Date: Fri, 09 Mar 2012 11:38:28 +0200 From: Alexander Motin <mav@FreeBSD.org> To: freebsd-hackers@freebsd.org Cc: jeff@freebsd.org Subject: [RFC] Thread CPU load estimation for ULE, not sampled by hardclock() Message-ID: <4F59CF94.7070506@FreeBSD.org>
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Hi. At this time ULE scheduler uses hardclock() ticks via sched_tick() to estimate threads CPU load. That strictly limits it's best possible precision and that is one more reason to call hardclock() on every HZ tick, even when there is no callouts scheduled. I've made a patch to get CPU load estimation in ULE in event-based way. It won't immediately increase precision as I still use ticks counter as time source, but it should be trivial to use more precise one later (if some global, fast and reliable one is available). What it gives now, is that sched_tick() function is now empty for ULE, same as it always was for 4BSD. With some more changes in other areas it should allow to run hardclock() with full HZ rate only on one non-idle CPU, not on each one as it is now. One more small step toward tick-less kernel. Patch can be found here: http://people.freebsd.org/~mav/sched.notick4.patch Any comments? Important theoretical question: what exactly we want to see as CPU load? ps(1) defines %cpu as: "decaying average over up to a minute of previous (real) time", but I think that definition is not exactly true now. As I understand, 4BSD calculates pure decaying average without time limit -- if not precision limitation, CPU load value would include data for all the thread life time with exponentially decreasing weight for old values. ULE behaves partially alike, but has strict idle deadline after which load considered equal to zero and in some cases it decays more linearly, that seems also depends on if/how often CPU load is read. So which way should be considered right? Should it be clear decaying average as 4BSD does, or should it be something closer to "average load for last N seconds", following ideas I see in ULE now? -- Alexander Motin
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