> 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: >>>> >>>> Solaris: http://dtrace.org/blogs/eschrock/2004/10/13/microstate-accounting-in-solaris-10/ >>>> 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@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