Re: Xen timing mode

[Priya <pbhat@acis.ufl.edu>]

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The Xen Interface manual for Xen v2.0 (attached) Page 4 defines Domain -
virtual time as

" The time that progresses at the same pace as system time, but only while a
domain is executing -- it stops while a domain is de-scheduled. Therefore
the share of the CPU that a domain receives is indicated by the rate at
which its virtual time increases."

I am interesting in finding out if any of the timing system
calls/instructions like gettimeofday(), hwclock(), rdtsc() can read this
domain virtual time.

Thanks, again !

PS: I could not find a similar documentation for Xen v3.0 or later


On Tue, Feb 23, 2010 at 12:18 PM, Keir Fraser <keir.fraser@eu.citrix.com>wrote:

> What do you mean by domain - virtual time?
>
>
> On 23/02/2010 17:12, "Priya" <pbhat@acis.ufl.edu> wrote:
>
> > Thanks !
> >
> > In that case, how does a para-virtualized user domain find the domain -
> > virtual time ?
> >
> > --Priya
> >
> > On Tue, Feb 23, 2010 at 12:08 PM, Keir Fraser <keir.fraser@eu.citrix.com
> >
> > wrote:
> >> Independent_wallclock is specific to Linux PV guests, and is set within
> the
> >> guest. Timer_mode is specific to HVM guests and is a domain build-time
> >> config option.
> >>
> >>  -- Keir
> >>
> >> On 23/02/2010 16:50, "PBhat" <pbhat@acis.ufl.edu> wrote:
> >>
> >>>
> >>> A question about timing modes in Xen came up after reading the
> documentation
> >>> on
> >>>
> http://wiki.xensource.com/xenwiki/XenConfigurationFileOptions?highlight=(int<http://wiki.xensource.com/xenwiki/XenConfigurationFileOptions?highlight=%28int>
> >>> er
> >>> <
> http://wiki.xensource.com/xenwiki/XenConfigurationFileOptions?highlight=%28
> >>> inter>
> >>> rupts)|(timer)
> >>> configuration file options  and I was wondering if you could help me
> with
> >>> it.
> >>>
> >>>     * I know that Xen has a facility on the user domains to change the
> >>> timing from something called the '
> http://openskill.info/infobox.php?ID=1451
> >>> independent wallclock ' to 'dependent clock' and vice versa. The way to
> do
> >>> this is to toggle the parameter /proc/sys/xen/independent_wallclock
> between
> >>> 0 (dependent) and 1 (independent)
> >>>
> >>>
> >>>     * Now, the documentation on the configuration options for xen
> virtual
> >>> machine under the heading of Timers contains no mention of the above
> >>> mentioned facility, but instead talks about something called a
> 'timer_mode'
> >>>
> >>>
> >>>  timer_mode: Timer mode (0=delay virtual time when ticks are missed;
> >>> 1=virtual time is always wallclock time
> >>>       timer_mode (default=1; Value='TIMER_MODE')
> >>>
> >>> Now my question is whether the timer mode  = 1 is equivalent to
> independent_
> >>> wallclock?
> >>>
> >>> I think the answer is no. While timer_mode = 1 decides whether the
> virtual
> >>> machine is able to read the Domain-0 time, the independent_wallclock
> decides
> >>> whether you need NTP sychronization on both (user and control) domains
> or
> >>> not.
> >>>
> >>> Namely, in the timer_mode = 1, the gettimeofday() called from the
> virtual
> >>> machine will return the domain virtual time, but time_mode = 0
> >>> gettimeofday() instruction from the virtual machine will return the
> system
> >>> time.
> >>>
> >>> The independent and dependent modes have an effect on whether the
> virtual
> >>> machines can use the NTP or any other synchronization of Domain-0.
> >>>
> >>> Is my understanding correct?
> >>>
> >>> I further want to know whether the working remains similar on Xen HVMs
> also.
> >>> In the sense that are Hardware based virtual machines capable of
> reading the
> >>> Domain-0 system time in an analogous way?
> >>>
> >>>
> >>> Your help would be much appreciated. I am new to this area and I
> apologize
> >>> if my questions seem too basic.
> >>
> >>
> >>
> >
> >
>
>
>
>

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