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Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage w
From: |
Luigi Rizzo |
Subject: |
Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing) |
Date: |
Thu, 10 Sep 2009 21:08:00 +0200 |
User-agent: |
Mutt/1.4.2.3i |
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 address@hidden 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
- Re: [Qemu-devel] Re: Breakage with local APIC routing, Juergen Lock, 2009/09/01
- FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing), Juergen Lock, 2009/09/07
- Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing), Luigi Rizzo, 2009/09/09
- Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing), Juergen Lock, 2009/09/10
- Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing),
Luigi Rizzo <=
- Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing), Juergen Lock, 2009/09/10
- Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing), John Baldwin, 2009/09/11
- Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing), Luigi Rizzo, 2009/09/12
- Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing), John Baldwin, 2009/09/11
- Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing), Luigi Rizzo, 2009/09/12