[Starlink] Fwd: Microstate Accounting and the Nyquist problem

[Mike Puchol]

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.

Has nobody attacked the JTAG connector on a Dishy yet?

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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