Date: Mon, 7 Sep 2009 22:17:23 -0400 From: Ryan Stone <rysto32@gmail.com> To: Juergen Lock <nox@jelal.kn-bremen.de> Cc: Mohammed Gamal <m.gamal005@gmail.com>, freebsd-current@freebsd.org, Jan Kiszka <jan.kiszka@web.de>, qemu-devel@nongnu.org, Avi Kivity <avi@qumranet.com> Subject: Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing) Message-ID: <bc2d970909071917n12256d9ucee24327aec3d527@mail.gmail.com> In-Reply-To: <20090907205955.GA91866@triton8.kn-bremen.de> References: <4A93AFF9.1060201@web.de> <52d4a3890908250321u746e5757u136030bcbc19208d@mail.gmail.com> <4A93BF0C.8040601@web.de> <20090826221001.GA1070@triton8.kn-bremen.de> <4A96C8D9.6070804@web.de> <20090829211848.GA59305@triton8.kn-bremen.de> <4A9B800F.1040209@web.de> <20090831212723.GA32448@triton8.kn-bremen.de> <20090901201248.GA60123@triton8.kn-bremen.de> <20090907205955.GA91866@triton8.kn-bremen.de>
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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.
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