From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: Make QEmu HPET disabled by default for KVM? Date: Thu, 11 Mar 2010 10:46:06 +0200 Message-ID: <4B98ADCE.5060206@redhat.com> References: <201003111552.54293.sheng@linux.intel.com> <4B98A99C.8020909@redhat.com> <20100311083157.GZ16909@redhat.com> <201003111638.48661.sheng@linux.intel.com> <20100311084252.GA16909@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Sheng Yang , Marcelo Tosatti , kvm@vger.kernel.org To: Gleb Natapov Return-path: Received: from mx1.redhat.com ([209.132.183.28]:53469 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752367Ab0CKIqK (ORCPT ); Thu, 11 Mar 2010 03:46:10 -0500 In-Reply-To: <20100311084252.GA16909@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 03/11/2010 10:42 AM, Gleb Natapov wrote: > On Thu, Mar 11, 2010 at 04:38:48PM +0800, Sheng Yang wrote: > >> On Thursday 11 March 2010 16:31:57 Gleb Natapov wrote: >> >>> On Thu, Mar 11, 2010 at 10:28:12AM +0200, Avi Kivity wrote: >>> >>>> On 03/11/2010 10:23 AM, Sheng Yang wrote: >>>> >>>>>>> I have kept --no-hpet in my setup for >>>>>>> months... >>>>>>> >>>>>> Any details about the problems? HPET is important to some guests. >>>>>> >>>>> Seems like HPET reaction is too slow to satisfy some guests(for it would >>>>> replace PIT). >>>>> >>>>> Here is the thread last time. >>>>> >>>>> http://thread.gmane.org/gmane.comp.emulators.kvm.devel/44899 >>>>> >>>> Thanks. We can address this in three ways: first, adjust the guest >>>> not to do timing related tests when virtualized (since no matter >>>> what we do, the tests may fail). Second, I think we should >>>> implement userspace ack notifiers (similar to tpr access notifiers >>>> already present). Third, we can implement a kernel hpet, which, >>>> after we solve the zillion bug it introduces, will also give a nice >>>> performance improvement for hpet intensive workloads. >>>> >>> Second will not solve the problem. Presence of ack notifiers will not >>> make HPET interrupt arrive faster. >>> >> The slow may also due to lost tick. And with the lost tick, hpet is still >> unusable... >> >> > If the problem it due to lost ticks reinjection may solve it, but only partially. > What if IO thread haven't run even once during the time vcpu did clock > source check? IIRC sometimes we trigger this even with in kernel PIT. > That is true. Reinjection can correct problems in the long term, but may fail in the short term. 10 ticks is easily short term in a heavily loaded system. How does it happen with kernel PIT? I could understand it if we had a work item doing the injection, but everything happens either from hrtimer context or vcpu context. -- error compiling committee.c: too many arguments to function