Date: Thu, 10 Sep 2009 21:08:00 +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: <20090910190800.GA14191@onelab2.iet.unipi.it> In-Reply-To: <20090910174640.GA30706@triton8.kn-bremen.de> References: <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> <20090909204616.GB93761@onelab2.iet.unipi.it> <20090910174640.GA30706@triton8.kn-bremen.de>
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On Thu, Sep 10, 2009 at 07:46:40PM +0200, Juergen Lock wrote: > On Wed, Sep 09, 2009 at 10:46:16PM +0200, Luigi Rizzo wrote: > > 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. > > > Thanx for the pointer! > > The proposed patch in that post didn't make a different here tho, > guest still sees only half host HZ clock irq rate. (i.e. ~500 Hz.) > > Here is the patch I used, to make sure I patched what you meant... > > Index: sys/kern/kern_timeout.c > @@ -323,7 +323,7 @@ softclock(void *arg) > steps = 0; > cc = (struct callout_cpu *)arg; > CC_LOCK(cc); > - while (cc->cc_softticks != ticks) { > + while (cc->cc_softticks-1 != ticks) { > /* > * cc_softticks may be modified by hard clock, so cache > * it while we work on a given bucket. > as mentioned in the followup message in that thread, you also need this change in callout_tick() mtx_lock_spin_flags(&cc->cc_lock, MTX_QUIET); - for (; (cc->cc_softticks - ticks) < 0; cc->cc_softticks++) { + for (; (cc->cc_softticks - ticks) <= 0; cc->cc_softticks++) { bucket = cc->cc_softticks & callwheelmask; cheers luigi
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