What you're seeing clearly isn't the TSC calibration problem that I was having, and Michael fixed (but which would not be fixed in 9.2, so it was a possible source) as what I was seeing was time actually running slow, and you're clearly not seeing that - just internal sleep timers running slow.
However, when I switched from using the (miscalibrated) TSC to using a different timer hardware source, after having started with a badly running TSC, I did observe behaviour much like you are reporting. That is, time was running at the right rate, but internal sleeps were slow - still running at the incorrect frequency. That was mentioned somwhere in the "Weird clock behaviour with current (amd64) kernel" thread. This one I never bothered looking for, as it seemed (to me) likely to be just a side effect of the miscalibrated TSC, but perhaps not - perhaps there is some other bug in the internal timing that is being triggered by something (if I had to guess, now, I'd say perhaps some overflow, just because of how long your system has been running without a reboot). I'd also assume this is something in (relatively) new code - clearly in the 9 series, but I have had older vintage NetBSD systems running much longer than 90 days without any issues, but I have never really run a 9.x system, except when that was HEAD - the long running systems tend to be older (early 8 at best), and on HEAD, I tend to reboot much more frequently to keep up to date. If this is what it is, a reboot would almost certainly fix things, for a while - but would also loose the ability to debug what is currently happening. kre
