From owner-freebsd-current@FreeBSD.ORG Thu Sep 10 20:45:47 2009 Return-Path: Delivered-To: freebsd-current@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id CCE111065679 for ; Thu, 10 Sep 2009 20:45:47 +0000 (UTC) (envelope-from nox@jelal.kn-bremen.de) Received: from smtp.kn-bremen.de (gelbbaer.kn-bremen.de [78.46.108.116]) by mx1.freebsd.org (Postfix) with ESMTP id 5CC4F8FC12 for ; Thu, 10 Sep 2009 20:45:47 +0000 (UTC) Received: by smtp.kn-bremen.de (Postfix, from userid 10) id 5A5991E005A0; Thu, 10 Sep 2009 22:45:46 +0200 (CEST) Received: from triton8.kn-bremen.de (noident@localhost [127.0.0.1]) by triton8.kn-bremen.de (8.14.3/8.14.3) with ESMTP id n8AKiXZ2002264; Thu, 10 Sep 2009 22:44:33 +0200 (CEST) (envelope-from nox@triton8.kn-bremen.de) Received: (from nox@localhost) by triton8.kn-bremen.de (8.14.3/8.14.3/Submit) id n8AKiVRq002263; Thu, 10 Sep 2009 22:44:31 +0200 (CEST) (envelope-from nox) From: Juergen Lock Date: Thu, 10 Sep 2009 22:44:31 +0200 To: Luigi Rizzo Message-ID: <20090910204431.GA2102@triton8.kn-bremen.de> References: <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> <20090910190800.GA14191@onelab2.iet.unipi.it> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090910190800.GA14191@onelab2.iet.unipi.it> User-Agent: Mutt/1.5.20 (2009-06-14) Cc: Juergen Lock , Avi Kivity , qemu-devel@nongnu.org, freebsd-current@freebsd.org, Jan Kiszka , Mohammed Gamal Subject: Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing) X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 10 Sep 2009 20:45:48 -0000 On Thu, Sep 10, 2009 at 09:08:00PM +0200, 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; > Ah I missed that, now guest clock irqs are back to HZ indeed. Thanx, :) Juergen