> 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