Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing)
Date:
Mon, 7 Sep 2009 22:17:23 -0400
I'm not entirely clear on why it's done this way, but the timer is run at twice hz for statistics-gathering purposes*. CPU usage statistics gathering is driven off of the timer interrupt. Running the timer at twice hz may be an attempt to eliminate clock-aliasing problems; if so, it's a poor way of doing so. In any case, seeing interrupts come in at twice hz is expected behaviour. This means that the guest will be requesting a timer interrupt rate of twice the granularity that the host's scheduler can support; this may be the cause of your other timing problems(although I have a hard time imagining how).
This timer is twice hz behaviour has existed at least since FreeBSD 6.1, so I can't explain why you see the new behaviour between 7 and 8. You do have hz set to 1000 on both the guest and host when running 7?
* Actually, from looking at the code the behaviour is dynamic. If hz >= 1500, the timer interrupt rate is set to hz. If 750 <= hz < 1500, the timer interrupt rate is set to 2 * hz. If hz < 750, the timer interrupt rate is set to 4 * hz.