Re: [Starlink] Microstate Accounting and the Nyquist problem

[Nathan Owens <nathan@nathan.io>]

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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 <davet@teklibre.net> wrote:

>
>
> On Jun 11, 2021, at 3:34 PM, Mike Puchol <mike@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?
>
>
> Has nobody attacked the JTAG connector on a Dishy yet?
>
> Best,
>
> Mike
> On Jun 12, 2021, 00:14 +0200, David Collier-Brown <davecb.42@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@gmail.com>
> *Subject:* *Microstate Accounting and the Nyquist problem*
> *Date:* June 9, 2021 at 4:44:14 AM PDT
> *To:* Dave Taht <davet@teklibre.net>
> *Cc:* Dave Collier-Brown <dave.collier-brown@indexexchange.com>
> *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/
>    <https://can01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fdtrace.org%2Fblogs%2Feschrock%2F2004%2F10%2F13%2Fmicrostate-accounting-in-solaris-10%2F&data=04%7C01%7C%7C7f7cd5aab2ca42e2e7e908d92d25e27f%7Cb07c069022b843668d8d7b845d088e18%7C1%7C0%7C637590463000477252%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=fdZDOtRCcBk%2BO1ksiTOSU%2FltR8IMwueHyj0kQG4UkHw%3D&reserved=0>
>    2. A failing Linux effort: https://lwn.net/Articles/127296/
>    <https://can01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flwn.net%2FArticles%2F127296%2F&data=04%7C01%7C%7C7f7cd5aab2ca42e2e7e908d92d25e27f%7Cb07c069022b843668d8d7b845d088e18%7C1%7C0%7C637590463000487248%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=uN0gq8vi0GJHMPpjKVYRjX6G5nQOc%2BugxUwUEk3%2BWJ8%3D&reserved=0>,
>    https://sourceforge.net/projects/microstate/
>    <https://can01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fsourceforge.net%2Fprojects%2Fmicrostate%2F&data=04%7C01%7C%7C7f7cd5aab2ca42e2e7e908d92d25e27f%7Cb07c069022b843668d8d7b845d088e18%7C1%7C0%7C637590463000497242%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=iMNi40Pl9hMmd1h7WrLFP5jmHQ60mJl7zehhO8miJv4%3D&reserved=0>
>
> --
> David Collier-Brown,         | Always do right. This will gratify
> System Programmer and Author | some people and astonish the restdavecb@spamcop.net           |                      -- Mark Twain
>
>
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