Ha, that’s a great point about the GPS/NTP thing. GPS chip, not sure, there’s only one tear down I’ve seen, here’s the gps chip: https://cdn.arstechnica.net/wp-content/uploads/2020/12/starlink-28-gps-receiver.jpg No idea about the software, maybe I’ll try to JTAG mine. On Fri, Jun 11, 2021 at 3:39 PM Dave Taht wrote: > > > On Jun 11, 2021, at 3:34 PM, Mike Puchol 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? > > > Has nobody attacked the JTAG connector on a Dishy yet? > > Best, > > Mike > On Jun 12, 2021, 00:14 +0200, David Collier-Brown , > 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 > *Subject:* *Microstate Accounting and the Nyquist problem* > *Date:* June 9, 2021 at 4:44:14 AM PDT > *To:* Dave Taht > *Cc:* Dave Collier-Brown > *Reply-To:* davecb@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 restdavecb@spamcop.net | -- Mark Twain > > > _______________________________________________ > Starlink mailing list > Starlink@lists.bufferbloat.net > https://lists.bufferbloat.net/listinfo/starlink > > _______________________________________________ > Starlink mailing list > Starlink@lists.bufferbloat.net > https://lists.bufferbloat.net/listinfo/starlink > > > _______________________________________________ > Starlink mailing list > Starlink@lists.bufferbloat.net > https://lists.bufferbloat.net/listinfo/starlink >