From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=55263 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1PmRL3-0001mi-UZ for qemu-devel@nongnu.org; Mon, 07 Feb 2011 08:41:11 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1PmRL1-0004jI-Qy for qemu-devel@nongnu.org; Mon, 07 Feb 2011 08:41:09 -0500 Received: from mx1.redhat.com ([209.132.183.28]:63212) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1PmRL1-0004im-AP for qemu-devel@nongnu.org; Mon, 07 Feb 2011 08:41:07 -0500 Date: Mon, 7 Feb 2011 15:41:04 +0200 From: Gleb Natapov Subject: Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift Message-ID: <20110207134104.GE14984@redhat.com> References: <1375835067.226263.1296740625327.JavaMail.root@zmail07.collab.prod.int.phx2.redhat.com> <4D4AC99A.2070803@siemens.com> <4D4B0B07.2040904@codemonkey.ws> <4D4B1CF8.8040800@web.de> <4D4B5F23.7040801@codemonkey.ws> <4D4BBF55.9060000@web.de> <4D4FE6BF.5080502@redhat.com> <4D4FEF81.1040603@codemonkey.ws> <4D4FF02F.2030309@redhat.com> <4D4FF24A.7000004@codemonkey.ws> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4D4FF24A.7000004@codemonkey.ws> List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: kvm , Glauber Costa , Ulrich Obergfell , qemu-devel , Jan Kiszka , Avi Kivity On Mon, Feb 07, 2011 at 07:23:22AM -0600, Anthony Liguori wrote: > On 02/07/2011 07:14 AM, Avi Kivity wrote: > >On 02/07/2011 03:11 PM, Anthony Liguori wrote: > >>On 02/07/2011 06:34 AM, Avi Kivity wrote: > >>>On 02/04/2011 10:56 AM, Jan Kiszka wrote: > >>>>> > >>>>> This should be a rare event. If you are missing 50% of your > >>>>> notifications, not amount of gradual catchup is going to > >>>>help you out. > >>>> > >>>>But that's the only thing this patch is after: lost ticks at > >>>>QEMU level. > >>> > >>>Most lost ticks will happen at the vcpu level. The iothread > >>>has low utilization and will therefore be scheduled promptly, > >>>whereas the vcpu thread may have high utilization and will > >>>thus be preempted. When it is preempted for longer than the > >>>timer tick, we will see vcpu-level coalescing. All it takes > >>>is 2:1 overcommit to see time go half as fast; I don't think > >>>you'll ever see that on bare metal. > >> > >>But that's not to say that doing something about lost ticks in > >>QEMU isn't still useful. > >> > > > >If it doesn't solve the majority of the problems it isn't very > >useful IMO. It's a good first step, but not sufficient for real > >world use with overcommit. > > Even if we have a way to detect coalescing, we still need to make > sure we don't lose ticks in QEMU. So regardless of whether it > solves the majority of problems, we need this anyway. > Actually it is very strange we lose them. Last time I checked vm_clock worked in such a way that if ticks were lost due to qemu not been scheduled for a long time timer callback was repeatedly fired to compensate for missed wakeups. -- Gleb.