest that the right place to record the information you want is right where the event happens.  Preferably in c code, as performance is easy to mess up, but perhaps with an eBPF mechanism to export it.
> > > > > > In previous Solaris work, I reliably found that exporting kstats was a darn sight harder than collecting them, and in Eric's blog post[1] he notes that converting time is expensive and best done long after collecting, when someone wanted to read the data.
> > > > > > There was an effort to do kstats in Linux[2], but it had supposedly poor performance, and actual trouble when the clock frequency changed.
> > > > > > Is there, in your opinion, a "natural" place to capture state changes to get the data you want, and if so, is it common or similar enough between drivers to be worthy of attention?
> > > > > > --dave
> > > > > >
> > > > > > References:
> > > > > >
> > > > > > 1. Solaris: http://dtrace.org/blogs/eschrock/2004/10/13/microstate-accounting-in-solaris-10/
> > > > > > 2. A failing Linux effort: https://lwn.net/Articles/127296/, https://sourceforge.net/projects/microstate/
> > > > > >
> > > > > > --
> > > > > > David Collier-Brown,         | Always do right. This will gratify
> > > > > > System Programmer and Author | some people and astonish the rest
> > > > > > davecb at spamcop.net           |                      -- Mark Twain
> > > > >
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