Date: Wed, 9 Sep 2009 22:46:16 +0200 From: Luigi Rizzo <rizzo@iet.unipi.it> 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: <20090909204616.GB93761@onelab2.iet.unipi.it> In-Reply-To: <20090907205955.GA91866@triton8.kn-bremen.de> References: <52d4a3890908250316l4de68725xa9d780e7d5b37205@mail.gmail.com> <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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On Mon, Sep 07, 2009 at 10:59:55PM +0200, Juergen Lock wrote: > [I'm copying freebsd-current@FreeBSD.org because ppl there might know > more about this...] > > qemu on FreeBSD hosts used to be able to run a (FreeBSD at least) guest > with the same HZ as the host (like, 1000) with (mostly) proper timing > once, but no longer. :( It seems there are two problems involved: > > a) use of apic seems to cause the clock irq rate to be doubled to 2 * HZ > (can anyone explain why?), i.e. a FreeBSD 7 guest on a FreeBSD 7 host > only gets proper timing after setting hint.apic.0.disabled=1 via the > loader. (as can be verified by `vmstat -i' and `time sleep 2' in an > installed guest or via the fixit->cdrom/dvd shell on a FreeBSD livefs > or dvd1 iso.) > > b) qemu running on FreeBSD 8 hosts (and most likely head) has the > additional problem of running its timers only at HZ/2 when using > setitimer(2) (called `-clock unix' in qemu), as seen below. (as also this problem in 8.x is caused by the bug i described here yesterday: http://lists.freebsd.org/pipermail/freebsd-current/2009-September/011393.html In qeumu, the setitimer call (in file vl.c) has a timeout of 1 tick which maps to callout_reset(..., 1, ...) and because (due to the bug) 8.x processes callouts 1 tick late, this effectively halves the clock rate. > seen below, timer_settime(2) aka `-clock dynticks' in qemu behaves > even worse, but that is similarly true on FreeBSD 7 which is why > I removed the patch that enabled that from our qemu port(s) a few > days ago.) And the only reason FreeBSD 8 guests are usually less > affected by these problems is they now reduce their HZ to 100 when > they detect being run in a VM. (which makes sense for other reasons > as well, don't get me wrong... but of course doesn't help when the > host is running with HZ=100 too.) cheers luigi
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