From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift Date: Mon, 07 Feb 2011 14:34:07 +0200 Message-ID: <4D4FE6BF.5080502@redhat.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Anthony Liguori , qemu-devel , Glauber Costa , Ulrich Obergfell , kvm To: Jan Kiszka Return-path: Received: from mx1.redhat.com ([209.132.183.28]:12096 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751474Ab1BGMeO (ORCPT ); Mon, 7 Feb 2011 07:34:14 -0500 In-Reply-To: <4D4BBF55.9060000@web.de> Sender: kvm-owner@vger.kernel.org List-ID: 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. -- error compiling committee.c: too many arguments to function