[Starlink] Fwd: Microstate Accounting and the Nyquist problem

[Dave Taht]

> 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:
> 
> Solaris: http://dtrace.org/blogs/eschrock/2004/10/13/microstate-accounting-in-solaris-10/ <http://dtrace.org/blogs/eschrock/2004/10/13/microstate-accounting-in-solaris-10/> 
> A failing Linux effort: https://lwn.net/Articles/127296/ <https://lwn.net/Articles/127296/>,https://sourceforge.net/projects/microstate/ <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 <mailto:davecb at spamcop.net>           |                      -- Mark Twain

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