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<div dir="auto">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: <a href="https://blog.beerriot.com/2021/02/14/starlink-raster-scan/" target="_blank">https://blog.beerriot.com/2021/02/14/starlink-raster-scan/</a> )<br />
<br />
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.<br />
<br />
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.<br />
<br />
Has nobody attacked the JTAG connector on a Dishy yet?</div>
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Best,<br />
<br />
Mike</div>
<div name="messageReplySection">On Jun 12, 2021, 00:14 +0200, David Collier-Brown <davecb.42@gmail.com>, wrote:<br />
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<p>OK, <i>Oh Smarter Colleagues</i>, 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?<br /></p>
<p>--dave<br /></p>
<div class="moz-cite-prefix">On 2021-06-09 9:15 a.m., Dave Taht wrote:<br /></div>
<blockquote type="cite" cite="mid:950B8EAF-90B9-41A6-951D-91821F591D41@teklibre.net"><br class="" />
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<blockquote type="cite" class="">
<div class="">Begin forwarded message:</div>
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<div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px;" class=""><span style="font-family: -webkit-system-font, Helvetica Neue, Helvetica, sans-serif; color:rgba(0, 0, 0, 1.0);" class=""><b class="">From:</b></span> <span style="font-family: -webkit-system-font, Helvetica Neue, Helvetica, sans-serif;" class="">David Collier-Brown <<a href="mailto:davecb.42@gmail.com" class="" moz-do-not-send="true">davecb.42@gmail.com</a>><br class="" /></span></div>
<div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px;" class=""><span style="font-family: -webkit-system-font, Helvetica Neue, Helvetica, sans-serif; color:rgba(0, 0, 0, 1.0);" class=""><b class="">Subject:</b></span> <span style="font-family: -webkit-system-font, Helvetica Neue, Helvetica, sans-serif;" class=""><b class="">Microstate Accounting and the Nyquist problem</b><br class="" /></span></div>
<div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px;" class=""><span style="font-family: -webkit-system-font, Helvetica Neue, Helvetica, sans-serif; color:rgba(0, 0, 0, 1.0);" class=""><b class="">Date:</b></span> <span style="font-family: -webkit-system-font, Helvetica Neue, Helvetica, sans-serif;" class="">June 9, 2021 at 4:44:14 AM PDT<br class="" /></span></div>
<div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px;" class=""><span style="font-family: -webkit-system-font, Helvetica Neue, Helvetica, sans-serif; color:rgba(0, 0, 0, 1.0);" class=""><b class="">To:</b></span> <span style="font-family: -webkit-system-font, Helvetica Neue, Helvetica, sans-serif;" class="">Dave Taht <<a href="mailto:davet@teklibre.net" class="" moz-do-not-send="true">davet@teklibre.net</a>><br class="" /></span></div>
<div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px;" class=""><span style="font-family: -webkit-system-font, Helvetica Neue, Helvetica, sans-serif; color:rgba(0, 0, 0, 1.0);" class=""><b class="">Cc:</b></span> <span style="font-family: -webkit-system-font, Helvetica Neue, Helvetica, sans-serif;" class="">Dave Collier-Brown <<a href="mailto:dave.collier-brown@indexexchange.com" class="" moz-do-not-send="true">dave.collier-brown@indexexchange.com</a>><br class="" /></span></div>
<div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px;" class=""><span style="font-family: -webkit-system-font, Helvetica Neue, Helvetica, sans-serif; color:rgba(0, 0, 0, 1.0);" class=""><b class="">Reply-To:</b></span> <span style="font-family: -webkit-system-font, Helvetica Neue, Helvetica, sans-serif;" class=""><a href="mailto:davecb@spamcop.net" class="" moz-do-not-send="true">davecb@spamcop.net</a><br class="" /></span></div>
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<p class="">A million years ago (roughly around Solaris 9), Sun was suffering from the same problems in measuring their dispatcher as you are with "sloshing".</p>
<p class="">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 <i class="">maybe</i> 10.1, if the sampler happened to sample right when the spike was happening.</p>
<p class="">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.<br class="" /></p>
<p class="">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.</p>
<p class="">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.</p>
<p class="">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.</p>
<p class="">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.</p>
<p class="">There was an effort to do kstats in Linux[2], but it had supposedly poor performance, and actual trouble when the clock frequency changed.<br class="" /></p>
<p class="">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?</p>
<p class="">--dave<br class="" /></p>
<p class=""><br class="" /></p>
<p class="">References:</p>
<ol class="">
<li class="">Solaris: <a class="moz-txt-link-freetext" href="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" originalsrc="http://dtrace.org/blogs/eschrock/2004/10/13/microstate-accounting-in-solaris-10/" shash="r++8CBJzd3EiHdQD4yln/JywHEgcQZcRkADLM58pY3y4GIQM79qWqmhLCC/gRJFmrZMcTsRTXYWsjJvwqLaUNTcyJvdbeC+sFPghSwQwf0ml5RWpT/hdeHE62U3EYo3yqhk0XWHHRmrDgD5wIcJPF8LNpbygu6zdFrcp5AUtudE=" moz-do-not-send="true">http://dtrace.org/blogs/eschrock/2004/10/13/microstate-accounting-in-solaris-10/</a><br class="" /></li>
<li class="">A failing Linux effort: <a class="moz-txt-link-freetext" href="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" originalsrc="https://lwn.net/Articles/127296/" shash="NeRrtMZSW/w5Z16EVp+L4mx76CnciKROysvnuvIYMwhpmHy1kPIu8UIvlSNPVJr3mrf6T8eg/A9RZFdY3ToSPDwOK9AXprKGFxF5bfPklETFT2/wyZDMQg+32h2Au2fNqlAk1p20ndsJ2B3+iEmm08ARfHCVl7c8Z3RpgKoan60=" moz-do-not-send="true">https://lwn.net/Articles/127296/</a>, <a class="moz-txt-link-freetext" href="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" originalsrc="https://sourceforge.net/projects/microstate/" shash="EaATi/ge264upYiz/waLkJTFhNT5ya2c8AUQqq5JoVbfuWm//GeW5inxQP8rULFqg7ezIt8agie84EjTPfOKSsmTlrVx7IVrdUIQdV3qSc0D2gNrGzTCYSEeYSd1AQhTNbTx3c8CCg2k4xiUArgx1w5vfMhPymyvv501lHYtKH4=" moz-do-not-send="true">https://sourceforge.net/projects/microstate/</a><br class="" /></li>
</ol>
<pre class="moz-signature" cols="72">--
David Collier-Brown, | Always do right. This will gratify
System Programmer and Author | some people and astonish the rest
<a class="moz-txt-link-abbreviated" href="mailto:davecb@spamcop.net" moz-do-not-send="true">davecb@spamcop.net</a> | -- Mark Twain
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