From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dan Magenheimer Subject: RE: Domain-Virtual time Date: Fri, 26 Feb 2010 09:46:03 -0800 (PST) Message-ID: References: <5c3550fe1002250803q21decea2j101c53fe390b856c@mail.gmail.com> <5c3550fe1002260846s6775ac4dsabf90bba503d7b@mail.gmail.com> <20100226170101.GV368@whitby.uk.xensource.com 5c3550fe1002260920n77c43e2cmbfd6ccc29be527c9@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: <5c3550fe1002260920n77c43e2cmbfd6ccc29be527c9@mail.gmail.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Priya , Tim Deegan Cc: xen-devel@lists.xensource.com, xen-users@lists.xensource.com List-Id: xen-devel@lists.xenproject.org The timer_mode=3DX can be set in the "VM configuration" file in which you specify things like the disk, memory, etc. Setting the Xen time source on your machine is done by adding clocksource=3Dxxxxx to your Xen boot line, but unless your system has one broken clocksource and another is fine, this is rarely the source of extreme drift in an HVM. (And, by the way, doing something like enabling HPET in your BIOS may cause Xen to choose a different clocksource. Xen tries to choose what it deems to be the best, and the clocksource=3D is just a manual override.) There are similar Linux kernel parameters that must be specified within the boot-time grub file for the HVM. These parameters vary from release to release (and between 32-bit and 64-bit). Many more recent kernels allow you to change the clocksource dynamically using a sysfs file. Be aware that in some cases, a misspelled option name or one that works fine on one version but not on the currently booting Xen/kernel, may be completely and silently ignored. And finally, time has been a moving target on Xen, Linux, VMware, etc, so problems that appear in older versions may have been resolved in later versions. ANY time you post questions/problems regarding time (or for that matter even non-time-related problems), it is best to describe what version of Xen and kernels you are using. =3D=3D=3D=3D=3D=3D From: Priya [mailto:pbhat@acis.ufl.edu]=20 Sent: Friday, February 26, 2010 10:20 AM To: Tim Deegan Cc: Dan Magenheimer; xen-devel@lists.xensource.com; xen-users@lists.xensour= ce.com Subject: Re: [Xen-devel] Domain-Virtual time Thanks Tim! I have some follow-up questions: 1. How can I check/change the timer_mode on a XenServer HVM and on a Xen HV= M? 2. How can I check/change the time-source on my machines? 3. No; they are showing system time modified according to the timer_mode setting, then extrapolated into a collection of virtual timers and reconstituted by the linux kernel. Can you please elaborate a little bit. Or please point me to a reference.= =20 Thanks again ! Pr On Fri, Feb 26, 2010 at 12:01 PM, Tim Deegan wrote: Hi, At 16:46 +0000 on 26 Feb (1267202798), Priya wrote: > The funny thing is that NTP is measuring a very different drift on my > three machines (-189.206, -108.373 and -71.321 parts per million). The > drift reported on Domain-0 is -11.393. So I don't think my machines > are showing the system time. No; they are showing system time modified according to the timer_mode setting, then extrapolated into a collection of virtual timers and reconstituted by the linux kernel. I don't know a lot about linux HVM because I always run Linux with PV kernels, but on Windows I've found that the (virtual) ACPI PM timer is a better time-source than the HPET or RTC. > In addition, the negative sign on the drift means that my machines are > running faster that the real time, which is again puzzling. Xen system time generally does drift forward, IIRC that's because xen always tends to catch up to the fastest CPU, so cross-CPU jitter turns into forward drift. =A0Linux may be doing the same thing inside the HVM VM, where the jitter is higher. Tim. -- Tim Deegan Principal Software Engineer, XenServer Engineering Citrix Systems UK Ltd. =A0(Company #02937203, SL9 0BG)