Date: Wed, 9 Sep 2009 22:15:56 +0200 From: Juergen Lock <nox@jelal.kn-bremen.de> To: Ryan Stone <rysto32@gmail.com> Cc: Juergen Lock <nox@jelal.kn-bremen.de>, Avi Kivity <avi@qumranet.com>, qemu-devel@nongnu.org, freebsd-current@freebsd.org, Jan Kiszka <jan.kiszka@web.de>, Mohammed Gamal <m.gamal005@gmail.com> Subject: Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing) Message-ID: <20090909201556.GA95426@triton8.kn-bremen.de> In-Reply-To: <bc2d970909071917n12256d9ucee24327aec3d527@mail.gmail.com> References: <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> <bc2d970909071917n12256d9ucee24327aec3d527@mail.gmail.com>
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On Mon, Sep 07, 2009 at 10:17:23PM -0400, Ryan Stone wrote: > 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). > Yes, this is the first issue I reported, and it causes e.g. a FreeBSD 7 guest on a 7 host to sleep ~4s on a `time sleep 2'. (unless when I disable apic as I said.) > 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? > Yes, and the behaviour on 8 is in addition to the guest expecting clock irqs at 2000 Hz, i.e. on 8 the guest gets only around 500 Hz with -clock unix instead of ~1000 Hz on a 7 host, and `time sleep 2' then consequently sleeps ~8s. > * 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. Aha, so another way to get around the first issue would be to run at least the host with HZ=2000... Worth a try, tho I suspect it won't help the additional rate halving on 8, unless maybe to use even HZ=4000. Oh well... Juergen
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