From mboxrd@z Thu Jan 1 00:00:00 1970 From: Priya Subject: Re: [Xen-devel] Domain-Virtual time Date: Fri, 26 Feb 2010 12:20:01 -0500 Message-ID: <5c3550fe1002260920n77c43e2cmbfd6ccc29be527c9@mail.gmail.com> References: <5c3550fe1002250803q21decea2j101c53fe390b856c@mail.gmail.com> <5c3550fe1002260846s6775ac4dsabf90bba503d7b@mail.gmail.com> <20100226170101.GV368@whitby.uk.xensource.com> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="===============0601010018==" Return-path: In-Reply-To: <20100226170101.GV368@whitby.uk.xensource.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-users-bounces@lists.xensource.com Errors-To: xen-users-bounces@lists.xensource.com To: Tim Deegan Cc: Dan Magenheimer , "xen-devel@lists.xensource.com" , "xen-users@lists.xensource.com" List-Id: xen-devel@lists.xenproject.org --===============0601010018== Content-Type: multipart/alternative; boundary=000e0cd5d1c60bfded0480841c7e --000e0cd5d1c60bfded0480841c7e Content-Type: text/plain; charset=ISO-8859-1 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 HVM? 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. 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. Linux 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. (Company #02937203, SL9 0BG) > > --000e0cd5d1c60bfded0480841c7e Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Thanks Tim!

I have some follow-up questions:
  1. How can I ch= eck/change the timer_mode on a XenServer HVM and on a Xen HVM?
    2. <= li>How can I check/change the time-source on my machines?
  2. 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 elaborat= e a little bit. Or please point me to a reference.
Than= ks again !
Pr



On Fri, Feb 26, = 2010 at 12:01 PM, Tim Deegan <Tim.Deegan@citrix.com> 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<= br> > 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 machine= s
> 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 <Tim.Deegan@citrix.c= om>
Principal Software Engineer, XenServer Engineering
Citrix Systems UK Ltd. =A0(Company #02937203, SL9 0BG)


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