Date: Fri, 11 Sep 2009 11:22:59 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-current@freebsd.org Cc: Juergen Lock <nox@jelal.kn-bremen.de>, Avi Kivity <avi@qumranet.com>, qemu-devel@nongnu.org, Jan Kiszka <jan.kiszka@web.de>, Mohammed Gamal <m.gamal005@gmail.com>, Luigi Rizzo <rizzo@iet.unipi.it> Subject: Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing) Message-ID: <200909111123.00257.jhb@freebsd.org> In-Reply-To: <20090910190800.GA14191@onelab2.iet.unipi.it> References: <4A93BF0C.8040601@web.de> <20090910174640.GA30706@triton8.kn-bremen.de> <20090910190800.GA14191@onelab2.iet.unipi.it>
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On Thursday 10 September 2009 3:08:00 pm Luigi Rizzo wrote: > 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; I would fix the style in the first hunk (spaces around '-') but I think you should commit this and get it into 8.0. I think a per-CPU ticks might prove very problematic as 'ticks' is rather widely used (though I would find that cleaner perhaps). -- John Baldwin
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