On Wed, 2012-07-11 at 03:36 -0500, Harti Brandt wrote: > On Wed, 11 Jul 2012, Peter Jeremy wrote: > > PJ>On 2012-Jul-10 10:03:08 -0500, Paul Albrecht <albre...@glccom.com> wrote: > PJ>>I have a question about the kqueue timer timeout period ... what's data > PJ>>supposed to be? I thought it was supposed to be the period in > PJ>>milliseconds, but I seem to off by one. > PJ>> > PJ>>For example, if I set date (the timeout period) to 20 milliseconds, I > PJ>>often wait 21 milliseconds which is very undesirable for my application. > PJ> > PJ>FreeBSD is not a real-time OS. The timeouts specified in various > PJ>syscalls (eg kevent(EVFILT_TIMER), nanosleep(), select(), poll()) > PJ>specify minimum timeouts. Once the timeout (rounded up to the next > PJ>tick) has expired, the process will be placed back into the queue > PJ>of processes eligible to be run by the scheduler - which may impose > PJ>a further arbitrary delay. > PJ> > PJ>Periodic timers are somewhat better behaved: Scheduler delays only > PJ>impact process scheduling after the timeout expires and the average > PJ>rate should be very close to that requested. > > While it is certainly true that FreeBSD is not a real-time OS, this does > not explain the timer problems. 2 or 3 month ago I did a simple test with > select and poll: I observed a systematic error of about 3-5% of the > waiting time. So when you wait for 20ms, you may get 21ms (if running with > a low HZ value) because of rounding. But if you wait for 100s, you get 103 > or even 105s on a completly idle machine (all services disabled). > > I think that this is not a problem with beeing non-realtime, but a problem > with time-keeping. Shouldn't it be possible to do this better? >
I don't think it has anything to do with realtime either. I've been using gentoo linux to run my application using timerfd_create/read for 20 millisecond timing without any problems. > harti -- Paul Albrecht _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"
