From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift Date: Mon, 07 Feb 2011 07:11:29 -0600 Message-ID: <4D4FEF81.1040603@codemonkey.ws> References: <480481933.225059.1296734409954.JavaMail.root@zmail07.collab.prod.int.phx2.redhat.com> <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> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Jan Kiszka , qemu-devel , Glauber Costa , Ulrich Obergfell , kvm To: Avi Kivity Return-path: Received: from mail-ww0-f44.google.com ([74.125.82.44]:55600 "EHLO mail-ww0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751200Ab1BGNLe (ORCPT ); Mon, 7 Feb 2011 08:11:34 -0500 Received: by wwa36 with SMTP id 36so4762474wwa.1 for ; Mon, 07 Feb 2011 05:11:32 -0800 (PST) In-Reply-To: <4D4FE6BF.5080502@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: 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. Regards, Anthony Liguori